Direct binding of p85 to sst2 somatostatin receptor reveals a novel mechanism for inhibiting PI3K pathway

Abstract

Phosphatidylinositol 3-kinase (PI3K) regulates many cellular functions including growth and survival, and its excessive activation is a hallmark of cancer. Somatostatin, acting through its G protein-coupled receptor (GPCR) sst2, has potent proapoptotic and anti-invasive activities on normal and cancer cells. Here, we report a novel mechanism for inhibiting PI3K activity. Somatostatin, acting through sst2, inhibits PI3K activity by disrupting a preexisting complex comprising the sst2 receptor and the p85 PI3K regulatory subunit. Surface plasmon resonance and molecular modeling identified the phosphorylated-Y 71 residue of a p85-binding pYXXM motif in the first sst2 intracellular loop, and p85 COOH-terminal SH2 as direct interacting domains. Somatostatin-mediated dissociation of this complex as well as p85 tyrosine dephosphorylation correlates with sst2 tyrosine dephosphorylation on the Y 71 residue. Mutating sst2-Y 71 disabled sst2 to interact with p85 and somatostatin to inhibit PI3K, consequently abrogating sst2's ability to suppress cell survival and tumor growth. These results provide the first demonstration of a physical interaction between a GPCR and p85, revealing a novel mechanism for negative regulation by ligand-activated GPCR of PI3K-dependent survival pathways, which may be an important molecular target for antineoplastic therapy.

Contribution of HIF-1α in 4E-BP1 Gene Expression

“…These data suggest that 4E-BP1 phosphorylation at S65 is inversely correlated to cell density, and is severely impaired by sst2 signaling. This latter observation is consistent with our previous reports describing how sst2 inhibits the PI3K/mTOR axis (19,20), the major signaling pathway that impinges upon 4E-BP1 phosphorylation (2).…”

KIM-1–mediated phagocytosis reduces acute injury to the kidney

“…In particular, regulation of NF-κB by the phosphoinositide 3-kinase (PI3K) pathway member p85 stood out, because KIM-1 is known to directly bind to and regulate p85, and p85 has been shown to mediate KIM-1 signaling (16). In addition, p85 can suppress NF-κB activity (29,30). Downmodulation of NF-κB can explain the dramatic reduction in TLR4 activity and the decreased proinflammatory cytokine release we observed in KIM-1-expressing cells.…”

Selective agonism in somatostatin receptor signaling and regulation

“…In addition to activating the Gi/Go family of G proteins, somatostatin receptors can act via pertussis toxin independent mechanisms. In some cases these mechanisms involve activation of pertussis toxin insensitive G proteins, such as G 12 or G 14 (Hou et al, 1994;Komatsuzaki et al, 2001;Lin et al, 1996;Liu and Wong, 2005) whereas in other instances a direct interaction between sst receptors and phosphatases or kinases has been proposed (Bousquet et al, 2006;Ferjoux et al, 2003;Lopez et al, 1996). The list of molecules that bind to activated somatostatin receptors to initiate intracellular signaling is still undoubtedly incomplete and will continue to expand with additional studies.…”