Glucagon Causes Disappearance of a T3 and Carbohydrate-Inducible Rat Hepatic mRNA (mRNAS14): Unexpected Circadian Dependency of Response
We have employed a rat hepatic mRNA sequence (mRNAS14) as a model for studies of hormonal and nutrient effects on the regulation of gene expression in vivo. Attention was drawn to this sequence, which encodes a cytosolic protein of Mr 17,010, pl 4.9, because of its rapid induction to 15X the basal hypothyroid level within 4 h after T3 administration (1). Subsequent studies showed that hepatic mRNAS14 expression is regulated by other factors as well. Starvation or experimental diabetes mellitus, for example, markedly attenuate mRNAS14 levels, whereas carbohydrate feeding augments mRNAS14 (2,3). Moreover, Mariash et al. in our laboratory have shown that induction of this mRNA by T3 and dietary carbohydrate exhibits the synergistic action which typifies their induction of several lipogenic enzymes in rat liver (2,4,5). The hypothesis has been advanced that a product of carbohydrate metabolism interacts in a multiplicative fashion with a signal generated by the association of T3 with its receptor to induce specific mRNA sequences encoding lipogenic enzymes. Participation of the S14 protein in lipogenesis is further suggested by its abundancy only in lipogenic tisses—liver, fat, and lactating mammary gland (5).
KeywordsDietary Carbohydrate Circadian Variation Lipogenic Enzyme Experimental Diabetes Mellitus Carbohydrate Feeding
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