The Med1 Subunit of the Mediator Complex Induces Liver Cell Proliferation and Is Phosphorylated by AMP Kinase

Mediator, a large multisubunit protein complex, plays a pivotal role in gene transcription by linking gene-specific transcription factors with the preinitiation complex and RNA polymerase II. In the liver, the key subunit of the Mediator complex, Med1, interacts with several nuclear receptors and transcription factors to direct gene-specific transcription. Conditional knock-out of Med1 in the liver showed that hepatocytes lacking Med1 did not regenerate following either partial hepatectomy or treatment with certain nuclear receptor activators and failed to give rise to tumors when challenged with carcinogens. We now report that the adenovirally driven overexpression of Med1 in mouse liver stimulates hepatocyte DNA synthesis with enhanced expression of DNA replication, cell cycle control, and liver-specific genes, indicating that Med1 alone is necessary and sufficient for liver cell proliferation. Importantly, we demonstrate that AMP-activated protein kinase (AMPK), an important cellular energy sensor, interacts with, and directly phosphorylates, Med1 in vitro at serine 656, serine 756, and serine 796. AMPK also phosphorylates Med1 in vivo in mouse liver and in cultured primary hepatocytes and HEK293 and HeLa cells. In addition, we demonstrate that PPAR alpha activators increase AMPK-mediated Med1 phosphorylation in vivo. Inhibition of AMPK by compound C decreased hepatocyte proliferation induced by Med1 and also by the PPAR alpha activators fenofibrate and Wy-14,643. Co-treatment with compound C attenuated PPAR alpha activator-inducible fatty acid beta-oxidation in liver. Our results suggest that Med1 phosphorylation by its association with AMPK regulates liver cell proliferation and fatty acid oxidation, most likely as a downstream effector of PPAR alpha and AMPK.