Muscle wasting in insulinopenic rats results from activation of the ATP-dependent, ubiquitin-proteasome proteolytic pathway by a mechanism including gene transcription.

Abstract

In normal subjects and diabetic patients, insulin suppresses whole body proteolysis suggesting that the loss of lean body mass and muscle wasting in insulinopenia is related to increased muscle protein degradation. To document how insulinopenia affects organ weights and to identify the pathway for accelerated proteolysis in muscle, streptozotocin-treated and vehicle-injected, pair-fed control rats were studied. The weights of liver, adipose tissue, and muscle were decreased while muscle protein degradation was increased 75% by insulinopenia. This proteolytic response was not eliminated by blocking lysosomal function and calcium-dependent proteases at 7 or 3 d after streptozotocin. When ATP synthesis in muscle was inhibited, the rates of proteolysis were reduced to the same level in insulinopenic and control rats suggesting that the ATP-dependent, ubiquitin-proteasome pathway is activated. Additional evidence for activation of this pathway in muscle includes: ( a ) an inhibitor of proteasome activity eliminated the increased protein degradation; ( b ) mRNAs encoding ubiquitin and proteasome subunits were increased two-to threefold; and ( c ) there was increased transcription of the ubiquitin gene. We conclude that the mechanism for muscle protein wasting in insulinopenia includes activation of the ubiquitin-proteasome pathway with increased expression of the ubiquitin gene. ( J.

Regulation of Muscle Protein Degradation

“…The Ub-P'some system in muscle is markedly activated in response to insulin deficiency (18). As shown in Figure 6, we confirmed an increase in the levels of mRNAs encoding the E3 ubiquitin conjugating enzyme, atrogin-1/MAFbx, and ubiquitin (5,18).…”

“…As shown in Figure 6, we confirmed an increase in the levels of mRNAs encoding the E3 ubiquitin conjugating enzyme, atrogin-1/MAFbx, and ubiquitin (5,18). Does reduced PI3K activity lead to coordinated regulation of the Ub-P'some system?…”

“…Acute diabetes was induced in anesthetized rats, and they were pair-fed as described (18). For the experiment, the rats were reanesthetized, and arterial blood was collected to measure blood glucose; the gastrocnemius muscles were removed, immediately frozen in liquid nitrogen, and then stored at Ϫ80°C.…”

“…Gastrocnemius muscles were studied because they contain both red and white muscle fibers, and protein turnover in the mixed muscle of the hindquarter exhibits similar responses to those of epitrochlears muscles that exhibit accelerated protein degradation in response to acute diabetes (18,19). Sections of gastrocnemius were examined for muscle atrophy.…”

“…As in uremia or metabolic acidosis, rats with insulin deficiency have suppressed PI3K activity in muscle, lose weight, and exhibit accelerated muscle proteolysis via activation of the Ub-P'some pathway (16,17,18,22,23). For these reasons and because insulin deficiency is more easily created and less expensive compared with models of metabolic acidosis or uremia, we studied how acute insulin deficiency affects apoptotic and Ub-P'some pathways in muscle (16,22).…”