SOCS (suppressor of cytokine signaling)-3 has recently been shown to be an insulin- and tumor necrosis factor (TNF)-alpha-induced negative regulator of insulin signaling. To further clarify a potential involvement of SOCS-3 in the development of insulin resistance, we measured differentiation-dependent SOCS-3 mRNA expression in 3T3-L1 adipocytes and studied its regulation by various hormones known to impair insulin signaling using quantitative real-time RT-PCR. There was a differentiation-dependent downregulation of SOCS-3 mRNA by 50% over the 9 day adipocyte differentiation course. Interestingly, besides insulin and TNF-alpha, chronic treatment of differentiated 3T3-L1 cells with 10 microM isoproterenol for 16 h stimulated SOCS-3 gene expression by about 3.5-fold. Furthermore, isoproterenol stimulated SOCS-3 mRNA expression in a dose-dependent manner with significant activation detectable at concentrations as low as 10 nM isoproterenol. Moreover, a strong 27- and 47-fold activation of SOCS-3 mRNA expression could be seen after 1 h of isoproterenol and GH treatment respectively. The stimulatory effect of isoproterenol could be almost completely reversed by pretreatment of 3T3-L1 cells with the beta-adrenergic antagonist propranolol. Finally, isoproterenol's action could be mimicked by stimulation of G(S)-proteins with cholera toxin and of adenylyl cyclase with forskolin and dibutyryl cAMP. Taken together, our results demonstrate a differentiation-dependent downregulation of SOCS-3 in adipocytes and suggest that SOCS-3 gene expression is stimulated by beta-adrenergic agents via activation of a G(S)-protein-adenylyl cyclase-dependent pathway. As SOCS-3 is a novel inhibitor of insulin signaling, the data support a possible role of this protein as a selectively regulated mediator of catecholamine-induced insulin resistance.
Isoproterenol is a positive regulator of the suppressor of cytokine signaling-3 gene expression in 3T3-L1 adipocytes
