Essential role of PSM/SH2‐B variants in insulin receptor catalytic activation and the resulting cellular responses

Abstract

The positive regulatory role of PSM/SH2-B downstream of various mitogenic receptor tyrosine kinases or gene disruption experiments in mice support a role of PSM in the regulation of insulin action. Here, four alternative PSM splice variants and individual functional domains were compared for their role in the regulation of specific metabolic insulin responses. We found that individual PSM variants in 3T3-L1 adipocytes potentiated insulin-mediated glucose and amino acid transport, glycogenesis, lipogenesis, and key components in the metabolic insulin response including p70 S6 kinase, glycogen synthase, glycogen synthase kinase 3 (GSK3), Akt, Cbl, and IRS-1. Highest activity was consistently observed for PSM alpha, followed by beta, delta, and gamma with decreasing activity. In contrast, dominant-negative peptide mimetics of the PSM Pro-rich, pleckstrin homology (PH), or src homology 2 (SH2) domains inhibited any tested insulin response. Potentiation of the insulin response originated at the insulin receptor (IR) kinase level by PSM variant-specific regulation of the Km (ATP) whereas the Vmax remained unaffected. IR catalytic activation was inhibited by peptide mimetics of the PSM SH2 or dimerization domain (DD). Either peptide should disrupt the complex of a PSM dimer linked to IR via SH2 domains as proposed for PSM activation of tyrosine kinase JAK2. Either peptide abolished downstream insulin responses indistinguishable from PSM siRNA knockdown. Our results implicate an essential role of the PSM variants in the activation of the IR kinase and the resulting metabolic insulin response. PSM variants act as internal IR ligands that in addition to potentiating the insulin response stimulate IR catalytic activation even in the absence of insulin.

SH2B1 regulation of energy balance, body weight, and glucose metabolism

“…Replacement of IR Tyr 1158 with Phe disrupts IR binding to SH2B1, and completely blocks the ability of SH2B1β to stimulate IR kinase activity [48] . SH2B1α similarly increases IR catalytic activity [49] . Additionally, SH2B1β directly binds to tyrosyl phosphorylated IRS1 and IRS2 and protects IRS proteins against dephosphoarylation, thus prolonging the ability of IRS proteins to activate their downstream pathways [48] .…”

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

“…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 [Zhang et al, 2008b]. However, here we show that it is also an inhibitor of the IR kinase-independent insulin signal, and disruption of SH2B1/PSM dimer binding to IR potentiates this signal.…”

“…We had earlier reported that PSM DD or SH2 domain mimetics inactivated the IR kinase, reflecting an essential requirement of PSM dimer binding for IR kinase activity [Zhang et al, 2008b]. The observation that these same conditions actually potentiated PHLPP1 Tyr phosphorylation and activity (Figs.…”

“…Intriguingly, insulin-mediated PHLPP1 activation was highly potentiated by PSM DD or SH2 domain peptide mimetics that disrupt the established IR-PSM signaling complex and inactivate the IR kinase [Zhang et al, 2008b], whereas a control Pro-rich domain PSM peptide mimetic had no effect. In contrast, increased levels of the PSM variants that potentiate IR kinase activity as reported earlier [Zhang et al, 2008b] highly attenuated insulin-meditated PHLPP1 activation with an inversely proportional pattern of activity. No wortmannin sensitivity was observed for insulinmediated PHLPP1 activation under any of the tested conditions ( Fig.…”

“…We had earlier reported the essential role of SH2B1/PSM variants in the activation of the IR kinase and the resulting metabolic insulin response that included the regulation of the central mediator Akt2 [Zhang et al, 2008b], the major isoform involved in insulin action [Cho et al, 2001;Bae et al, 2003]. A PH domain leucine-rich repeat protein phosphatase termed PHLPP1/2 had been reported to regulate Akt by dephosphorylating its hydrophobic motif and for Akt2 (Ser 474) PHLPP1 was identified as the preferred isoform [Gao et al, 2005;Brognard et al, 2007;Mendoza and Blenis, 2007].…”