beta-Adrenergic signaling and thyroid hormones affect HSP72 expression during heat acclimation

Area of Science:

• Physiology
• Molecular Biology
• Endocrinology

Background:

• Heat acclimation (AC) upregulates 72-kDa heat shock protein (HSP72), enhancing the heat shock response (HSR).
• The precise roles of beta-adrenergic signaling and thyroxine in mediating these AC-induced adaptations remain unclear.

Purpose of the Study:

• To investigate the involvement of beta-adrenergic signaling and plasma thyroxine levels in HSP72 upregulation and HSR modulation during heat acclimation.
• To elucidate the distinct mechanisms by which thyroxine influences heat shock protein gene expression and response.

Main Methods:

• Rats were subjected to various conditions: heat acclimation (34°C for 2 or 30 days), heat acclimation with beta-adrenergic blockade, euthyroid, hypothyroid, hyperthyroid, and control states.
• hsp72 mRNA and HSP72 protein levels were quantified using RT-PCR and Western blot, respectively.
• Responses were assessed before and after acute heat stress (41°C for 2 hours), with rectal temperature monitoring.

Main Results:

• Beta-adrenergic blockade during AC prevented HSP72 accumulation but did not impair HSR activation.
• Low thyroxine levels blunted the HSR at a posttranscriptional level.
• Thyroxine administration in hyperthyroid and AC-euthyroid rats suppressed heat stress-induced hsp72 transcription.

Conclusions:

• Beta-adrenergic signaling is crucial for the elevated HSP72 levels observed in the heat-acclimated state.
• Thyroxine exerts dual effects: direct repression of rapid hsp72 transcription and indirect stimulation via beta-adrenergic pathways.
• Low thyroxine may reduce HSP72 synthesis by decreasing heat production and lowering the thermoregulatory set point.