A comparative study on gene expression of calpains, calpastatin, cathepsin D, and proteasome in tissues of fed and fasted rabbits

Abstract

Molecular mechanisms underlying changes in muscle protein turnover are not fully understood. In this study, effects of fasting on mRNA concentrations encoding several proteinases in skeletal muscle were investigated. Proteinases included calpains I and II, cathepsin D, and proteasome. We also examined effects of fasting on the calpain small subunit, calpastatin, and on β-actin mRNAs for comparative purposes. Fasting increased mRNAs encoding all proteinases, calpain small subunit, and calpastatin in skeletal muscle but reduced β-actin mRNA. This effect was most pronounced for cathepsin D. To determine whether the changes observed in skeletal muscle occurred in other tissues, we examined effects of fasting on proteinase mRNA concentrations in liver, lung, and kidney. Fasting either had no effect or reduced proteinase, calpastatin, and calpain small subunit mRNA concentrations in these tissues. Fasting reduced β-actin mRNA concentration in all tissues. Therefore, proteinase gene expression in skeletal muscle differs from other tissues during fasting. Despite the changes in calpain mRNA concentrations in muscle, changes in calpain activities were not detected. This suggests that either calpain synthesis was concomitantly reduced or calpain turnover was increased during fasting. Differences in calpain mRNA concentrations were detected among tissues and these were related to differences in calpain concentrations and activities among tissues. We conclude that calpain and other proteinase genes are co-regulated in muscle in a manner that differs from other tissues, and that fasting-dependent changes in muscle calpain mRNA serve to maintain calpain concentrations at fixed levels at a time when muscle protein synthesis is reduced or calpain stability is reduced. Finally, we conclude that differences in calpain mRNAs among tissues underlie the differences in tissue calpain concentrations and activities.