INSULIN-LIKE GROWTH-FACTOR-I STIMULATES DEGRADATION OF AN MESSENGER-RNA TRANSCRIPT ENCODING THE 14 KDA UBIQUITIN-CONJUGATING ENZYME

Upon fasting, the ubiquitin-dependent proteolytic system is activated in skeletal muscle in parallel with the increases in rates of proteoly sis. Levels of mRNA encoding the 14 kDa ubiquitin-conjugating enzyme ( E2(14k)), which can catalyse the first irreversible reaction in this p athway, rise and fall in parallel with the rates of proteolysis [Wing and Banville (1994) Am. J. Physiol. 267, E39-E48], indicating that the conjugation of ubiquitin to proteins is a regulated step. To characte rize the mechanisms of this regulation, we have examined the effects o f insulin, insulinlike growth factor I (IGF-I) and des(1-3) insulin-li ke growth factor I (DES-IGF-I), which does not bind IGF-binding protei ns, on E2(14k) mRNA levels in L6 myotubes. Insulin suppressed levels o f E2(14k) mRNA with an IC50 of 4 x 10(-9) M, but had no effects on mRN As encoding polyubiquitin and proteasome subunits C2 and C8, which, li ke E2(14k), also increase in skeletal muscle upon fasting. Reduction o f E2(14k) mRNA levels was more sensitive to IGF-I with an IC50 of appr ox. 5 x 10(-10) M. During the incubation of these cells for 12 h there was significant secretion of IGF-I-binding proteins into the medium. DES-IGF-I, which has markedly reduced affinity for these binding prote ins, was found to potently reduce E2(14k) mRNA levels with an IC50 of 3 x 10(-11) M. DES-IGF-I did not alter rates of transcription of the E 2(14k) gene, but enhanced the rate of degradation of the 1.2 kb mRNA t ranscript. The half-life of the 1.2 kb transcript was approximately on e-third that of the 1.8 kb transcript and can explain the more marked regulation of this transcript observed previously. This indicates that the additional 3' non-coding sequence in the 1.8 kb transcript confer s stability. These observations suggest that IGF-I is an important reg ulator of E2(14k) expression and demonstrate, for the first time, stim ulation of degradation of a specific mRNA transcript by this hormone, while overall RNA accumulates.