Down-regulation of β3 adrenoreceptor gene expression in brown fat cells is transient and recovery is dependent upon a short-lived protein factor

Abstract

The regulation of the expression of the β3 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 β3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of β3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of β3 gene expression, with a lag phase of 30 min and with an apparent half-life of β3 mRNA of ~30 min. This down- regulation was stimulated via the β3 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 ~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 ~10 nM). It is concluded that a down-regulated state cannot explain the functional desensitization of β3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the β3 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.