Multiple regulation of S14 gene expression during brown fat differentiation

Abstract

S14 is a gene known to be under thyroid hormone control. Its mRNA concentration is very high in lipogenic tissues, and although the precise function of the protein is still unknown, indirect data suggest its implication in triglyceride synthesis. S14 gene expression is up-regulated by thyroid hormone in liver, white adipose tissue, and lactating mammary gland. However, in brown fat, the level of this sequence is increased 3-fold in the hypothyroid animal. We have used primary cultures of brown preadipocytes differentiated to fully mature brown adipocytes to investigate the influence of cellular differentiation and hormonal stimulation on S14 gene expression. Steady state levels of S14 mRNA rose from nondetectable levels in preadipocytes to reach a maximum in fully mature adipocytes. Treatment of brown adipocytes cultures with T3 did induce S14 gene expression. This induction reflects in part a posttranscriptional stabilization of the messenger by T3. Insulin, insulin-like growth factor-I, and inositol phosphate-glycan also increase the level of S14 mRNA. Norepinephrine (NE) plays a major role in the regulation of S14 gene, and 24 h after its addition, NE elicited a 20-fold decrease in mRNA S14 concentrations. An elevated intracellular concentration of cAMP is a strong negative effector of S14 gene expression, and neither NE nor cAMP action is totally overcome by T3. As happens in vivo, glucose is a potent stimulator of S14 mRNA; however, there is a lag time of several hours before its effects can be detected. The increase in S14 gene expression with the maturation stage of the cell suggests an important role for S14 in adipocyte differentiation.