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HSP90, a major molecular chaperone, plays an essential role in the maintenance of several signaling molecules. Inhibition of HSP90 by inhibitors such as 17-allylamino-demethoxy-geldanamycin (17AAG) is known to induce apoptosis in various cancer cells by decreasing the activation or expression of pro-survival molecules such as protein kinase B (Akt). While we did not observe either decrease in expression or activation of pro-survival signaling molecules in human breast cancer cells upon inhibiting HSP90 with 17AAG, we did observe a decrease in cell motility of transformed cells, and cell motility and invasion of cancer cells. We found a significant decrease in the number of filopodia and lamellipodia, and in the F-actin bundles upon HSP90 inhibition. Our results show no change in the active forms or total levels of FAK and Pax, or in the activation of Rac-1 and Cdc-42; however increased levels of HSP90, HSP90α and HSP70 were observed upon HSP90 inhibition. Co-immuno-precipitation of HSP90 reveals interaction of HSP90 with G-actin, which increases upon HSP90 inhibition. FRET results show a significant decrease in interaction between actin monomers, leading to decreased actin polymerization upon HSP90 inhibition. We observed a decrease in the invasion of human breast cancer cells in the matrigel assay upon HSP90 inhibition. Over-expression of αB-crystallin, known to be involved in actin dynamics, did not abrogate the effect of HSP90 inhibition. Our work provides the molecular mechanism by which HSP90 inhibition delays cell migration and should be useful in developing cancer treatment strategies with known anti-cancer drugs such as cisplatin in combination with HSP90 inhibitors.

Heat shock response is associated with the synthesis of heat shock proteins (Hsps) which is strictly regulated by different members of heat shock transcription factors (HSFs). We previously reported that a rat histiocytoma, BC-8 failed to synthesize Hsps when subjected to typical heat shock conditions (42 degrees C, 60 min). The lack of Hsp synthesis in these cells was due to a failure in HSF1 DNA binding activity. In the present study we report that BC-8 tumor cells when subjected to heat shock at higher temperature (43 degrees C, 60 min) or incubation for longer time at 42 degrees C, exhibited necrosis characteristics; however,under mild heat shock (42 degrees C, 30 min) conditions cells showed activation of autophagy. Mild heat shock treatment induced proteolysis of HSF1, and under similar conditions we observed an increase in HSF2 expression followed by its enhanced DNA binding activity. Inhibiting HSF1 proteolysis by reversible proteasome inhibition failed to inhibit heat shock induced autophagy. Compromising HSF2 expression but not HSF1 resulted in the inhibition of autophagy, suggesting HSF2 dependent activation of autophagy. We are reporting for the first time that HSF2 is heat inducible and functions in heat shock induced autophagic cell death in BC-8 tumor cells.

Pharmacological inhibition of Hsp90 has emerged as a novel anticancer treatment. In this study we have investigated the effect of Hsp90 inhibitor drug 17AAG combination with curcumin on human neuroblastoma cells. The 17AAG treatment of cells for 18 h induced G1/S cell cycle arrest associated with cyclin D1 down regulation, and degradation of Raf-1 and inactivation of Akt. However, 17AAG treatment activated the mitogen kinase, ERK1, and induced the expression of stress proteins, Hsp70 and p53. The curcumin treatment resulted in G2/M cell cycle arrest and activation of both Raf1 and ERK1 kinases. The drugs in combination induced proteolytic degradation of Raf1 and Akt, and surpassed curcumin induced G2/M arrest. The combination treatment additionally inactivated MEK, inhibited activation and nuclear localization of ERK1, and also inhibited the stress protein induction. EGF stimulation induced re-activation of mitogen signaling with individual drug treatments but not in combination.This study highlights that 17AAG combination with curcumin selectively targets mitogen signal transduction mechanism through ERK1 inactivation. In conclusion, our study proposes the beneficial effects of 17AAG combination with curcumin in combating cancer.