AMP-activated protein kinase (AMPK), a critical signaling molecule for regulating energy homeostasis, might bi-directionally regulate inflammation, and its action mechanism leading to inflammation is not fully understood. We utilized 5-aminoimidazole-4-carboxamide riboside (AICAR) as a pharmacological activator of AMPK to unveil the effects of and signaling cascades mediated by AMPK on cyclooxygenase (COX)-2 gene expression in rat aortic vascular smooth muscle cells (VSMCs), murine macrophage cell line (J774), and human umbilical vein endothelial cells (HUVECs). Biochemical approaches were further conducted to elucidate interactions among signaling molecules. We found that AICAR could induce COX-2 protein expression in the cell types tested. This event was mediated by COX-2 gene transcription, and abrogated by compound C and 5'-iodotubercidin, suggesting the essential role of AMPK in COX-2 induction. Pharmacological and biochemical studies indicated that p38 mitogen-activated protein kinase (MAPK) activation is the common downstream signal of AMPK in COX-2 expression in all three cell types. Furthermore, we also found that TAK1 is associated with AMPKalpha2, and this binding requires an interaction between the kinase domains of both molecules. Notably data of TAK1 phosphorylation indicate that the activating state is enhanced upon AMPK activation in vivo and in vitro. Our data for the first time prove a pivotal role of TAK1 in the AMPK signaling axis. Such interaction gives AMPK an additional pathway for regulating cellular functions. Via a downstream p38 MAPK signaling cascade, AMPK-dependent TAK1 activation leads to the expression of the inflammatory COX-2 gene in various cell types.
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