Analysis of inhibition by H89 of UCP1 gene expression and thermogenesis indicates protein kinase A mediation of beta(3)-adrenergic signalling rather than beta(3)-adrenoceptor antagonism by H89.

Although it has generally been assumed that protein kinase A (PKA) is essential for brown adipose tissue function, this has not as yet been clearly demonstrated. H89, an inhibitor of PKA, was used here to inhibit PKA activity. In cell extracts, it was confirmed that norepinephrine stimulated PKA activity, which was abolished by H89 treatment. In isolated brown adipocytes, H89 inhibited adrenergically induced thermogenesis (with an IC(50) of approx. 40 microM), and in cultured cells, adrenergically stimulated expression of the uncoupling protein-1 (UCP1) gene was abolished by H89 (full inhibition with 50 microM). However, H89 has been reported to be an adrenergic antagonist on beta(1)/beta(2)-adrenoceptors (AR). Although adrenergic stimulation of thermogenesis and UCP1 gene expression are mediated via beta(3)-ARs, it was deemed necessary to investigate whether H89 also had antagonistic potency on beta(3)-ARs. It was found that EC(50) values for beta(3)-AR-selective stimulation of cAMP production (with BRL-37344) in brown adipose tissue membrane fractions and in intact cells were not affected by H89. Similarly, the EC(50) of adrenergically stimulated oxygen consumption was not affected by H89. As H89 also abolished forskolin-induced UCP1 gene expression, and potentiated selective beta(3)-AR-induced cAMP production, H89 must be active downstream of cAMP. Thus, no antagonism of H89 on beta(3)-ARs could be detected. We conclude that H89 can be used as a pharmacological tool for elucidation of the involvement of PKA in cellular signalling processes regulated via beta(3)-ARs, and that the results are concordant with adrenergic stimulation of thermogenesis and UCP1 gene expression in brown adipocytes being mediated via a PKA-dependent pathway.

As the proliferation promoter noradrenaline induces expression of ICER (induced cAMP early repressor) in proliferative brown adipocytes, ICER may not be a universal tumour suppressor.

The CREM (cAMP-response-element modulator) gene product ICER (induced cAMP early repressor) has been proposed to function as a tumour (cell proliferation) suppressor. To investigate the generality of this concept, the expression pattern of ICER in brown adipocytes was followed; this was critical because brown adipocytes are one of few cell types in which cAMP is associated positively with cell proliferation but negatively with apoptosis. In response to the physiological stimulus of cold (which induces cell proliferation), ICER mRNA levels were increased in brown adipose tissue in vivo. In brown adipocytes in primary culture, ICER gene expression was induced by noradrenaline (norepinephrine) not only in the mature state (where noradrenaline potentiates differentiation), but also in the proliferative state of the cell cultures (where noradrenaline enhances cell proliferation). The induction was mediated via beta-receptors and the cAMP/protein kinase A pathway. The induced ICER appeared to repress its own expression and that of the beta2-adrenoceptor. It is thus evident that also in cell types in which cAMP induces proliferation, and even when these cells are in the proliferative state, ICER expression is induced by the same agents that stimulate proliferation. This can either mean that ICER is not a general tumour suppressor, or that brown adipocytes temporally or spatially avoid this role of ICER.

Norepinephrine utilizes alpha 1- and beta-adrenoreceptors synergistically to maximally induce c-fos expression in brown adipocytes.

In order to examine how norepinephrine stimulates proliferation and differentiation in brown fat cells, we have investigated the ability of brown fat cells to respond to norepinephrine stimulation with an increase in the expression of the proto-oncogene c-fos. Stimulation of brown fat precursor cells (isolated from young mice and grown for 4 days in culture) with norepinephrine led to a marked but transient (maximal approximately 30 min) induction of c-fos expression. The magnitude of this induction was similar in pre- and postconfluent cells. The norepinephrine effect could be blocked by both alpha 1- and beta-adrenergic antagonists. Forskolin had a small inductive ability, as had the selective alpha 1-agonist cirazoline, but with both together a high induction was obtained. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) could in itself induce c-fos expression, but pretreatment with TPA did not abolish the ability of norepinephrine to induce c-fos expression, indicating that TPA-sensitive protein kinase C was not a primary 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 interaction between a cAMP pathway and a [Ca2+]i pathway. This interaction 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, in agreement with its fundamental role in the control of brown fat cell growth and development, was able to induce c-fos expression, that this induction was not exclusively linked to promotion of either proliferation or differentiation, and that the induction was mediated via a distal 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.