HSP90 Affects the Stability of BMAL1 and Circadian Gene Expression

The mammalian circadian clock comprises a system of interconnected transcriptional and translational feedback loops. Proper oscillator function requires the precisely timed synthesis and degradation of core clock proteins. Heat shock protein 90 (HSP90), an adenosine triphosphate (ATP)-dependent molecular chaperone, has important functions in many cellular regulatory pathways by controlling the activity and stability of its various client proteins. Despite accumulating evidence for interplay between the heat shock response and the circadian system, the role of HSP90 in the mammalian core clock is not known. The results of this study suggest that inhibition of the ATP-dependent chaperone activity of HSP90 impairs circadian rhythmicity of cultured mouse fibroblasts whereby amplitude and phase of the oscillations are predominantly affected. Inhibition of HSP90 shortened the half-life of BMAL1, which resulted in reduced cellular protein levels and blunted expression of rhythmic BMAL1-CLOCK target genes. Furthermore, the HSP90 isoforms HSP90AA1 and HSP90AB1, and not HSP90B1-GRP94 or TRAP1, are responsible for maintaining proper cellular levels of BMAL1 protein. In summary, these findings provide evidence for a model in which cytoplasmic HSP90 is required for transcriptional activation processes by the positive arm of the mammalian circadian clock.