Adrenergic regulation of the mitochondrial uncoupling protein gene in brown fat tumor cells

Abstract

A tumor appeared on the back of a transgenic mouse carrying the SV40 T-antigen under control of a mouse major urinary protein promoter. High levels of mRNA for the mitochondrial uncoupling protein (UCP) indicated that the tumor was a hibernoma. The tumor has been established as a transplantable tumor line in nude (nu/nu) mice and used as a source of cells to develop a tissue culture system for analyzing brown fat development and differentiation. Ucp expression in tumor cells cultured in Dulbecco's modified Eagle's medium and 10% fetal calf serum was virtually undetectable. Addition of 10-7 M norepinephrine resulted in approximately a 30-fold induction of Ucp mRNA within 4 h. The induction by norepinephrine was independent of cell density and also independent of thyroid hormone and insulin during the first 5 days in culture. However, in order to maintain the inducibility of Ucp during prolonged culture periods, it was necessary to supplement the medium with insulin. In contrast to Ucp, the expression of Gdc-1, which encodes the cytoplasmic glycerol-3-phosphate dehydrogenase and which is also induced in brown fat by cold exposure, was repressed by norepinephrine and induced by the addition of insulin. Characterization of the adrenergic receptors required for Ucp induction with agonists and antagonists indicated that β1 receptors are predominantly utilized; there is no evidence for utilization of β3 and α1 receptors for Ucp induction.