2.1 Lipid Metabolism
Ovarian cancer is unique in abdominal implantation, a way of metastasis compared with other epithelial malignancies, and the most common site of metastasis is the omentum rich in fat. Omental adipocytes can secrete adipokines to promote the selective orientation of cancer cells[14]. Then, ECO cells interact with and convert them into “cancer-associated adipocytes”[15], absorbing them to meet their demands for energy. Mukherjee A et al.[15] report that adipocytes can support requirements of ovarian cancer cells through the reprogramming of glucose metabolism. In addition, cholesterol levels in tumor cells were elevated compared to normal cells, and OC cells rely more on uptake of exogenous lipids and cholesterol[16]. De Wolf et al. found the up-regulation of HMGCR in ECO, compared with normal ovarian epithelial cells[17]. Interestingly, in platinum-resistant ovarian cancer cells, HUANG X Y, et al. discover the up-regulation of low-density lipoprotein receptor (LDLR) expression and down-regulation of HMGCR expression[18]. The factors inscribed into changes in the two drug targets is necessary to be further studied. In addition, high cholesterol levels in tumor cells is associated with platinum resistance in ovarian cancer[19], possibly due to preventing the entrence of platinum by reduction of the permeability and fluidity of the cell membranes of cancer cells.
These studies suggest that lipid metabolism plays an important role in the metastasis and drug resistance of ovarian cancer, and statins has great potential in anti-ovarian cancer because it inhibits HMGCR in the mevalonate pathway (MVAP) to reduce the production of cholesterol.
While Criscuolo,D.et al. declared that statins in OC cells would induce platinum resistance by stimulating exogenous cholesterol to increase intake due to inhibition of endogenous cholesterol synthesis in the MVAP, the conclution exists some paradox theoretically[19]. Because it was based on the study of HUANG X Y, et al. [18] about the up-regulation of low-density lipoprotein receptor (LDLR) expression and down-regulation of HMGCR expression in platinum-resistant ovarian cancer cell, which merely indicated that platinum-resistant cells have increased uptake intensity of exogenous cholesterol. If statins would increase intracellular cholesterol by activating more LDLRs, the speed should be faster than that of its inhibition of endogenous cholesterol synthesis, then the result would contradict their lipid-lowering effects and anticancer results in clinic. Hence, the key may be the reconstruction of balance of endogenous and exogenous cholesterol synthesis, and the main strategy for this problem should focus on two aspects, decreasing endogenous cholesterol synthesis and reducing the concentration of exogenous cholesterol or targeting LDL receptors. In addition, statins can re-polarize tumor-associated macrophages (TAM) via reducting cholesterol to increase tumor necrosis factor (TNF)-α, but to decrease transforming growth factor (TGF)-β, which suppresses epithelial-to-mesenchymal transition (EMT) (a biological process by epithelial cells losing their cellular identity and acquiring a mesenchymal phenotype, involving cancer metastasis)[20]. Moreover, combination of cholesterol and its G protein-coupled receptor (GPCR) can activate the Hedgehog (HH) pathway that has been proved an extraordinary promotion for the development of EOC[21]. Statins is confirmed to reduce proliferation of medulloblastoma cells by reduction of cholesterol systhesis to inhibit HH pathway[22], but whether statins fighting against ovarian cancer by the mechanism is necessary to establish further.
2.2 MVAP beyond cholesterol systhesis
In addition to cholesterol biosynthesis, the MVAP is a crucial metabolic pathway for numerous cellular activities[23], and HMGCR on which is a drug target of statins. In EOC, HMGCR is also established as a metabolic oncogene, improving tumor development, and commonly seen in OC cells with the TP53 mutation[24]. Thus the inhibition of statins for it should achieve a good anti-ovarian cancer effect in theory. Meanwhile, statins decreases the production of farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) by inhibiting HMGCR[25], which effect guanosine-triphosphate hydrolase (GTPases), such as Ras and Rho, that participate in proliferation, migration and cell invasion[26], and then farnesylation is unable to complete, which can make the protein non-polar to achieve cell membrane anchoring. Indeed, MVAP can also restrain EMT without cholesterol. Kato S.et al. demonstrated that statins revamped the stemness and EMT marker expression patterns (both in mRNA and protein levels) in ovarian cancer by the MVAP inactivating of the Hippo/YAP/Rho pathway[27]. Therefore, statin can restrict the proliferation and metastasis of EOC. Moreover, the results after knocking out the hydroxymethylglutaryl (HMG) metabolic target were similar to those of statins, further confirming that statins can inhibit ovarian cancer progression through the MVAP[28].
2.3Voltage-dependent anion channel 1 ( VDAC1)
The mitochondrial porins, called voltage dependent anionic channels (VDAC) plays a main role in selective permeability, which contributes to constant exchange of metabolites and ions with the cytoplasm through the external mitochondrial membrane (MOM)[29]. Among VDAC isoforms in mammals, namely VDAC1, VDAC2, and VDAC3, VDAC1 is the most characteristic one, serving as an essential gate for metabolites in the MOM (ATP/ADP, NAD+/NADH, Krebs cycle’s intermediates, cholesterol and glutamate), and taking part in cholesterol distribution and in fatty acid transport across the MOM, meanwhile, modulating the flow of small ions (Cl−, K+, Na+, and Ca2+)[30]. What’s more, it is acknowledged as a regulator of apoptosis. Under the stimulation of apoptosis, VDAC1 interacts with the pro-apoptotic protein Bax to form a channel that promotes the release of cytochrome c (CYT c) to the cytoplasm to activate apoptosis[31].
It is found that VDAC1 is upregulated in OC cell lines but not in normal cell line and has been proved as a gene related positively with statin response in OC cells[32]. Meanwhile, VDAC1 can be regulated by statins, which binds to hexokinase (HK), the rate-limiting enzyme of glycolysisIt, to play a role in the interconnection between mitochondrial respiration and the regulation of glycolysis, likely to be a potential therapeutic target in OC[33].
2.4 The PI3K/AKT/ mTOR signal pathway
The control of cell survival, growth, proliferation, angiogenesis, transcription, translation, and metabolism is largely dependent on the PI3K/AKT/mTOR signaling pathway[34]. It is overexpressed in 45% of high-grade serous ovarian cancer (HGSOC) and can promote OC cells proliferation and anti-apoptosis[35]. The study of J. Huang, L and co-workers implies a close connection of specific genetic aberrations in OC cell lines with siRNA targeting components in the PI3K/AKT/mTOR signaling pathway[36]. The PI3K/Akt/mTOR signaling pathway also activates sterol regulatory element binding protein (SREBPs) transcription to promote cholesterol uptake and synthesis to meet the needs of cancer cells[37]. Exactly, statins have a link with this pathway, which are observed that act on cancer cells can inhibit PI3K/AKT activation to promote the expression of PTEN[38]. And they can depend on mTOR to inhibit Akt phosphorylation and nuclear translocation, then to sensitize p53-deficient cells to cytostatic drugs in hepatocellular carcinoma HepG2 cells and non-small cell lung cancer cells[39]. Moreover, Stine et al. point out that simvastatin blocks the PI3K/AKT pathway in SKOV3 and HEY cellst to increase the active oxygen level to cause DNA damage and to reduce the vascular endothelial growth factor (VEGF) expression, to induce endoplasmic reticulum (ER) stress, having an anti-proliferation and metastasis effect on ovarian cancer[40].
The synergies of statins and antitumor therapies in OC
3.1 Paclitaxel and platinum
Paclitaxel and platinum are well-known anticancer agents, the former by interfering with spindle formation to promote tubulin polymerization and to block mitosis at the metaphase-anaphase transition[41], the latter binding to DNA by the formation of intra-stranded and inter-stranded crosslinks to kill tumor cells as main mechanism. Cisplatin, carboplatin, and oxaliplatin, as platinum-based anticancer drugs, with remarkable therapeutic effects, are widely used in the clinic.
Statins combined with cisplatin can arrest cell cycle and induce premature apoptosis, with accompanied dysregulation of Ras pathway proteins, finally resulting in synergistic reduction in ovarian cancer cell proliferation[42]. Robinson,E.et al. found that if simvastatin was administered together with either carboplatin or paclitaxel, additive effects were seen in the inhibition of autophagosome trafficking in human ovarian cancer cell lines; however, when simvastatin was given before carboplatin, strong antagonism was observed[43]. Actually, the plasma concentration of statin obtained in this context is inadequate to counteract the effect of carboplatin, implying that this is not a significant issue[43]. It also remind us that would be better to study how to increase the concentration of statins in the ovaries.
There have also been many experimental studies of statins combined with paclitaxel or platinum-based drugs for other cancers. The simvastatin’s combination with paclitaxel remarkably augments efficacy in cellular system and xenograft mouse model of cervical cancer by by depleting GGPP, inhibiting prenylation, decreasing GTPases activities[44]. Lovastatin can sensitize cells to paclitaxel and dampen metastatic spread by depleting caveolin-1 or inhibiting MVAP from destroying lipid rafts, then decreasing the metastatic-potential and chemoresistance in CD133 Hipancreatic tumor initiating cells[45]. Combination therapy of low-dose paclitaxel and fluvastatin reduced cell viability and induced apoptosis by forming DNA fragmentation in primary meningioma cell culture[46]. It is discovered that combined action of lovastatin and paclitaxel resulted in upgraded mitosin levels and that lovastatin changed the association of mitosin with condensed chromosomes, in the human leukemia K562 and HL-60 cell lines[47].
What’s more, with regard to drug resistence, it was revealed that simvastatin can inhibit FAK signaling pathway to resensitize the drug-resistant cancer cells to paclitaxel, by destroying lipid rafts, cholesterol-rich domains, and suppressing integrin-β3 and focal adhesion formation[48].
In terms of drug side effects, rosuvastatin and duloxetine bring down mechanical allodynia and thermal hyperalgesia in mice treated with paclitaxel.[49] Simvastatin and rosuvastatin may have a protective effect against cisplatin-induced kidney and liver damage via amelioration of lipid peroxidation as well as due to improvement of kidney and liver function, and lipid-lowering effects in rats[50]. These potential beneficial results imply a bonus to protect normal organs for statins except for anti-cancer effects.
3.2 Bevacizumab
Bevacizumab is a monoclonal antibody to inhibit VEGF. By linking with VEGF-A, bevacizumab obstructs the link between VEGF-A and VEGFR, thus preventing the VEGF signalling pathways that stimulate neovascularization[51]. It prolongs progression free survival (PFS) when added to first-line chemotherapy, and to platinum-sensitive disease. Bevacizumab has also shown activity in platinum-resistant ovarian cancer (PROC). Early treatment of PROC with a combination of bevacizumab and chemotherapy allows most patients to benefit from anti-angiogenic therapy[52].
The investigation of the relationship between statins and angiogenesis has been done, and interestingly, the effects of HMG-CoA reductase inhibition on angiogenesis is biphasic dose-dependent, proangiogenic at low therapeutic concentrations but reversed by GGPP at high, which is related with alterations in VEGF signaling and endothelial apoptosis[53]. Even though low-dose statins may have a little effect on angiogenesis or tumor growth, they are still thought to increase the sensitivity of cancer cells to signalling chemotherapy[54]. This may explain why some clinical studies favor low-dose, continuous statin therapy over short-dose, high-dose statin therapy, but this does not lead to more complete and valid conclusions[55].
Therefore, theoretically, it can be inferred that bevacizumab and statins have a certain synergistic effect on anti-cancer. In a study of Lee et al., statins (simvastatin, lovastatin, atorvastatin, and pravastatin) in combination with bevacizumab directly suppress angiogenic mediators, such as angiopoietin2, binding immunoglobulin protein (BiP), and Hsp90a, to reduce the cell viability, migration, invasion, and tube formation of human umbilical vein endothelial cell. The synergy of bevacizumab with simvastatin obviously weakens the growth and metastases of xenograft tumours in contrast to with bevacizumab alone[56]. Although there is a lack of relevant studies on the treatment of ovarian cancer by statin combined with bevacizumab, the above theories and studies imply the potential of this combination therapy as a new research direction.
3.3 Poly ADP-ribose polymeraseb (PARP) inhibitors ( PARP-is)
PARP-is became the first targeted treatment for HGSOC, selectively active for women with double-strand DNA break repair (BRCA) mutations and/or a homologous recombination (HR) deficiency (HRD) phenotype up to 50% of cases[57]. Mostly, PARP inhibitors eliminate selectively cancer cells through DNA damage accumulation, after block the single-strand DNA break repair process in BRCA1/2 and/or HRD cancer cells, while unaffecting normal cells with an intact double-strand DNA break repair system[57]. With further clinical trials of olaparib, niraparib and rucaparib, they usually cause a good initial response, and treatment landscape hasn’t been limited to a subgroup of women with relapsed ovarian cancer, which has shifted to the first-line maintenance therapy setting[60], but many patients develop resistance to PARP-is and appear disease progression or relapse of tumor cells[57].
According to the experimental and clinical results on PARP-is activating AKT under oxidative stress conditions[61], it emphasizes Akt activation that is critical in the cytoprotective properties of PARP1 inhibition, which can undermine the cellular inhibitory effect of PARP1 inhibitor on cancers carrying BRCA mutations. Reports about the increase of PARP-is’ cytotoxicity by Akt inhibitor support this notion[62]. As experimental evidences in vitro and in vivo indicate that inhibition of PARP1 can activate the cytoprotective PI3K-Akt pathway, which induce mitochondrial protection and apoptosis resistance that contribute to the limitation of cytostatic efficacy in PARP-is therapy[63].
Actually, many studies on the anti-cancer mechanism of statins have proved that they are inhibitors of AKT, that is, they reduce the phosphorylation of AKT to inhibit the proliferation of cancer cells[64]. A study indicates that statin-induced inhibition of Akt phosphorylatio can sensitize cells to cytostatic drugs. This mechanism appears to occur only in cancer cells, whereas in normal cells statins activate the AKT pathway for cell protection. To a certain extent, the overlapping results of these studies suggest that statins may counteract parp inhibitor resistance through AKT signaling pathway, and the potential as a new combination therapy for ovarian cancer, which needs to be confirmed in further experiments and clinical trials.