Role for Hsp90-associated cochaperone p23 in estrogen receptor signal transduction

The mechanism of signal transduction by the estrogen receptor (ER) is compl ex and not fully understood. In addition to the ER, a number of accessory p roteins are apparently required to efficiently transduce the steroid hormon e signal. In the absence of estradiol, the ER, like other steroid receptors , is complexed with Hsp90 and other molecular chaperone components, includi ng an immunophilin, and p23. This Hsp90-based chaperone complex is thought to repress the ER's transcriptional regulatory activities while maintaining the receptor in a conformation that is competent for high-affinity steroid binding. However, a role for p23 in ER signal transduction has not been de monstrated. Using a mutant ER (G400V) with decreased hormone binding capaci ty as a substrate in a dosage suppression screen in yeast cells (Saccharomy ces cerevisiae), we identified the yeast homologue of the human p23 protein (yhp23) as a positive regulator of ER function. Overexpression of yhp23 in yeast cells increases En transcriptional activation by increasing estradio l binding in vivo. Importantly, the magnitude of the effect of yhp23 on ER transcriptional activation is inversely proportional to the concentration o f both ER and estradiol in the cell. Under conditions of high ER expression , ER transcriptional activity is largely independent of yhp23, whereas at l ow levels of ER expression, ER transcriptional activation is primarily depe ndent on yhp23. The same relationship holds for estradiol levels. We furthe r demonstrate that yhp23 colocalizes with the ER in vivo. Using a yhp23-gre en fluorescent protein fusion protein, we observed a redistribution of yhp2 3 from the cytoplasm to the nucleus upon coexpression with ER. This nuclear localization of yhp23 was reversed by the addition of estradiol, a finding consistent with yhp23's proposed role as part of the aporeceptor complex. Expression of human p23 in yeast partially complements the loss of yhp23 fu nction with respect to ER signaling. Finally, ectopic expression of human p 23 in MCF-7 breast cancer cells increases both hormone-dependent and hormon e-independent transcriptional activation by the ER. Together, these results strongly suggest that p23 plays an important role in ER signal transductio n.