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\begin{document}

\title{Probing the Protrusions: Lamellipodia and~ Filopodia in Cancer Invasion
and Beyond}



\author[1]{Laras Pratiwi}%
\author[1]{Elisa Elisa}%
\author[1]{Henry Sutanto}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{June 09, 2024}


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\textbf{Abstract}

The dynamic protrusions of lamellipodia and filopodia have emerged as
crucial players in tumor progression and metastasis. These membrane
structures, governed by intricate actin cytoskeletal rearrangements,
facilitate cancer cell migration, invasion, and interaction with the
tumor microenvironment. This review provides a comprehensive examination
of the structural and functional attributes of lamellipodia and
filopodia, shedding light on their pivotal roles in mediating cancer
invasion. Navigating through the intricate landscape of cancer biology,
the review illuminates the intricate signaling pathways and regulatory
mechanisms orchestrating the formation and activity of these
protrusions. The discussion extends to the clinical implications of
lamellipodia and filopodia, exploring their potential as diagnostic and
prognostic markers, and delving into therapeutic strategies that target
these structures to impede cancer progression. As we delve into the
future, the review outlines emerging technologies and unexplored facets
that beckon further research, emphasizing the need for collaborative
efforts to unravel the complexities of lamellipodia and filopodia in
cancer, ultimately paving the way for innovative therapeutic
interventions.

\textbf{Keywords:} Lamellipodia; filopodia; cancer invasion; metastasis;
mechanobiology



\textbf{1. Introduction}

Cancer cell migration is a critical factor in cancer progression,
facilitating the invasion and dissemination of cancer cells from primary
tumors to establish metastases in distant organs. This process is
complex and involves the integration of signaling pathways that regulate
cell adhesion, cytoskeleton reorganization, and interactions with the
tumor microenvironment {[}1--3{]}. Among others, collective movement,
mesenchymal migration, and amoeboid migration are crucial aspects of
cancer migration, each characterized by distinct mechanisms and
implications in the progression of cancer. Collective cell movement
involves the coordinated migration of groups of cancer cells. it is
characterized by cells moving as sheets, strands, clusters, or ducts,
and is regulated by cadherin-based junctions maintaining supracellular
properties like collective polarization and force generation {[}4{]}. It
has been seen in various cancers and relies on cell-cell adhesion
mechanisms to maintain the integrity and directionality of the group.
Studies have shown that beta1 integrins play a critical role in the
invasive migration of multicellular clusters, such as in primary
melanoma explants. Disruption of beta1-integrin function can lead to the
detachment of individual cells and switch to amoeboid migration,
highlighting the plasticity in tumor cell migration strategies {[}5{]}.
Whilst, mesenchymal migration enables cancer cells to move individually.
It is characterized by the formation of focal adhesions and the
elongation of the cell body. The transition from mesenchymal to amoeboid
migration can be induced by changes in the microenvironment, such as
confinement and low adhesion, allowing mesenchymal cells to switch to a
fast amoeboid phenotype {[}6{]}. Amoeboid migration is characterized by
high plasticity, allowing cancer cells to move independently of
adhesions, often through squeezing and deforming their cell body. This
mode can be induced under certain conditions, such as hypoxia, which
triggers a collective-to-amoeboid transition promoting the dissemination
of amoeboid-moving single cells from collective invasion strands. This
process involves hypoxia-inducible factors (HIF-1) and demonstrates the
adaptive capability of cancer cells to environmental challenges {[}7{]}.

Cellular dynamics in cancer progression have become a focal point of
research, unveiling the intricate roles played by subcellular structures
like lamellipodia and filopodia. Lamellipodia, broad, sheet-like
protrusions, and filopodia, slender, finger-like extensions, are dynamic
membrane structures crucial for cellular movement and interaction within
the complex tumor microenvironment. They are dynamic extensions
emanating from the leading edge of migrating cells, orchestrating
directed cell movement through the intricate interplay of the actin
cytoskeleton. Lamellipodia, characterized by a broad, flattened
morphology, drive cell migration by promoting adhesion to the
extracellular matrix (ECM) and facilitating the establishment of focal
contacts. On the other hand, filopodia, with their slender, finger-like
appearance, are involved in cellular probing, sensing the
microenvironment, and guiding directional movement {[}8,9{]}. Comprising
a dense network of actin filaments, lamellipodia and filopodia exhibit
distinct molecular and structural features. Lamellipodia are enriched
with branched actin networks, largely regulated by the Actin-Related
Protein 2/3 (Arp2/3) complex and the Wiskott-Aldrich Syndrome Protein
(WASP) {[}8,10,11{]}. In contrast, filopodia exhibit bundled actin
filaments regulated by proteins like fascin and the
Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family members
{[}9,12--14{]}. These structural variances contribute to their
specialized functions in cellular motility and invasion.

The relevance of lamellipodia and filopodia in cancer progression lies
in their pivotal roles in the tumor invasion and metastatic cascade
{[}9{]}. Cancer cells exploit these structures to navigate through the
intricate matrix of the tumor microenvironment, invade surrounding
tissues, and disseminate to distant sites {[}15{]}. Lamellipodia-driven
migration facilitates the invasion of cancer cells by promoting ECM
degradation and enabling efficient interaction with neighboring cells
{[}16,17{]}. Moreover, filopodia play a crucial role in guiding cancer
cells through the complex extracellular milieu, aiding in processes such
as intravasation and extravasation during metastasis {[}18{]}. Beyond
their physical contributions to invasion, these membrane protrusions
actively participate in signal transduction pathways. They respond to
extracellular cues, translating environmental signals into intracellular
responses that modulate cellular behaviors. The dysregulation of these
signaling processes contributes to aberrant cancer cell motility and
invasiveness, making lamellipodia and filopodia promising targets for
therapeutic intervention {[}10,19,20{]}.

This review aims to provide a comprehensive synthesis of current
knowledge regarding the structural and functional intricacies of
lamellipodia and filopodia in the context of cancer progression. By
critically evaluating existing literature and integrating findings from
diverse studies, we seek to offer a holistic understanding of how these
protrusions contribute to cancer cell invasion and metastasis.
Furthermore, this review serves as a platform for discussing the
regulatory mechanisms governing lamellipodia and filopodia dynamics in
cancer cells. Insights into the signaling pathways and key molecular
players involved will be dissected, providing a roadmap for potential
therapeutic interventions. Beyond elucidating their roles in cancer
progression, this review will explore the clinical implications of
lamellipodia and filopodia, considering their potential as diagnostic
and prognostic markers.

\textbf{2. Lamellipodia and Filopodia: Structure and Function}

\emph{2.1. Definition and characteristics of lamellipodia}

Lamellipodia are characterized by a complex and dynamic actin
cytoskeleton organization. The actin filaments within lamellipodia form
a dense meshwork, creating a broad, sheet-like protrusion at the leading
edge of migrating cells. This organization is largely regulated by the
Arp2/3 complex and WASP, orchestrating the polymerization of actin
filaments in a branched pattern {[}8{]}. The dynamic interplay between
actin assembly and disassembly within lamellipodia allows for rapid
membrane protrusion and retraction, facilitating efficient cell
movement. Lamellipodia are essential for promoting cell motility by
facilitating adhesion to the ECM and establishing focal contacts. The
leading edge of lamellipodia contains membrane ruffles, allowing for
dynamic interactions with the surrounding environment {[}19{]}. The
continuous extension and retraction of lamellipodia create a wave-like
pattern, propelling the cell forward. This dynamic behavior is crucial
for processes such as chemotaxis, where cells navigate gradients of
signaling molecules, and haptotaxis, where cells move along
concentration gradients of matrix-bound factors {[}10,21,22{]}.

\emph{2.2. Definition and characteristics of filopodia}

Filopodia, in contrast to lamellipodia, exhibit a slender, finger-like
morphology characterized by the formation of tightly bundled actin
filaments. These actin bundles are organized in a parallel fashion,
providing structural stability to the protrusion. Key regulatory
proteins involved in filopodia formation include fascin and members of
the Ena/VASP family {[}9,23,24{]}. The bundling of actin filaments
imparts rigidity to filopodia, allowing them to serve as exploratory
antennae for the cell. Filopodia play a critical role in cellular
sensing and directional migration. These structures are enriched with
receptors and sensors that enable cancer cells to perceive the
surrounding microenvironment. Through interactions with the ECM and
neighboring cells, filopodia contribute to the transmission of external
signals into the cell, influencing migratory responses {[}25,26{]}.
Moreover, filopodia are involved in guiding cells during directional
migration, facilitating the navigation of cancer cells through complex
tissue architectures and aiding in processes such as intravasation and
extravasation during metastasis {[}18{]}.

Together, lamellipodia and filopodia contribute to the directional
motility of cancer cells by extending the cell's leading edge and
interacting with the extracellular matrix, thus aiding in metastasis and
invasion. Lamellipodia act as a motor pulling the cell forward, while
filopodia serve sensory or exploratory functions {[}9{]}.

\textbf{3. Role of Lamellipodia and Filopodia in Cancer Progression}

\emph{3.1. Lamellipodia- and filopodia-driven cancer cell migration and
invasion}

Lamellipodia are instrumental in mediating cancer cell invasion by
orchestrating the degradation of the ECM. Lamellipodia play a role in
forming invadopodia -- specialized membrane structures that focus
proteolytic activity, facilitating localized ECM degradation {[}9{]}.
This process is facilitated by the secretion of matrix
metalloproteinases (MMPs) and other proteolytic enzymes, enabling cancer
cells to breach physical barriers and invade surrounding tissues
{[}27{]}. The dynamic protrusions of lamellipodia create focal points
for the assembly of integrins, transmembrane receptors that connect the
cell to the ECM, fostering adhesion and promoting localized matrix
degradation {[}28,29{]}. In aggressive cancers, elevated expression of
MMPs in lamellipodia correlates with increased invasive potential. The
spatiotemporal regulation of ECM degradation by lamellipodia is critical
for tumor progression, allowing cancer cells to navigate through the
intricate matrix and invade neighboring tissues {[}30{]}. Additionally,
lamellipodia extend beyond their role in ECM degradation to facilitate
dynamic interactions with neighboring cells. Adhesion structures formed
by lamellipodia, such as focal adhesions, not only promote stable
attachments to the ECM but also enable communication between cancer
cells and surrounding stromal or immune cells {[}31{]}. This
intercellular crosstalk influences collective migration, allowing groups
of cancer cells to coordinate their invasive behavior {[}9{]}. Through
these interactions, lamellipodia contribute to the creation of a
permissive microenvironment that supports invasion and further
potentiates metastasis.

Filopodia, with their slender and elongated morphology, serve as sensory
extensions that guide cancer cells through the complex terrain of the
tumor microenvironment. Rich in receptors and adhesion molecules,
filopodia sense and respond to chemotactic gradients, allowing cancer
cells to navigate toward specific regions within the tumor. Through
dynamic probing and sensing, filopodia facilitate the recognition of
guidance cues, such as growth factors and chemokines, directing cancer
cells towards blood vessels or areas of increased stromal support. This
guidance is critical for the spatial organization of cancer cells within
the tumor, influencing invasion patterns and ultimately shaping the
metastatic potential of the cancer {[}25,26{]}. Filopodia play a crucial
role in the metastatic cascade, particularly during extravasation, where
cancer cells exit the bloodstream and invade distant tissues {[}18{]}.
As cancer cells intravasate into the bloodstream, filopodia aid in their
interaction with endothelial cells, facilitating the adhesion to and
transmigration through the vascular endothelium {[}32{]}. Once in the
extravascular space, filopodia continue to contribute to metastasis by
guiding cancer cells through the foreign microenvironment. The ability
of filopodia to establish contacts with neighboring cells, including
stromal cells and other cancer cells, enhances their invasive potential.
Filopodia-driven invasion is thus a key determinant in the establishment
of secondary tumor foci, shaping the metastatic spread of cancer cells
{[}9{]}.

\emph{3.2. The roles of lamellipodia and filopodia in immune cells'
activity during malignancy}

Lamellipodia and filopodia on macrophages and T cells play crucial roles
in malignancy, largely due to their involvement in cell movement,
environmental sensing, and immune responses. Macrophage filopodia assist
in the phagocytic uptake of particles, aiding in pathogen clearance.
They capture pathogens by various mechanisms, including surfing along
the filopodial shaft towards the cell body and sweeping actions. These
mechanisms are crucial for the phagocytic uptake of particles and, by
extension, plays a significant role in tumor immunity where phagocytosis
of cancer cells is involved {[}33{]}. In T cells, lamellipodia and
filopodia play a crucial role in crossing endothelial barriers, a
process vital during immune surveillance and inflammation. Rho GTPases,
which regulate cytoskeletal dynamics, are essential for T-cell
polarization and migration {[}34{]}. A study by Doh, Song, and Kwon
(2013) found that T cells utilize lamellipodia to sense topography of
endothelial cell layers and filopodia to sense nuclei of endothelial
cells, crucial for optimal intraluminal path finding during
transendothelial migration {[}35{]}. Ward (2009) described
shear-facilitated chemokine-induced adhesive filopodia on crawling T
lymphocytes that scan the endothelial surface for potential sites of
transendothelial migration, indicating a novel mode of lymphocyte
locomotion over endothelial cells before extravasation {[}36{]}. Shulman
et al. (2009) found that endothelial-presented chemokines triggered
high-affinity lymphocyte function-associated antigen 1 (LFA-1) and
adhesive filopodia underneath crawling lymphocytes, which were critical
for lymphocyte crawling and probing for transendothelial migration sites
{[}37{]}.

\textbf{4. Regulation of Lamellipodia and Filopodia in Cancer Cells}

The intricate dynamics of lamellipodia and filopodia in cancer cells are
tightly regulated by a complex network of signaling pathways and
regulatory proteins (\textbf{Figure 1} ) {[}38--40{]}. At the forefront
of regulating cytoskeletal dynamics are the Rho family of small GTPases,
particularly RhoA, Rac1, and Cdc42. These GTPases act as molecular
switches, cycling between an active GTP-bound state and an inactive
GDP-bound state. The activation of Rho GTPases orchestrates the
intricate processes of lamellipodia and filopodia formation
{[}38,39,41{]}. Activation of RhoA promotes actomyosin contractility,
influencing the rear retraction of the cell during migration. This
counteracts the protrusive forces generated by lamellipodia, ensuring
coordinated cell movement. Rac1 activation stimulates lamellipodia
formation by promoting actin polymerization at the leading edge
{[}9,29,39,42{]}. WASP and Arp2/3 complex are critical regulators of
lamellipodia formation, acting downstream of Rac1 activation. Activated
by Rac1, WASP binds to the Arp2/3 complex, promoting the nucleation of
new actin filaments. This nucleation initiates the formation of branched
actin networks characteristic of lamellipodia. Through the branching of
actin filaments, Arp2/3 complex facilitates the creation of a dendritic
network that drives the protrusion of the plasma membrane during
lamellipodia formation. Overall, the WASP-Arp2/3 pathway exemplifies the
intricate molecular machinery that governs the dynamics of lamellipodia,
allowing cancer cells to extend and retract their leading edges during
migration {[}43--45{]}. Next, activation of Cdc42 induces filopodia
formation by promoting the bundling of actin filaments. It engages with
proteins like Ena/VASP, facilitating the elongation of filopodia
{[}9,39{]}. Ena/VASP family of proteins enhance actin filament
elongation and bundling, promoting the formation of parallel actin
bundles characteristic of filopodia. They interact with actin barbed
ends, inhibiting capping and facilitating filament growth {[}14,46{]}.
Ena/VASP proteins contribute to filopodia elongation, stability, and
navigation through the tumor microenvironment, ultimately influencing
cancer cell invasion and metastasis {[}47,48{]}. Overall, the
coordinated activity of regulatory proteins, such as WASP and Ena/VASP,
ensures the precise orchestration of lamellipodia and filopodia
dynamics, allowing cancer cells to respond to external stimuli and
navigate through complex tissue environments.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1-new-new/Figure-1-new-new}
\caption{{Molecular Pathways in Tumor Cell Invasion and Migration. This diagram
illustrates the complex intracellular signaling pathways that govern the
invasion and migration of tumor cells. Key components include the
endothelin-1 (ET-1) signaling through endothelin A receptor (ETAR) and
endothelin B receptor (ETBR), which leads to actin polymerization via
multiple pathways. Proteins like RhoC, RhoA, Rac1, and Cdc42 are small
GTPases that play pivotal roles in cytoskeletal dynamics. ROCK1/2 are
Rho-associated protein kinases involved in actin-myosin contraction.
LIMK and p-cofilin are implicated in actin filament stabilization and
turnover. mDia2, IQGAP, N-WASP, WAVE, and the Arp2/3 complex are crucial
for actin nucleation and polymerization, forming structures such as
filopodia and lamellipodia. These processes are aided by the release of
matrix metalloproteinases (MMPs) like MMP1, MMP13, MMP14, and MMP10,
which remodel the extracellular matrix to facilitate tumor cell
movement. (ET-1: Endothelin-1; ETAR: Endothelin A receptor; RhoC, RhoA,
Rac1, Cdc42: Small GTPases involved in cytoskeletal organization;
ROCK1/2: Rho-associated protein kinases; LIMK: LIM domain kinase;
p-cofilin: phosphorylated cofilin; mDia2: Mammalian diaphanous-related
formin 2; IQGAP: IQ motif containing GTPase activating protein; N-WASP:
Neural Wiskott-Aldrich syndrome protein; WAVE: WASP family
verprolin-homologous protein; Arp2/3: Actin-related protein 2/3 complex;
MMPs: Matrix metalloproteinases)
{\label{831201}}%
}}
\end{center}
\end{figure}

The delicate balance and spatiotemporal regulation of Rho GTPase
activity determine the type and extent of protrusions formed, playing a
central role in the migratory and invasive behavior of cancer cells.
These GTPases activate downstream effectors such as Rho-associated
protein kinases (ROCK1/2) that facilitate the phosphorylation of myosin
light chains (MLC), leading to increased actomyosin contractility.
Simultaneously, LIM kinases (LIMK) phosphorylate cofilin (p-cofilin),
which prevents actin filament depolymerization, thus stabilizing the
cytoskeleton. Meanwhile, proteins like mDia2 and N-WASP, in conjunction
with the Arp2/3 complex, promote actin nucleation, a process fundamental
to the polymerization of actin filaments, giving rise to the dynamic
restructuring of the cytoskeleton. Concurrently, the cell coordinates
the trafficking and releasing of matrix metalloproteinases (MMPs) via
molecules such as IQGAP. MMPs are proteolytic enzymes that degrade
components of the extracellular matrix, clearing a path through the
tissue and enabling the invasive behavior of the tumor cell {[}49{]}.

The proteins WASP and WASP-family Verprolin-homologous protein (WAVE),
integral to Rho GTPase signaling, are key regulators of the actin
cytoskeleton, playing crucial roles in the formation of lamellipodia and
filopodia. They are activated by Rho family GTPases, such as Rho, Rac1,
and Cdc42, leading to the formation of cellular protrusions essential
for cell motility. In cancer, this mechanism becomes pivotal, as the
dynamic rearrangement of the actin cytoskeleton facilitates invasive and
metastatic behaviors of tumor cells. These proteins serve as critical
activators of the actin cytoskeleton, which plays a central role in the
morphological and motile capabilities of cancer cells. The activation of
specific Rho GTPases - Rho, Rac1, and Cdc42 - by WASP and WAVE leads to
the formation of actin stress fibers, membrane ruffles, lamellipodia,
and filopodia. These actin structures are not merely components of
cellular architecture but are actively involved in the directional
motility of cancer cells, a key process in the invasion of surrounding
tissues and the progression to metastasis. The actin cytoskeleton's
dynamic reorganization, facilitated by these proteins, enables cancer
cells to form cellular protrusions essential for their movement and
interaction with the extracellular matrix. This interaction is
particularly important for the cells' ability to degrade barriers, a
prerequisite for invasive behavior. Moreover, these structures enable
the cells to navigate complex extracellular environments, aiding in
their metastatic spread. Furthermore, by influencing the actin dynamics
and cell motility, these proteins emerge as potential targets for
therapeutic interventions aimed at mitigating cancer metastasis
{[}50,51{]}.

Colorectal cancer, the third leading cause of cancer-related deaths
globally, has been linked to WAVE2 expression. Studies indicate WAVE2's
association with liver metastasis, disease progression, and the
activation of TGF-\selectlanguage{greek}β\selectlanguage{english}1 and YAP1 signaling pathways in colorectal cancer.
WAVE2's role is critical in colorectal liver metastasis, particularly
through its regulation by TGF-\selectlanguage{greek}β\selectlanguage{english}1 in the cancer immune microenvironment.
In cervical cancer, which predominantly affects women aged 35-44,
overexpression of SH3BP1 has been found to increase Rac1 and WAVE2
activity, enhancing invasion, migration, and chemoresistance. WAVE2 has
also been implicated in the invasiveness and motility of pancreatic
cancer cells. It is shown that WAVE2 interacts with alpha-actinin 4
(ACTN4), affecting cell movement and invasiveness. Prostate cancer
research indicates the involvement of WAVE2 in cell invasion and
metastasis, particularly through its interaction with PIP3 and
Rac1-induced actin reorganization. WAVE2 is also significant in breast
cancer, the most common cancer among women worldwide. It contributes to
the formation of lamellipodial protrusions in cancer cells and is
associated with aggressive cancer types like triple-negative breast
cancer (TNBC). The binding of WAVE2 to the Arp2/3 complex plays a
crucial role in breast cancer cell migration and invasion, with
implications for potential therapeutic targets {[}51{]}.

Integrins, transmembrane receptors that link the ECM to the actin
cytoskeleton, also play a pivotal role in regulating lamellipodia and
filopodia dynamics {[}29,52{]}. Integrin engagement with the ECM
initiates signaling cascades that influence focal adhesion formation and
cytoskeletal rearrangements. Integrins activate focal adhesion kinase
(FAK) and Src kinases, initiating downstream signaling events that
regulate the activity of Rho GTPases. Focal adhesions serve as anchoring
points for actin filaments and contribute to the stability of
lamellipodia and filopodia {[}53{]}. The turnover of these focal
adhesions is essential for dynamic cell movement. The interplay between
integrins, focal adhesion signaling, and Rho GTPases coordinates the
adhesive and protrusive forces required for effective cell migration and
invasion in the tumor microenvironment {[}16{]}.

\textbf{5. Clinical Implications}

As these cellular protrusions play pivotal roles in cancer progression,
their dysregulation holds potential as diagnostic and prognostic markers
and provides a foundation for developing targeted therapeutic
strategies.

\emph{5.1. Diagnostic and prognostic markers}

Examining the expression levels of key proteins associated with
lamellipodia and filopodia dynamics holds promise as diagnostic and
prognostic markers in various cancers. Aberrant expression or
overactivation of proteins involved in the formation and regulation of
these protrusions may serve as indicators of invasive potential and
disease progression. Elevated expression of Rho GTPases, such as Rac1
and Cdc42, or overexpression of regulatory proteins like WASP and
Ena/VASP, may correlate with more aggressive cancer phenotypes.
Increased levels of these proteins might be indicative of enhanced
migratory and invasive capabilities in cancer cells {[}47,54,55{]}.
Additionally, techniques such as immunohistochemistry and molecular
profiling can be employed to assess the expression levels of these
proteins in tumor tissues. High-throughput analyses can provide a
comprehensive profile of the molecular landscape, aiding in the
identification of potential biomarkers for specific cancer types.

Advanced imaging techniques offer a non-invasive means to visualize and
quantify lamellipodia and filopodia in cancer cells, providing valuable
information for diagnosis and prognosis. Real-time observation of cancer
cells using live-cell imaging allows for the dynamic visualization of
lamellipodia and filopodia. This technique enables researchers and
clinicians to monitor the migratory behavior of cancer cells and assess
the impact of therapeutic interventions {[}56,57{]}. Super-resolution
microscopy techniques, such as stimulated emission depletion (STED)
microscopy and stochastic optical reconstruction microscopy (STORM),
surpass the diffraction limit, providing detailed insights into the
subcellular structures of lamellipodia and filopodia {[}58{]}. This can
aid in precise characterization and quantification of these protrusions,
offering valuable information for diagnostic purposes. Utilizing a
combination of expression profiling and advanced imaging techniques,
clinicians can gain a more comprehensive understanding of the invasive
potential of cancer cells, enabling tailored therapeutic strategies.

\emph{5.2. Therapeutic targeting strategies}

Targeting key proteins involved in the regulation of lamellipodia and
filopodia dynamics presents a promising avenue for therapeutic
intervention. Small molecules and inhibitors designed to disrupt the
formation or activity of these cellular protrusions have the potential
to impede cancer cell invasion. Small molecules targeting Rho GTPases,
such as Rac1 and Cdc42, can interfere with the signaling pathways
responsible for lamellipodia and filopodia formation (\textbf{Table 1}
). These inhibitors aim to disrupt the cytoskeletal dynamics, inhibiting
the protrusive forces driving cancer cell migration {[}59--61{]}.
Meanwhile, targeting the WASP-Arp2/3 complex pathway with specific
inhibitors can disrupt the nucleation and branching of actin filaments,
inhibiting lamellipodia formation. Such inhibitors may attenuate the
invasive potential of cancer cells {[}45{]}. Therapeutic strategies can
also be designed to target signaling pathways that regulate lamellipodia
and filopodia dynamics. By modulating these pathways, it is possible to
influence the migratory behavior of cancer cells and impede their
invasive capabilities. Small molecules targeting integrins or FAK can
disrupt the signaling cascades that link extracellular signals to the
actin cytoskeleton. This interference may hinder the formation and
stability of lamellipodia and filopodia {[}62--64{]}. Developing
inhibitors specific to Ena/VASP proteins could hinder the bundling and
elongation of actin filaments in filopodia. This approach may prove
effective in curtailing the formation and function of filopodia in
cancer cells {[}65{]}. Tailoring therapeutic strategies to interfere
with the molecular machinery governing lamellipodia and filopodia
dynamics represents a novel approach in cancer treatment, particularly
for malignancies with a strong invasive component.

\textbf{Table 1. Characteristics, Regulation and Implications of
Lamellipodia and Filopodia in Cancer Cell Progression}\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
\begin{minipage}[b]{0.30\columnwidth}\raggedright\strut
\textbf{Feature}\strut
\end{minipage} & \begin{minipage}[b]{0.30\columnwidth}\raggedright\strut
\textbf{Lamellipodia}\strut
\end{minipage} & \begin{minipage}[b]{0.30\columnwidth}\raggedright\strut
\textbf{Filopodia}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
\textbf{Definition}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Broad, sheet-like protrusions at the leading edge of migrating cells,
essential for cell movement.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Thin, spike-like protrusions from the leading edge of migrating cells,
playing roles in sensing the cellular environment and
directionality.\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
\textbf{Key Cytoskeletal Components}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Actin filaments arranged in a branched network, primarily regulated by
the Arp2/3 complex.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Tightly bundled actin filaments, elongated by formins and enabled by
fascin.\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
\textbf{Primary Regulators}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Rac1 GTPase stimulates the Arp2/3 complex to initiate actin
polymerization.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Cdc42 GTPase activates formins to promote actin polymerization and
bundling.\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
\textbf{Signaling Molecules}\strut
\end{minipage} & \begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
- Rac1 - Arp2/3 complex - WAVE complex\strut
\end{minipage} & \begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
- Cdc42 - Ena/VASP proteins - Formins\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
\textbf{Pathways Involved}\strut
\end{minipage} & \begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
\textbf{Rho GTPase signaling}: - Rac1 activation leads to WAVE complex
recruitment, activating the Arp2/3 complex for actin nucleation.
\textbf{PI3K/Akt signaling}: - Promotes Rac1 and Arp2/3 complex
activities, enhancing lamellipodia formation and cell migration. -
Influences the activity of proteins that control actin polymerization
and depolymerization, regulating the dynamic rearrangement of the actin
cytoskeleton for lamellipodia extension.\strut
\end{minipage} & \begin{minipage}[t]{0.32\columnwidth}\raggedright\strut
\textbf{Rho GTPase and PI3K/Akt signaling}: - Cdc42 activation triggers
formin-mediated actin elongation. Formins are actin-binding proteins
that nucleate the elongation of unbranched actin filaments. The PI3K/Akt
pathway can influence the activity of formins directly or indirectly
through Cdc42. \textbf{FAK-Src signaling}: - Facilitates
integrin-mediated signaling, enhancing filopodia formation for cell
adhesion and migration.\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
\textbf{Role in Cancer}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Lamellipodia are crucial for cancer cell migration, invasion, and
metastasis by facilitating cell movement through the ECM.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Filopodia contribute to cancer cell invasion and metastasis by probing
the environment, forming contacts with the ECM, and directing
migration.\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
\textbf{Targeted Therapies}\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Inhibitors targeting Rac1 or the Arp2/3 complex to disrupt lamellipodia
formation and hinder cancer cell migration.\strut
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright\strut
Small molecules or peptides inhibiting Cdc42 activity or formin function
to prevent filopodia formation and impair metastatic potential.\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

Cellular protrusions of immune cells can also be employed as a
therapeutic modality to treat cancer. Weiskopf and Weissman (2015)
explored the role of macrophages in antibody therapies for cancer,
highlighting their ability to perform antibody-dependent phagocytosis
{[}66{]}. This process significantly involves the use of filopodia for
initial contact and engagement with target cells, emphasizing the
critical role of macrophage filopodia in cancer therapies. Pathria,
Louis, and Varner (2019) discussed the critical pathways regulating the
recruitment, polarization, and metabolism of tumor-associated
macrophages (TAMs) during tumor progression. The ability of TAMs to
phagocytose tumor cells, a process that involves the use of filopodia,
was highlighted as a potential therapeutic strategy {[}67{]}. Jaiswal,
Chao, Majeti, and Weissman (2010) discussed how macrophages act as
mediators of tumor immunosurveillance. They emphasized the role of
macrophages in the recognition and phagocytic clearance of cancer cells,
a process that involves the dynamic use of filopodia {[}68{]}.

\textbf{6. Future Perspectives}

\emph{6.1. Emerging technologies for studying lamellipodia and
filopodia}

First, the advent of single-cell omics technologies holds tremendous
promise for dissecting heterogeneity within cancer cell populations
{[}69{]}. By analyzing individual cells, researchers can uncover
variations in lamellipodia and filopodia dynamics that may be masked in
bulk analyses. Single-cell RNA sequencing and proteomics can provide a
nuanced understanding of how individual cancer cells modulate protrusion
dynamics in response to microenvironmental cues. Second, continuous
advancements in live-cell imaging technologies offer unprecedented
opportunities to capture the dynamic behavior of lamellipodia and
filopodia in real-time. High-speed, high-resolution microscopy combined
with super-resolution techniques enables detailed visualization of these
cellular protrusions. Additionally, the integration of multi-dimensional
imaging modalities, such as fluorescence and label-free imaging,
enhances the spatiotemporal resolution, allowing for a comprehensive
exploration of protrusion dynamics in diverse physiological contexts
{[}56{]}. Third, computational tools for quantitative image analysis are
evolving rapidly, providing researchers with the ability to extract
precise measurements and quantitative data from imaging experiments.
Automated algorithms can track the dynamics of lamellipodia and
filopodia, enabling the quantification of parameters such as protrusion
length, speed, and branching patterns. These tools facilitate
large-scale data analysis and contribute to a more systematic
understanding of the factors influencing protrusion dynamics
{[}70--72{]}.

\emph{6.2. Unexplored aspects and potential areas for research}

The interplay between cancer cells and the immune system remains an
underexplored aspect of protrusion dynamics. Investigating how
lamellipodia and filopodia influence immune cell interactions within the
tumor microenvironment could uncover novel mechanisms of immune evasion
and immune-mediated control of cancer cell invasion. The role of
extracellular vesicles (EVs) in mediating communication between cancer
cells and the tumor microenvironment is also an emerging area of
interest. Understanding how lamellipodia and filopodia contribute to the
release and uptake of EVs may unveil new dimensions of intercellular
communication that impact cancer progression and metastasis.
Additionally, the influence of mechanical forces on lamellipodia and
filopodia dynamics is an area ripe for exploration. Investigating how
physical cues, such as substrate stiffness and fluid shear stress,
modulate protrusion formation and function could provide insights into
the biomechanics of cancer cell invasion. While much of the existing
research relies on two-dimensional cell cultures, the transition to
three-dimensional models more closely mimics the \emph{in
vivo}microenvironment. Examining lamellipodia and filopodia dynamics
within 3D cultures or organoids can offer a more realistic
representation of cancer cell invasion, providing valuable information
for translational research {[}73--75{]}.

\emph{6.3. Integration of computational models in understanding
protrusion dynamics}

Agent-based models (ABMs) simulate the behavior of individual agents
(cells) within a defined environment {[}76{]}. Integrating ABMs allows
researchers to simulate and analyze the emergent properties of
lamellipodia and filopodia in response to various stimuli. These models
provide a platform for exploring how individual cells contribute to
collective invasive behavior and how perturbations at the cellular level
propagate through the system. Developing mechanistic computational
models that incorporate biochemical and biomechanical processes involved
in protrusion dynamics enables a more detailed understanding of the
underlying regulatory networks. Computational simulations can predict
the effects of genetic or pharmacological interventions on lamellipodia
and filopodia dynamics, guiding experimental design and hypothesis
generation. Leveraging machine learning and data-driven approaches can
uncover hidden patterns within large datasets generated from imaging
experiments. Integrating computational algorithms with experimental data
facilitates the identification of novel correlations and predictive
models, enhancing our understanding of the factors influencing
protrusion dynamics {[}77{]}.

\textbf{7. Conclusion}

In this comprehensive exploration of lamellipodia and filopodia in
cancer, key findings underscore the critical roles these cellular
protrusions play in invasion, metastasis, and intercellular
communication within the tumor microenvironment. The intricate
regulation of lamellipodia and filopodia by signaling pathways and
regulatory proteins influences cancer cell behavior, emphasizing their
significance as therapeutic targets. The insights gained from studying
lamellipodia and filopodia have profound implications for cancer
research and treatment. Diagnostic and prognostic markers associated
with these protrusions offer potential avenues for personalized
medicine. Therapeutic targeting strategies aimed at disrupting the
dynamics of lamellipodia and filopodia may curb the invasive potential
of cancer cells, presenting novel approaches in the fight against
metastatic disease. As we conclude this exploration, a call for
continued investigation and collaboration resonates strongly. The
uncharted territories of immune interactions, extracellular vesicle
communication, mechanical influences, and three-dimensional modeling
present exciting opportunities for future research. The integration of
computational models and advanced technologies is essential for
unraveling the complexities of lamellipodia and filopodia dynamics, and
collaboration across disciplines will drive transformative advancements
in our understanding of cancer biology.

\textbf{Funding}

This research receives no external funding.

\textbf{Declaration of competing interest}

The authors have no competing interests to declare.

\textbf{Declaration of generative AI and AI-assisted technologies in the
writing process}

During the preparation of this work the author used ChatGPT version 3.5
in order to improve readability and language. After using this
tool/service, the author reviewed and edited the content as needed and
takes full responsibility for the content of the publication.

\textbf{Acknowledgments}

None

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