Triiodothyronine regulates beta 3-adrenoceptor expression in 3T3-F442A differentiating adipocytes

Opposing effects of glucocorticoids on beta(3)-adrenergic receptor expression in HIB-1B brown adipocytes

Glucocorticoids (GC) have been reported to promptly repress beta(3)-adrenergic receptor (beta(3)-AR) gene transcription in a white adipose tissue cell line. However, the effect of these hormones on beta(3)-AR expression in brown adipose tissue in vivo suggests a more complex mechanism of action. To avoid potential in vivo confounding variables, we investigated the effect of GC on the beta(3)-AR of HIB-1B brown adipocytes. While beta(3)-AR mRNA had same rapid turnover as in white fat cells, 1.5-2 h, the time course of its descent following dexamethasone was complex. A rapid initial descent beta(3)-AR mRNA with t(1/2) approximately 1.6 h was consistent with a prompt, complete inhibition of transcription. Such rapid initial phase was followed approximately 2 h later by a plateau or even an increase of beta(3)-AR mRNA, to descend thereafter following a slower single exponential (t(1/2) approximately 10 h). The change in the time course was abrogated by cycloheximide, and was not due to dexamethasone degradation or stabilization of beta(3)-AR mRNA at later times after dexamethasone. In vivo, a sufficiently large dose of dexamethasone was associated with a transient approximately 70% reduction of brown adipose tissue beta(3)-AR mRNA by 4 h and full recovery by 24 h. These findings suggest that GC have two opposing effects on beta(3)-AR gene expression: they rapidly and directly inhibit transcription but also induce a rapidly turned-over protein (C/EBPbeta?) that stimulates gene transcription. The relative magnitude of these two effects may vary explaining apparently discrepant observations.

Regulation of beta 1- and beta 3-adrenergic agonist-stimulated lipolytic response in hyperthyroid and hypothyroid rat white adipocytes

1. This study examined the effects of thyroid status on the lipolytic responses of rat white adipocytes to beta-adrenoceptor (beta-AR) stimulation. The beta 1- and beta 3-AR mRNAs and proteins were measured by Northern and saturation analyses, respectively. Glycerol production and adenyl cyclase (AC) activity induced by various non-selective and selective beta 1/beta 3-AR agonists and drugs which act distal to the receptor in the signalling cascade were measured in cells from untreated, triiodothyronine (T3)-treated and thyroidectomized rats. 2. The beta 3-AR density was enhanced (72%) by T3-treatment and reduced (50%) by introduction of a hypothyroid state while beta 1-AR number remained unaffected. The beta 1- and beta 3-AR density was correlated with the specific mRNA level in all thyroid status. 3. The lipolytic responses to isoprenaline, noradrenaline (beta 1/beta 3/beta 3-AR agonists) and BRL 37344 (beta 3-AR agonist) were potentiated by 48, 58 and 48%, respectively in hyperthyroidism and reduced by about 80% in hypothyroidism. 4. T3-treatment increased the maximal lipolytic response to the partial beta 3-AR (CGP 12177) and beta 1-AR (xamoterol) agonists by 234 and 260%, respectively, increasing their efficacy (intrinsic activity: 0.95 versus 0.43 and 1.02 versus 0.42). The maximal AC response to these agonists was increased by 84 and 58%, respectively, without changing their efficacy. 5. In the hypothyroid state, the maximal lipolytic and AC responses were decreased with CGP (0.17 +/- 0.03 versus 0.41 +/- 0.08 mumol glycerol/10(6) adipocytes; 0.048 +/- 0.005 versus 0.114 +/- 0.006 pmol cyclic AMP min-1 mg-1) but not changed with xamoterol. 6. The changes in lipolytic responses to postreceptor-acting agents (forskolin, enprofylline and dibutenyl cyclic AMP, (Bu)2cAMP) suggest the modifications on receptor coupling and phosphodiesterase levels in both thyroid states. 7. Thyroid status affects lipolysis by modifying beta 3-AR density and postreceptor events without changes in the beta 1-AR functionality.

Differential regulation by tumor necrosis factor-alpha of beta1-, beta2-, and beta3-adrenoreceptor gene expression in 3T3-F442A adipocytes

Modulation of beta-adrenoreceptor expression by tumor necrosis factor-alpha (TNF-alpha) was investigated in murine 3T3-F442A adipocytes. TNF-alpha treatment of mature adipocytes decreased beta3-adrenoreceptor mRNA content in a time- and concentration-dependent manner, with a 8.5-fold decrease observed after a 6-h exposure to 300 pM TNF-alpha. beta1-Adrenoreceptor mRNA abundance was slightly decreased by TNF-alpha treatment, while beta2-adrenoreceptor mRNA levels were potently induced (6-fold increase at 6 h). (-)-[125I]Iodocyanopindolol saturation and competition binding experiments indicated that TNF-alpha induced a 2-fold decrease in beta3-adrenoreceptor number, a nonsignificant reduction in beta1-subtype population, and a approximately 4.5-fold increase in beta2-adrenoreceptor density. This correlated with a lower EC50 value measured for epinephrine in stimulating adenylyl cyclase, whereas the EC50 value for norepinephrine increased. Nuclear run-on assays on isolated nuclei and mRNA stability measurements showed that TNF-alpha increased both beta2-adrenoreceptor gene transcription and beta2-adrenoreceptor mRNA half-life, while beta1- and beta3-adrenoreceptor gene expression was modulated only at the transcriptional level by the cytokine. These findings demonstrate a differential modulation by TNF-alpha of the three beta-adrenoreceptor subtypes in adipocytes, which may contribute to metabolic disorders induced by the cytokine in the adipocyte.

Down-regulation of beta3-adrenergic receptor expression in rat adipose tissue during the fasted/fed transition: evidence for a role of insulin

The beta3-adrenergic receptor (beta3-AR) exerts a central role in the transduction of catecholamine effects in white and brown adipose tissue (WAT and BAT). A recent report has documented that insulin strongly down-regulates beta3-AR expression and catecholamine responsiveness in 3T3-F442A adipocytes [Fève, El Hadri, Quignard-Boulangé and Pairault (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 5677-5681]. In the present report we show that the rise in plasma insulin levels elicited by the fasted/fed transition is associated with a reduction in beta3-AR mRNA levels and beta-adrenergic responsiveness in WAT and BAT. beta3-AR transcripts are also decreased in adipose tissue from animals subjected for 6 h to euglycaemic hyperinsulinaemic glucose clamps. Moreover, insulin acts directly on cultured rat white and brown adipocytes to decrease beta3-AR gene expression and adenylate cyclase activity in response to beta3-AR-selective agonists. These results suggest that there is a close relationship between food intake, plasma insulin levels and beta3-AR expression.

Regulation of expression of leptin mRNA and secretion of leptin by thyroid hormone in 3T3-L1 adipocytes

Leptin, the obese gene product, is secreted exclusively by adipocytes and regulates energy balance. We examined the effects of thyroid hormones on the regulation of leptin in 3T3-L1 adipocytes. Fully differentiated adipocytes were incubated with thyroid hormones, and the expression of leptin mRNA was measured by quantitative reverse transcription-polymerase chain reaction. After a 24-h incubation, triiodothyronine (T3) at 10-1000 nmol/l significantly increased the expression of leptin mRNA (237-337%). These stimulatory effects were not observed in preadipocytes. By contrast, thyroxine (T4) at 1-1000 nmol/l did not affect leptin mRNA expression in adipocytes. We also measured the levels of secreted leptin in conditioned media using radioimmunoassay. T3 at 10 and 1000 nmol/l significantly increased the levels of secreted leptin (132% and 126%, respectively) after a 24-h incubation. Our present data suggest that thyroid hormone is a novel regulator of leptin mRNA expression and protein secretion in 3T3-L1 adipocytes.