Bengtsson T, Redegren K, Strosberg A D, Nedergaard J, Cannon B
The Wenner-Gren Institute, The Arrhenius Laboratories F3, Stockholm University, S-106 91 Stockholm, Sweden.
J Biol Chem. 1996 Dec 27;271(52):33366-75. doi: 10.1074/jbc.271.52.33366.
The regulation of the expression of the beta3 adrenoreceptor gene was examined in the brown adipose tissue of intact mice and in murine brown fat primary cell cultures. Both in vivo and in vitro, high levels of beta3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of beta3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of beta3 gene expression, with a lag phase of 30 min and with an apparent half-life of beta3 mRNA of approximately 30 min. This down-regulation was stimulated via the beta3 receptors themselves and mediated via cAMP; the apparent affinity of norepinephrine was extremely high (<1 nM). The degradation rate after actinomycin was identical to that after norepinephrine and was not affected by the presence of norepinephrine; thus, the down-regulation was due to cessation of transcription but not to an increased rate of degradation. Notably, inhibition of protein synthesis by cycloheximide also led to down-regulation. The norepinephrine-induced down-regulation was transient; spontaneous recovery occurred after approximately 18 h and was not due to depletion of adrenergic agent. Recovery did not occur in the presence of cycloheximide. After recovery, the cells showed a functional desensitization of the down-regulation process itself (EC50 now approximately 10 nM). It is concluded that a down-regulated state cannot explain the functional desensitization of beta3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the beta3 receptor is not subject to desensitization via phosphorylation processes, no satisfactory explanation for the functional desensitization exists as yet. A model is presented for the down-regulation/recovery process, involving the participation of a phosphorylatable short-lived transcription factor.
在完整小鼠的棕色脂肪组织和小鼠棕色脂肪原代细胞培养物中，研究了β3肾上腺素能受体基因表达的调控。无论是在体内还是体外，均观察到高水平的β3受体mRNA。小鼠急性冷暴露导致β3基因表达显著且快速下调；然而，这种下调是短暂的。同样，在棕色脂肪细胞培养物中，添加去甲肾上腺素导致β3基因表达下调，有30分钟的延迟期，β3 mRNA的表观半衰期约为30分钟。这种下调是通过β3受体自身刺激并通过cAMP介导的；去甲肾上腺素的表观亲和力极高（<1 nM）。放线菌素处理后的降解速率与去甲肾上腺素处理后的相同，且不受去甲肾上腺素存在的影响；因此，下调是由于转录停止而非降解速率增加所致。值得注意的是，用环己酰亚胺抑制蛋白质合成也导致下调。去甲肾上腺素诱导的下调是短暂的；约18小时后自发恢复，且不是由于肾上腺素能介质的耗竭。在环己酰亚胺存在的情况下未发生恢复。恢复后，细胞对下调过程本身表现出功能性脱敏（现在的EC50约为10 nM）。结论是下调状态不能解释从冷适应动物分离的棕色脂肪细胞（即生理上长期受肾上腺素能刺激的棕色脂肪细胞）中观察到的β3肾上腺素能反应性的功能性脱敏；由于β3受体不会通过磷酸化过程发生脱敏，因此目前尚无对功能性脱敏的令人满意的解释。提出了一个下调/恢复过程的模型，涉及一种可磷酸化的短命转录因子的参与。
