Phosphatase 2 A represses the human alpha myosin heavy chain promoter activity

Abstract

Phosphatases are an important component of the signal transduction pathway. Recently, various studies have shown that phophatase (e.g. Map Kinase Phosphatases 1 and 2 (MKP 1 and 2), calcineurin, PP1 and PP2A) levels and/or activity are up-regulated in the failing heart. These various phophatases are implicated in a variety of cellular and molecular processes. Heart failure is also characterized by changes in the expression levels of many of the components of the contractile apparatus. Of the changes that are observed in failing hearts, increases in b-MyHC, skeletal a-actin, and ANF, with coordinate decreases in a-MyHC and SRCA2a, termed fetal gene induction, are perhaps the most well-known. Changes in MyHC are likely to play an important role in the contractile dysfunction observed in the failing heart. Recent experiments have shown that in single cell experiments, cardiac myocytes expressing only 12% of their MyHC as alpha have 52% greater power output than those expressing only beta. More recently, findings that a mutation in the aMyHC gene can cause either hypertrophic or dilated cardiomyopathies in humans, emphasize the importance of the regulation of aMyHC mRNA and protein expression that is observed in the human failing heart. Recent findings from our laboratory suggest that PP2A mRNA levels are increased in the human failing heart. In order to understand the significance of this increase we sought to determine the regulation of the fetal gene program in response to PP2A. At low concentrations Okadaic Acid is a specific inhibitor of PP2A. Here we report that aMyHC promoter activity is up-regulated in Okadaic Acid-treated neonate rat ventricular myocytes (NRVMs) and over expression of a wild type PP2A construct results in repression of aMyHC promoter activity. Moreover, over expression of a dominant negative PP2A construct dramatically up-regulates aMyHC promoter activity. Our results suggest that PP2A plays an important role in mediating aMyHC expression levels, and that its up-regulation in the failing heart is likely to have a dramatic effect on the regulation of the fetal gene program. Future studies are needed to determine other proteins involved in the PP2A regulation of the fetal gene program.