NORMAL DIFFERENTIATION OF RAT BROWN ADIPOCYTES IN PRIMARY CULTURE JUDGED BY THEIR EXPRESSIONS OF UNCOUPLING PROTEIN AND THE PHYSIOLOGICAL ISOFORM OF GLUCOSE-TRANSPORTER

We examined the effects of dexamethasone (DEX) on the expressions of k ey proteins concerned with energy metabolism in brown adipocytes durin g their differentiation in primary culture. Transcripts of the uncoupl ing protein (UCP), lipoprotein lipase (LPL) and CCAAT enhancer binding protein alpha (C/EBP alpha) genes were observed in brown adipocytes c ultured in the presence of insulin and thyroid hormones but in the abs ence of DEX. However, the mRNA level of UCP decreased with the culture period after confluence, and significant mRNA encoding type-1 glucose transporter (GLUT1) was detected in brown adipocytes cultured without DEX, whereas type-4 glucose transporter (GLUT4) was predominantly exp ressed in mature brown adipocytes in vivo. In contrast, DEX added afte r confluence consistently elevated the mRNA levels of UCP, LPL and C/E BP alpha, and repressed the level of GLUT1 in a manner synchronized wi th increase in the level of GLUT4. Therefore, it is concluded that DEX as well as insulin and thyroid hormones is essential for differentiat ion of brown adipose precursor cells into mature cells that are simila r to brown adipocytes in vivo.