G(s)alpha repression of adipogenesis via Syk

G(s)alpha regulates the differentiation of 3T3-L1 mouse embryonic fibroblasts to adipocytes, a process termed adipogenesis. Inducers of adipogenesis lead to a loss of G(s)alpha and derepress differentiation to adipocytes. The broad spectrum tyrosine kinase inhibitor genistein is shown to block induction of adipogenesis, suggesting an early role of tyrosine phosphorylation in adipogenesis. Staining of phosphotyrosine identified prominent staining of a approximately 70-kDa protein, hypothesized to be the tyrosine kinase Syk. Reverse transcription and polymerase chain reaction amplification established the expression of Syk mRNA in these embryonic fibroblasts. Immunoprecipitations with Syk-specific antibodies demonstrated the presence of Syk in fibroblasts and a rapid increase in the amount of phospho-Syk, peaking at 24 h post induction. Clones constitutively expressing G(s)alpha, which can no longer be induced to differentiate, no longer display increased phospho-Syk levels in response to inducers. The linkage between G(s)alpha and Syk was probed by immunoprecipitations revealing association of Syk with G(s)alpha in the absence of induction. Upon induction of adipogenesis, G(s)alpha levels decline and phospho-Syk levels as well as Syk kinase activity increase. Expression of wild-type Syk both potentiates the ability of inducers to act as well as induces adipogenesis itself. Expression of the kinase-deficient Syk had no such effects on adipogenesis. These data provide a new insight into the control of adipogenesis, suggesting that G(s)alpha represses adipogenesis via Syk. Treatment with the inducers promotes a decline in G(s)alpha, increases in levels of phospho-Syk, and adipogenesis.