NOREPINEPHRINE UTILIZES ALPHA(1)-ADRRENOCEPTORS AND BETA-ADRENOCEPTORS SYNERGISTICALLY TO MAXIMALLY INDUCE C-FOS EXPRESSION IN BROWN ADIPOCYTES

In order to examine how norepinephrine stimulates proliferation and di fferentiation in brown fat cells, we have investigated the ability of brown fat cells to respond to norepinephrine stimulation with an incre ase in the expression of the proto-oncogene c-fos. Stimulation of brow n fat precursor cells (isolated from young mice and grown for 4 days i n culture) with norepinephrine led to a marked but transient (maximal approximate to 30 min) induction of c-fos expression. The magnitude of this induction was similar in pre- and postconfluent cells, The norep inephrine effect could be blocked by both alpha(1)- and beta-adrenergi c antagonists. Forskolin had a small inductive ability, as had the sel ective alpha(1)-agonist cirazoline, but with both together a high indu ction was obtained, The phorbol ester 12-O-tetradecanoylphorbol-13-ace tate (TPA) could in itself induce c-fos expression, but pretreatment w ith TPA did not abolish the ability of norepinephrine to induce c-fos expression, indicating that TPA sensitive protein kinase C was not a p rimary mediator in this pathway. Also the Ca2+ ionophore A23187 had in itself an inductive ability, but A23187 in combination with forskolin led to a large increase in c-fos expression, indicating synergistic i nteraction between a cAMP pathway and a [Ca2+](i) pathway. This intera ction was not proximal, i.e. alpha(1) stimulation or increase in [Ca2](i) by A23187 did not augment forskolin induced cAMP levels, and beta stimulation or forskolin did not affect [Ca2+](i) levels; and it did not require protein synthesis. It was concluded that norepinephrine, i n agreement with its fundamental role in the control of brown fat cell growth and development, was able to induce c-fos expression, that thi s induction was not exclusively linked to promotion of either prolifer ation or differentiation, and that the induction was mediated via a di stal synergism between beta/cAMP and alpha(1)/[Ca2+](i) pathways, thus conferring to the alpha(1)-adrenoreceptors on the cell a potentially significant role in the control of cell growth and development.