Differentiation of ovine brown adipocyte precursor cells in a chemically defined serum-free medium. Importance of glucocorticoids and age of animals.

The rapid apparent conversion of brown adipose tissue into white adipose tissue in newborn offspring of large mammals, such as sheep and cattle is not explained at the cellular level. To study the differentiation of lamb brown adipocyte, a genomic fragment corresponding to the uncoupling protein was cloned from an ovine DNA library. Stromal vascular fibroblasts isolated from the perirenal adipose tissue of newborn lambs completely differentiated into brown adipocytes expressing the uncoupling protein gene, in a chemically defined serum-free medium. Dexamethasone was necessary for the expression of the uncoupling protein gene. When stromal vascular fibroblasts were isolated from 3-week-old lambs, the glucocorticoid analog still promoted in vitro differentiation of adipocytes. However those adipocytes were unable to express uncoupling mRNA and could be considered as white adipocytes. The data indicate that dexamethasone is necessary but not sufficient clone for the complete differentiation of brown adipocytes, and that the preadipocytes are committed to differentiation into brown or white adipocytes before culture.