Discussion
In this study we investigated the potential sPLA2 inhibitory activity of
several plant extracts that are used in traditional medicine in Sri
Lanka and found that T. hispida aqueous and butanol soluble
fraction had potent sPLA2 inhibitory activities. In fact, it had
significantly higher sPLA2 inhibitory activity than the commercial sPLA2
inhibitor CAY10590, when assessed in dengue patient sera. The HPLC
analysis showed that T.
hispida butanol fraction contained many flavonoid compounds, which have
previously shown to inhibit sPLA2 activity 22. Snake
venom is known to be a rich source of sPLA2 enzymes and aqueous solution
of leaves and roots of Tragia involucrata have been taken orally
by certain tribes in Tamil Nadu to treat snake bites23. However, the presence of sPLA2 inhibitory activity
of these plant extracts had not been characterized previously.
The phospholipase A2 enzymes have many inflammatory actions in addition
to generation of PAF, which has shown to be an important mediator of
endothelial dysfunction in dengue 10. sPLA2 is an
acute phase protein, with a wide range of inflammatory effects18. Lipopolysaccharide (LPS), hypoxia and cytokines
have shown to induce its activity 24,25. We have
previously shown that the sPLA2 activity is significantly higher during
early illness (72 to 84 hours since onset of illness) in patients who
subsequently proceed to develop DHF 9. In addition to
its widely known inflammatory effects, sPLA2 has also shown to activate
cytoplasmic PLA2 in mast cells, which in turn could contribute to
generation of PAF and other arachidonic acid metabolites such as
leukotrienes 26. PAF acted synergistically with sPLA2
to induce neutrophil exocytosis, thereby is likely to further contribute
to endothelial dysfunction and elevated cytokines seen in dengue27. Leukotrienes (LTE4) levels were shown to be higher
in patients who proceeded to develop DHF, and the LTE4 levels continued
to rise in patients with DHF19. Since sPLA2 appears to
have a wide range of activities in the pathogenesis of endothelial
dysfunction and severe dengue, drugs which inhibit sPLA2 could be of
potential therapeutic value.
In this study we have shown that the leave extract of T. hispidawhich has been used for centuries to treat fever and inflammatory
diseases in traditional medicine has potent sPLA2 inhibitory activity
and in the cell cytotoxicity assays, it was shown to have
minimum toxicity for mammalian
cell lines. However, we could not isolate the exact compound in the
extract of T. hispida that had these functions. Since the whole
leave extract appears to be safe due to long term consumption and based
on the evidence of in vitro studies, it would be important to carry out
double-blind placebo controlled clinical trials to evaluate its efficacy
in treating patients with acute dengue. In addition, it would be most
important to also identify the exact chemical structure of this compound
that had sPLA2 inhibitory activity.