Contraction Accelerates Myosin Heavy Chain Synthesis Rates in Adult Cardiocytes by an Increase in the Rate of Translational Initiation

Charles T Ivester,William J Tuxworth,George Cooper,Paul J Mcdermott

doi:10.1074/jbc.270.37.21950

Charles T Ivester, William J Tuxworth + Show 2 more

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https://doi.org/10.1074/jbc.270.37.21950

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Abstract

The purpose of this study was to determine the mechanism by which contraction acutely accelerates the synthesis rate of the contractile protein myosin heavy chain (MHC). Laminin-adherent adult feline cardiocytes were maintained in a serum-free medium and induced to contract at 1 Hz via electrical field stimulation. Electrical stimulation of contraction accelerated rates of MHC synthesis 28%, p < 0.05 by 4 h as determined by incorporation of [3H]phenylalanine into MHC. MHC mRNA expression as measured by RNase protection was unchanged after 4 h of electrical stimulation. MHC mRNA levels in messenger ribonucleoprotein complexes and translating polysomes were examined by sucrose gradient fractionation. The relative percentage of polysomebound MHC mRNA was equal at 47% in both electrically stimulated and control cardiocytes. However, electrical stimulation of contraction resulted in a reproducible shift of MHC mRNA from smaller polysomes into larger polysomes, indicating an increased rate of initiation. This shift resulted in significant increases in MHC mRNA levels in the fractions containing the larger polysomes of electrically stimulated cardiocytes as compared with nonstimulated controls. These data indicate that the rate of MHC synthesis is accelerated in contracting cardiocytes via an increase in translational efficiency.

Highlights

The purpose of this study was to determine the mechanism by which contraction acutely accelerates the synthesis rate of the contractile protein myosin heavy chain (MHC)
These studies demonstrated that: 1) rates of MHC synthesis are accelerated by electrical stimulation without a corresponding change in steady state mRNA levels, and 2) the mechanism for accelerating MHC synthesis in contracting cardiocytes is an increase in translational efficiency as reflected by a shift of MHC mRNA into larger polysomes
We have demonstrated previously that contraction induced by electrical stimulation accelerated rates of total protein synthesis and fractional rates of MHC synthesis

Summary

Introduction

The purpose of this study was to determine the mechanism by which contraction acutely accelerates the synthesis rate of the contractile protein myosin heavy chain (MHC). This shift resulted in significant increases in MHC mRNA levels in the fractions containing the larger polysomes of electrically stimulated cardiocytes as compared with nonstimulated controls These data indicate that the rate of MHC synthesis is accelerated in contracting cardiocytes via an increase in translational efficiency. MHC was used as a cardiocyte-specific marker to determine whether translational mechanisms are involved in the acute acceleration of the rate of protein synthesis in response to electrically stimulated contraction of adult cardiocytes. Comparisons were made between quiescent cardiocytes and cardiocytes electrically stimulated to contract by measuring the synthesis rate of MHC protein, the size of the MHC mRNA pool, and the translational efficiency of MHC mRNA These studies demonstrated that: 1) rates of MHC synthesis are accelerated by electrical stimulation without a corresponding change in steady state mRNA levels, and 2) the mechanism for accelerating MHC synthesis in contracting cardiocytes is an increase in translational efficiency as reflected by a shift of MHC mRNA into larger polysomes

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