Signaling pathways mediating β 3-adrenergic receptor-induced production of interleukin-6 in adipocytes

Abstract

The β 3-adrenergic receptor (β 3AR) is an essential regulator of metabolic and endocrine functions. A major cellular and clinically significant consequence of β 3AR activation is the substantial elevation in interleukin-6 (IL-6) levels. Although the β 3AR-dependent regulation of IL-6 expression is well established, the cellular pathways underlying this regulation have not been characterized. Using a novel method of homogenous reporters, we assessed the pattern of activation of 43 transcription factors in response to the specific β 3AR agonist CL316243 in adipocytes, cells that exhibit the highest expression of β 3ARs. We observed a unique and robust activation of the CRE-response element, suggesting that IL-6 transcription is regulated via the G s-protein/cAMP/protein kinase A (PKA) but not nuclear factor kappa B (NF-κB) pathway. However, pretreatment of adipocytes with pharmacologic inhibitors of PKA pathway failed to block β 3AR-mediated IL-6 up-regulation. Additionally, stimulation of adipocytes with the exchange protein directly activated by cAMP (Epac) agonist did not induce IL-6 expression. Instead, the β 3AR-mediated transcription of IL-6 required activation of both the p38 and PKC pathways. Western blot analysis further showed that transcription factors CREB and ATF-2 but not ATF-1 were activated in a p38- and PKC-dependent manner. Collectively, our results suggest that while stimulation of the β 3AR leads to a specific activation of CRE-dependent transcription, there are several independent cellular pathways that converge at the level of CRE-response element activation, and in the case of IL-6 this activation is mediated by p38 and PKC but not PKA pathways.