Abstract
We previously reported that Sildenafil enhances apoptosis and antitumor efficacy of doxorubicin (DOX) while attenuating its cardiotoxic effect in prostate cancer. In the present study, we investigated the mechanism by which sildenafil sensitizes DOX in killing of prostate cancer (PCa) cells, DU145. The death receptor Fas (APO-1 or CD95) induces apoptosis in many carcinoma cells, which is negatively regulated by anti-apoptotic molecules such as FLIP (Fas-associated death domain (FADD) interleukin-1-converting enzyme (FLICE)-like inhibitory protein). Co-treatment of PCa cells with sildenafil and DOX for 48 hours showed reduced expression of both long and short forms of FLIP (FLIP-L and -S) as compared to individual drug treatment. Over-expression of FLIP-s with an adenoviral vector attentuated the enhanced cell-killing effect of DOX and sildenafil. Colony formation assays also confirmed that FLIP-S over-expression inhibited the DOX and sildenafil-induced synergistic killing effect as compared to the cells infected with an empty vector. Moreover, siRNA knock-down of CD95 abolished the effect of sildenafil in enhancing DOX lethality in cells, but had no effect on cell killing after treatment with a single agent. Sildenafil co-treatment with DOX inhibited DOX-induced NF-κB activity by reducing phosphorylation of IκB and nuclear translocation of the p65 subunit, in addition to down regulation of FAP-1 (Fas associated phosphatase-1, a known inhibitor of CD95-mediated apoptosis) expression. This data provides evidence that the CD95 is a key regulator of sildenafil and DOX mediated enhanced cell death in prostate cancer.
Highlights
Sildenafil citrate (Viagra), a highly selective inhibitor of cGMP-specific phosphodiesterase type 5 (PDE5), is used clinically for treating erectile dysfunction (ED) and pulmonary hypertension
FLIP-s was overexpressed in DU145 cells by adenoviral infection (Figure 3 upper panel). cell death assessed by trypan blue exclusion assay confirmed that sildenafil potentiated DOX-induced cell killing in cells infected with control virus (Ad-CMV)
Prostate cancer is resistant to Fraction Affected (Fa)-mediated apoptosis despite high levels of Fas surface expression and no mutation in the Fas gene [24,25,26]
Summary
Sildenafil citrate (Viagra), a highly selective inhibitor of cGMP-specific phosphodiesterase type 5 (PDE5), is used clinically for treating erectile dysfunction (ED) and pulmonary hypertension. Several studies have shown that PDE5 expression is increased in multiple human carcinomas including metastatic breast cancers, colon adenocarcinoma, bladder squamous carcinoma, and lung cancers as compared to adjacent normal tissues [1,2,3,4,5,6], suggesting its potential role in controlling tumor cell growth and death. PDE5 was detected as a predominant isoform of cGMP-PDEs in many carcinoma cells lines in culture, including colonic adenocarcinoma (SW480, HCT116, HT29, T84), breast cancer (HTB26, MCF-7), lung cancer, bladder and prostate cancer (LNCAP, PC-3), and leukemia [6,7,8]. Sildenafil potentiated DOX-induced killing of androgen independent human prostate cancer cells and inhibited tumor growth in mice bearing prostate tumor xenografts [14]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
