Role of calcium activated kinases and phosphatases in heat shock factor-1 activation

HSF-1 is regulated at multiple molecular levels through intra- and intermol ecular protein-protein interactions as well as by post-translational modifi cation through phosphorylation. We have found that elevating intracellular calcium ion levels by exposure to the ionophore A23187 or thapsigargin inhi bits the conversion of HSF-1 from a latent cytoplasmic form to its nuclear/ DNA binding form. To examine a role for calcium/calmodulin regulated enzyme s in this process, we examined the ability of specific inhibitors to abroga te the effects of calcium elevation. While the inhibitor of calmodulin depe ndent kinase II, KCN62 enhanced activation of HSF-1 during heat shock, it f ailed to block the inhibitory effects of calcium increase. By contrast, the immunosuppresant drugs cyclosporin A and FK506 abolished the effects of ca lcium elevation on HSF-1 activation. As the biological effects of the drugs are effected through inhibition of the calcium/calmodulin regulated phosph atase calcineurin, this suggests a role for calcineurin in antagonizing HSF -1 activity. The experiments suggest the existence of phosphorylated residu e(s) in HSF-1 important in one or more of the processes that lead to activa tion (trimerization, nuclear localization, DNA binding) and which becomes d ephosphorylated due to the activation of a calcium/calmodulin/calcineurin c omplex.