PSM/SH2B1 Splice Variants: Critical Role in Src Catalytic Activation and the Resulting STAT3-Mediated Mitogenic Response
A role of PSM/SH2B1 had been shown in mitogenesis and extending to phenotypic cell transformation, however, the underlying molecular mechanism remained to be established. Here, four alternative PSM splice variants and individual function a I protein domains were compared fort heir role in I lie regulation of Src: activity. We found that elevated cellular levels of PSM variants resulted in phenotypic cell transformation and potentiated cell proliferation and survival in response to serum withdrawal. PSM variant activity presented a consistent signature pattern for any tested response of highest activity observed for gamma, followed by delta, alpha, and beta with decreasing activity. PSM-potentiated cell proliferation was sensitive to Src inhibitor herbimycin and PSM and Src were found in the same immune complex. PSM variants were substrates of the: Sic Tyr kinase and potentiated Src catalytic activity by increasing the V,, and decreasing the K, for ATP with the signature pattern of variant activity. Dominant-negative PSM peptide mimetics including the SH2 or PH domains inhibited Src catalytic activity as well as Src-mediated phenotypic cell transformation. Activation of major SIC substrate STAT3 was similarly potentiated by the PSM variants in a Src-dependent fashion or inhibited by PSM domain-specific peptide mimetics. Expression of a dominant-negative STAT3 mutant blocked PSM variant-mediated phenotypic cell transformation. Our results implicate an essential role of the PSM variants in the activation of the Src: kinase and the resulting mitogenic response-extending to phenotypic cell transformation and involving the established Src substrate STAT3. J. Cell. Biochem. 104: 105-118, 2008. (C) 2008 Wiley-Liss, Inc.