Hsp90 chaperone facilitates E2F1/2-dependent gene transcription in human breast cancer cells.

The 90 kDa heat shock protein, Hsp90, is involved in the conformational stabilization and functional maturation of diverse cancer-promoting proteins. To date, more than 300 Hsp90 clients have identified, suggesting that Hsp90 plays a central role in deciding cancer cell fate. In this study, we present the nuclear functions of Hsp90 in regulating the E2F-dependent gene transcription. We show that the conformation specific Hsp90 inhibitor, 17AAG decreases the total cellular E2F levels more selectively in cancer cells than transformed cells. With the help of coimmunoprecipitation experiments, we show that Hsp90 interacts with E2F1 and E2F2 in cancer cells, whereas in transformed cells, only E2F1 interacts with Hsp90. Retention of E2F2 in the nucleus of cancer cells upon MG132 combination with 17AAG has suggested that Hsp90 is required for E2F2 stability and function. The HDAC6 inhibitor tubacin treatment did not interfere with E2F1/2 stability and nuclear accumulation. However, the HDAC3 inhibitor, RGFP966 treatment, decreased nuclear E2F1/2 and its target gene expression. The nuclear accumulation of E2F1 and E2F2 upon cell cycle inhibition correlated with decreased acetylated Hsp90. We expose the nuclear functions of Hsp90 in facilitating the cell cycle progression through stabilizing E2F1/2.

Altered molecular pathways decides the treatment outcome of Hsp90 inhibitors against breast cancer cells.

The altered molecular pathways in response to chemotherapeutic interventions impose limitations on breast cancer treatments. Therefore, understanding the outcome of these alternative pathways may help in improving the chemotherapy. In this study, using hormone responsive and hormone independent breast cancer cells, MCF-7 and MDAMB-231 respectively, we studied some of the molecular pathways that contribute to cancer progression. Since the cancer chaperone, Hsp90 inhibitors have entered the clinical trials, we used Hsp90 inhibitor, 17AAG to examine the outcome of altered molecular pathways. The observed differential sensitivity in MCF7 and MDAMB-231 cells to 17AAG treatment is then attributed to both tumor microenvironment mediated by hypoxia and acquired alterations in the endogenous stem cell pool. Interestingly, tumor cells are able to retain epithelial characteristics in addition to gaining mesenchymal characteristics in response to 17AAG treatment. We observed MCF-7 cells exhibiting induced cellular differentiation, whereas MDAMB-231 cells exhibiting reduced cellular differentiation in response to 17AAG treatment. These changes are subsequently found to be the sporadic outcome of altered epigenetic landscape. The mice tumor xenograft studies have revealed that decreased metastatic potential of MCF-7 and increased metastatic potential with altered homing properties of MDAMB-231 are the outcome of altered molecular pathways. Our findings expose the interference of altered molecular pathways influencing the therapeutic outcome.