A Novel Interplay between Rap1 and PKA Regulates Induction of Angiogenesis in Prostate Cancer

Abstract

Angiogenesis inhibition is an important therapeutic strategy for advanced stage prostate cancer. Previous work from our laboratory showed that sustained stimulation of Rap1 by 8-pCPT-2'-O-Me-cAMP (8CPT) via activation of Epac, a Rap1 GEF, or by expression of a constitutively active Rap1 mutant (cRap1) suppresses endothelial cell chemotaxis and subsequent angiogenesis. When we tested this model in the context of a prostate tumor xenograft, we found that 8CPT had no significant effect on prostate tumor growth alone. However, in cells harboring cRap1, 8CPT dramatically inhibited not only prostate tumor growth but also VEGF expression and angiogenesis within the tumor microenvironment. Subsequent analysis of the mechanism revealed that, in prostate tumor epithelial cells, 8CPT acted via stimulation of PKA rather than Epac/Rap1. PKA antagonizes Rap1 and hypoxic induction of 1α protein expression, VEGF production and, ultimately, angiogenesis. Together these findings provide evidence for a novel interplay between Rap1, Epac, and PKA that regulates tumor-stromal induction of angiogenesis.

PKA turnover by the REGγ-proteasome modulates FoxO1 cellular activity and VEGF-induced angiogenesis

“…Activation of PKA signaling can not only attenuate VEGF signaling in physiological conditions by blocking Raf activation [21] or through their actions on pp60Src [23], but also prohibit VEGF-induced increase in vascular permeability during cancer progression [22]. Paradoxically, distinct from previous demonstration that PKA promotes VEGF secretion, PKA may inhibit VEGF production by antagonizing Rap1 to regulate tumorstromal induction of angiogenesis in prostate cancer [24]. Besides the inhibitory role of PKA on VEGF signaling, direct activation of PKA signaling can induce EC apoptosis, resulting in the inhibition of angiogenesis in vivo [25].…”

Protein Kinase A-dependent Phosphorylation of Rap1 Regulates Its Membrane Localization and Cell Migration

“…For example, PKA can inhibit Rap1-dependent angiogenesis, acting downstream of Rap1 (40). cAMP-dependent activation of Epac can regulate Rap1-dependent adhesion and migration (27,(41)(42)(43)(44).…”

The pleiotropic role of exchange protein directly activated by cAMP 1 (EPAC1) in cancer: implications for therapeutic intervention

“…The inhibitory effects of 007 were not affected by EPAC1 and EPAC2 silencing using siRNA, but could be rescued by PKA inhibitors H89 and PKI [59]. This is not completely surprising because while 007 exerts about 100-fold selectivity towards EPAC1 over PKA, it still has the ability to activate PKA directly.…”

“…In prostate cancer, while some results suggest that EPAC1 promotes metastasis and proliferation [56][57][58], one study contradicts these results by showing that activation of EPAC1 by 007, an EPAC-selective agonist, inhibits migration and proliferation of human prostate carcinoma cells [46]. Subsequently, it has been shown that the inhibitory effect observed for EPAC1 in the aforementioned contradictory study was actually the result of PKA [56][57][58]; [46] a Subsequent study suggests that the apparent suppressive effect observed for EPAC1 may be attributed to PKA activation, as the EPAC-selective agonist used was shown to activate PKA [59].…”