Insulin receptor kinase‐independent signaling via tyrosine phosphorylation of phosphatase PHLPP1

Abstract

Most insulin responses correlate well with insulin receptor (IR) Tyr kinase activation; however, critical exceptions to this concept have been presented. Specific IR mutants and stimulatory IR antibodies demonstrate a lack of correlation between IR kinase activity and specific insulin responses in numerous independent studies. IR conformation changes in response to insulin observed with various IR antibodies define an IR kinase-independent signal that alters the C-terminus. IR-related receptors in lower eukaryotes that lack a Tyr kinase point to an alternative mechanism of IR signaling earlier in evolution. However, the implied IR kinase-independent signaling mechanism remained obscure at the molecular level. Here we begin to define the molecular basis of an IR-dependent but IR kinase-independent insulin signal that is equally transmitted by a kinase-inactive mutant IR. This insulin signal results in Tyr phosphorylation and catalytic activation of phosphatase PHLPP1 via a PI 3-kinase-independent, wortmannin-insensitive signaling pathway. Dimerized SH2B1/PSM is a critical activator of the IR kinase and the resulting established insulin signal. In contrast it is an inhibitor of the IR kinase-independent insulin signal and disruption of SH2B1/PSM dimer binding to IR potentiates this signal. Dephosphorylation of Akt2 by PHLPP1 provides an alternative, SH2B1/PSM-regulated insulin-signaling pathway from IR to Akt2 of opposite polarity and distinct from the established PI 3-kinase-dependent signaling pathway via IRS proteins. In combination, both pathways should allow the opposing regulation of Akt2 activity at two phosphorylation sites to specifically define the insulin signal in the background of interfering Akt-regulating signals, such as those controlling cell proliferation and survival.

Molecular Basis of Signaling Specificity of Insulin and IGF Receptors: Neglected Corners and Recent Advances

“…Although there is overwhelming evidence supporting the essential role of RTK activity in almost all insulin/IGF actions, kinase-independent signaling by both IR and IGFR has been reported (Povsic et al, 2003; Zhang and Riedel, 2009). Most recently it has been shown that the unliganded IR and IGFR have a permissive effect on apoptosis that appears to be independent of PI3K/Akt and Ras/ERK signaling, in contrast to anti-apoptotic signaling by the ligand-activated receptors (Boucher et al, 2010a).…”

Suppression of survival signalling pathways by the phosphatase PHLPP

“…Both isozymes have an abundance of predicted phosphorylation sites, and phosphorylation is a likely candidate to control the catalytic activity of the enzyme. Indeed, it has been suggested that factors downstream of the insulin receptor may modulate PHLPP activity [64], and serum stimulation or overexpression of Akt1, 2 or 3 was reported to decrease the activity of tagged, exogenously expressed PHLPP [9]. Further study of the regulation of PHLPP activity in vivo in the coming years is likely to unveil mechanisms that control the catalytic activity of PHLPP.…”

Time to Open the Blood–Brain Barrier Gate for Biologics?

“…In this study, a standard Y-shaped antibody was used against the IR. This format could potentially explain some of the findings since a Y-shaped antibody may cause receptor dimerization [18]. This would lead to downstream cell signalling and potentially also downregulation of the IR.…”