Function of 90-kDa heat shock protein in cellular differentiation of humanembryonal carcinoma cells

Heat shock proteins (HSPs) have been recognized as molecules that maintain cellular homeostasis during changes in the environment. Here we report that HSP90 functions not only in stress responses but also in certain aspects o f cellular differentiation. We found that HSP90 showed remarkably high expr ession in undifferentiated human embryonal carcinoma (EC) cells, which Here subsequently dramatically down-regulated during in vitro cellular differen tiation, following retinoic acid (RA) treatment, at the protein level. Surp risingly, heat shock treatment also triggered the clown-regulation of HSP90 within 48 h at the protein level. Furthermore, the heat treatment induced cellular differentiation into neural cells. This down-regulation of HSP90 b y heat treatment was shifted to an up-regulation pattern after cellular dif ferentiation in response to RA treatment. In order to clarify the functions of HSP90 in cellular differentiation, we conducted various experiments, in cluding overexpression of HSP90 via gene transfer. We showed that the RA-in duced differentiation of EC cells into a neural cell lineage was inhibited by overexpression of the HSP90 alpha or -beta isoform via the gene transfer method. On the other hand, the overexpression of HSP90 beta alone impaired cellular differentiation into trophoectoderm. These results show that down -regulation of HSP90 is a physiologically critical event in the differentia tion of human EC cells and that specific HSP90 isoforms may be involved in differentiation into specific cell lineages.