Published Online
on February 4, 2008

A more recent version of this article appeared on February 19, 2008

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Submitted on October 9, 2007
Accepted on December 14, 2007

Phosphorylation of LKB1 at Serine 428 by Protein Kinase C- Is Required for Metformin-Enhanced Activation of the AMP-Activated Protein Kinase in Endothelial Cells

Zhonglin Xie MD, PhD, Yunzhou Dong PhD, Roland Scholz BS, Dietbert Neumann PhD, and Ming-Hui Zou MD, PhD^*

From the Division of Endocrinology and Diabetes, Department of Medicine, University of Oklahoma Health Sciences Center, Oklahoma City (Z.X., Y.D., M.Z.); and Institute of Cell Biology, ETH Zurich, Zurich, Switzerland (R.S., D.N.).

^* To whom correspondence should be addressed. E-mail: ming-hui-zou{at}ouhsc.edu.

Background—Metformin, one of most commonly used antidiabetes drugs, is reported to exert its therapeutic effects by activating AMP-activated protein kinase (AMPK); however, the mechanism by which metformin activates AMPK is poorly defined. The objective of the present study was to determine how metformin activates AMPK in endothelial cells.

Methods and Results—Exposure of human umbilical vein endothelial cells or bovine aortic endothelial cells to metformin significantly increased AMPK activity and the phosphorylation of both AMPK at Thr172 and LKB1 at Ser428, an AMPK kinase, which was paralleled by increased activation of protein kinase C (PKC)-, as evidenced by increased activity, phosphorylation (Thr410/403), and nuclear translocation of PKC-. Consistently, either pharmacological or genetic inhibition of PKC- ablated metformin-enhanced phosphorylation of both AMPK-Thr172 and LKB1-Ser428, suggesting that PKC- might act as an upstream kinase for LKB1. Furthermore, adenoviral overexpression of LKB1 kinase-dead mutants abolished but LKB1 wild-type overexpression enhanced the effects of metformin on AMPK in bovine aortic endothelial cells. In addition, metformin increased the phosphorylation and nuclear export of LKB1 into the cytosols as well as the association of AMPK with LKB1 in bovine aortic endothelial cells. Similarly, overexpression of LKB1 wild-type but not LKB1 S428A mutants (serine replaced by alanine) restored the effects of metformin on AMPK in LKB1-deficient HeLa-S3 cells, suggesting that Ser428 phosphorylation of LKB1 is required for metformin-enhanced AMPK activation. Moreover, LKB1 S428A, like kinase-dead LKB1 D194A, abolished metformin-enhanced LKB1 translocation as well as the association of LKB1 with AMPK in HeLa-S3 cells. Finally, inhibition of PKC- abolished metformin-enhanced coimmunoprecipitation of LKB1 with both AMPK1 and AMPK2.

Conclusions—We conclude that PKC- phosphorylates LKB1 at Ser428, resulting in LKB1 nuclear export and hence AMPK activation.

Key words: diabetes mellitus • endothelium • metabolism • molecular biology

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Circulation 2008 117: 857-859. [Extract] [Full Text]

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