Norepinephrine-dependent selection of brown adipocyte cell lines.
A transgenic mouse carrying a simian virus-40 T-antigen gene under control of a mouse urinary protein promoter induced the formation of a brown fat tumor. In tissue culture, these tumor cells expressed the brown fat-specific mitochondrial uncoupling protein (Ucp) gene when stimulated by norepinephrine. To develop cell lines for the analysis of Ucp expression, we investigated growth conditions that would maintain expression. We found that the addition of 10(-7)-10(-6) M norepinephrine was critical for establishing cells with high Ucp expression, mitochondrial content, and adipogenesis. Norepinephrine exerts its effects by selectively stimulating the proliferation of brown fat cells. Results with receptor-specific agonists and antagonists indicate that norepinephrine interacts with the beta 1-adrenergic receptor to stimulate cell proliferation. The capacity of these brown fat cells to respond to norepinephrine by proliferating seems to be a manifestation of a normal physiological response of brown fat cells, because exposing an animal to the cold increases cell proliferation. Thus, this system indicates that two independent pathways for cell proliferation coexist in parallel, one based on transformation of the cell by the simian virus-40 T-antigen and the other on the normal response of the cell to stimulation by norepinephrine. Several clonal lines have been isolated that have similar levels of marker gene expression for adipogenesis and mitochondriogenesis, but differ in the level of Ucp expression. The results underscore the extreme sensitivity of mechanisms controlling Ucp expression and the fact that the expression of Ucp in brown fat is not coordinately linked to that of other nuclear genes which encode proteins associated with thermogenesis.