The steroid hormone progesterone is an essential regulator of the cellular processes that are required for the development and maintenance of reproductive function. The diverse effects of progesterone are mediated by the progesterone receptor (PR). The functions of the PR are regulated not only by ligands but also by modulators of various cell signaling pathways. However, it is not clear which energy state regulates PR activity. AMP-activated protein kinase (AMPK), a serine/threonine protein kinase, is a key modulator of energy homeostasis. Once activated by an increasing cellular AMP:ATP ratio, AMPK switches off ATP-consuming processes and switches on ATP-producing processes. We found that both 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) and metformin, traditional pharmacological activators of AMPK, inhibited the PR pathway, as evidenced by progesterone response element (PRE)-driven luciferase activity and PR target gene expression. Compound C, an inhibitor of AMPK, partly but significantly reversed the anti-PR effects of AICAR and metformin. The downregulation of endogenous AMPK by small interfering RNAs (siRNAs) stimulated PR activity. AMPK activation by AICAR decreased the progesterone-induced phosphorylation of PR at serine 294 and inhibited the recruitment of PR to an endogenous PRE. Taken together, our data suggest that AMPK, an energy sensor, is involved in the regulation of PR signaling. (C) 2011 Elsevier Inc. All rights reserved.