ALDOSTERONE AND DEXAMETHASONE STIMULATE CALCINEURIN ACTIVITY THROUGH A TRANSCRIPTION-INDEPENDENT MECHANISM INVOLVING STEROID RECEPTOR-ASSOCIATED HEAT-SHOCK PROTEINS

Heat shock proteins (HSP) are components of the steroid receptor compl ex and are released into the cell cytosol after hormone binding. We te sted whether HSPs released from steroid receptors mediate an increase in calcineurin phosphatase activity by steroid hormones. Aldosterone i ncreased calcineurin activity in microdissected rat cortical collectin g ducts (CCD) and connecting tubules, but not in proximal tubules, med ullary thick ascending limb, or outer medullary collecting ducts. In c ontrast, 5 mu M dexamethasone increased calcineurin activity in both C CD and proximal tubules. Aldosterone increased CCD calcineurin activit y after 30 min and this response was blocked by spironolactone, but no t by actinomycin D. An antibody recognizing HSP-56 did not change basa l calcineurin activity, but completely blocked the stimulation of calc ineurin by aldosterone, Rapamycin, an immunosuppressive drug that stab ilizes the HSP-steroid receptor complex, also blocked the aldosterone response, whereas HSP-90 or HSP-70 increased calcineurin activity in p ermeabilized CCD. In summary, (a) aldosterone increases calcineurin ac tivity in CCD through a transcription-independent process; (b) maneuve rs inactivating HSP-56 or slowing HSP disassociation from the receptor complex blocks stimulation of calcineurin by steroid hormones; (c) HS P-90 and HSP-70 increase CCD calcineurin activity in the absence of st eroid hormone. We conclude that HSPs released from transformed steroid receptors can stimulate calcineurin activity through a transcription- independent pathway.