c-myc mRNA is Stabilized by Deprivation of Amino Acids in Primary Cultured Rat Hepatocytes.

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We investigated whether the expression of growth-related genes could be changed in primary cultured hepatocytes in response to changes in the nutritional environment. Hepatocytes were isolated from the liver of growing rats after collagenase perfusion and cultured in Williams' E medium (WE medium) containing 5% calf serum, 10-7 M insulin and 10-6 M dexamethasone for 24h. When amino acids were removed from the culture, the level of c-myc mRNA increased more than 18-fold within 2-3h, whereas replenishment of the amino acids to the medium caused rapid decrease in the mRNA level. We found that the half-life of the c-myc mRNA was prolonged more than 6-fold in the absence of amino acids. The mRNA levels of other proteins, such as ornithine decarboxylase, c-Ha-ras and actin, and their half-lives were not affected by amino acids. It is known that a short-lived protein is involved in the degradation of c-myc mRNA. In fact, the addition of cyclohexi-mide to cultured hepatocytes increased the level of c-myc mRNA either in the presence or absence of amino acids, though the levels of other mRNAs were not changed significantly. These results suggest that the synthesis of the short-lived protein is suppressed and the c-myc mRNA is thereby stabilized in the absence of amino acids.
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