Down-regulation of beta 3 adrenoreceptor gene expression in brown fat cells is transient and recovery is dependent upon a short-lived protein factor

The regulation of the expression of the beta 3 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 beta 3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of beta 3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of beta 3 gene expression, with a lag phase of 30 min and with an apparent half-life of beta 3 mRNA of approximately 30 min. This down-regulation was stimulated via the beta 3 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 beta 3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the beta 3 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 shortlived transcription factor.