Alterations in dietary intake, especially of protein, may produce changes in the hepatic levels of the branched-chain α-keto acid dehydrogenase (BCKAD) complex. The possible role of insulin in the regulation of these observed changes in hepatic capacity for BCKAD expression was therefore examined. Steady-state RNA levels encoding three of the subunits, E1α, E1β and E2, increased by 2–4-fold in the livers of mice starved for 3 days, a known hypoinsulinaemic state. In contrast, the levels of E1β and E2, but not E1α, RNA were decreased when mice were fed 0% protein diets compared with the levels observed in mice fed standard (23%) or higher protein isocaloric diets. BCKAD subunit protein levels under these conditions changed co-ordinately even though the changes in RNA were not co-ordinate. The effects of hormonal changes that might be associated with these dietary changes were examined, using the rodent hepatoma cell line H4IIEC3. In these cells, the levels of E1α protein and mRNA were significantly depressed in the presence of insulin. In contrast, the levels of E1β and E2 RNAs were not decreased by insulin. The half-lives of the E1α and E2 RNAs were determined to be quite long, from 13 to 18 h, with insulin having no dramatic overall effect on the half-lives determined over 24 h. Therefore, it is likely that insulin directly affects the transcription of the E1α gene rather than RNA stability in exerting its negative regulatory effect. This effect is specific to the E1α subunit. The differences in BCKAD subunit RNA levels observed under various nutritional and developmental conditions may therefore be the result of the differential effects of insulin and other hormones on the multiple regulatory mechanisms modulating BCKAD subunit expression.

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