Euglycemic hyperinsulinemia and hyperaminoacidemia decrease skeletal muscle ubiquitin mRNA in goats

Abstract

Insulin inhibits protein breakdown at the whole body level, but neither the tissues nor the proteolytic pathways on which insulin exerts its antiproteolytic effect are well characterized. We measured the effects of insulin on mRNA levels for cathepsin D and m-calpain (a lysosomal and Ca2(+)-dependent proteinase, respectively) and ubiquitin (a component of ubiquitin-dependent proteolysis) in skeletal muscle, skin, liver, and intestine. We used a 6-h hyperinsulinemic, euglycemic, and hyperaminoacidemic clamp in goats, a species in which insulin markedly inhibited whole body protein breakdown under similar conditions [S. Tesseraud, J. Grizard, E. Debras, I. Papet, Y. Bonnet, G. Bayle, and C. Champredon. Am. J. Physiol. 265 (Endocrinol. Metab. 28): E402-E413, 1993]. Hyperinsulinemia and hyperaminoacidemia had no effect on cathepsin D, m-calpain, and ubiquitin mRNA levels in liver, skin, and jejunum. In contrast, depressed ubiquitin mRNA levels were seen in skeletal muscle without any concomitant reduction in mRNA levels for cathepsin D, m-calpain, and other components of the ubiquitin-dependent proteolytic pathway. The reduced ubiquitin mRNA levels in skeletal muscle may represent a possible mechanism explaining the antiproteolytic effect of insulin in vivo.

The effect of supplemental propionate on insulin responsiveness to glucose and tissue responsiveness to insulin in relation to feeding in sheep

“…Moreover, gluconeogenesis from propionate was not influenced by insulin [3]. This [13]. Insulin infusion reduces whole body protein degradation and concentrations of plasma free amino acids [7,25].…”

Ubiquitin-proteasome-dependent proteolysis in skeletal muscle

“…Glucocorticoids [10,33] and TNF-a [3] up-regulate mRNA levels for 20S proteasome subunits. By contrast, the expression of a and (3 subunits is not down-regulated by insulin [22].…”

“…In vitro, insulin and insulin-like growth factor I decrease the stability of the 14-kDa E2 mRNAs [49]. Such down-regulation, however, does not prevail in insulininfused animals [22].…”

“…The 14-kDa E2 has two transcripts (1.8 and 1.2 kb) arising from different polyadenylation sites [48]. The 1.8-kb transcript, which corresponds to the major mRNA species in rat muscle, is barely detectable in humans [26] and goats [22] [33] or acutely septic [45] rats, although in both conditions the activation of a ubiquitin-dependent proteolytic process was responsible for increased proteolysis. Furthermore, increased expression of the 14-kDa E2 also does not always correlate with enhanced protein breakdown [40].…”

“…However, there is increased production of glucocorticoids in several catabolic states [35,42,47], and glucocorticoid responsive elements have been detected in the promotors of the chicken and human polyubiquitin genes [4]. Ubiquitin expression is also regulated by insulin [22], and presumably indirectly by several cytokines including tumor necrosis factor-a (TNF-a) [24].…”