It is widely believed that targeting Hsp90 with small molecule inhibitors is able to directly interfere with the physical interaction between Hsp90 and survivin, leading to the decrease of survivin protein level and induction of cancer cell death [5, 15]. Interestingly, this study demonstrated for the first time that targeting Hsp90 with small molecule inhibitors will affect the expression of survivin at various stages, resulting in an increase of the amount of survivin protein presented in cancer cells. Furthermore, this study demonstrated that survivin plays an important role in the sensitivity to the Hsp90 inhibitor, 17-AAG, in cancer cells.
Here, we showed that targeting Hsp90 with small molecule inhibitor affected the amount of survivin mRNA transcript presented in cancer cells. It is not surprising that targeting Hsp90 induces different effect at the level of gene transcription in different cancer cells. Literatures revealed that the rate of survivin gene transcription is positively regulated by molecules such as sp1, sp3 and Myc [29, 30]. In contrast, the gene transcription process of survivin is negatively regulated by molecules such as p53, retinoblastoma (Rb) and prostate-derived Ets transcription factor (PDEF) [31–33]. Importantly, Hsp90 interferes with sp1, sp3, p53 and Rb simultaneously [34, 35]. Hence, differences in the response of survivin gene transcription may reflect different dependencies of various Hsp90-interfered and Hsp90-unrelated transcriptional factors on the expression of survivin in different cell types. Therefore, depending on the cellular context, targeting Hsp90 might indirectly up-regulate/down-regulate the process of survivin gene transcription through the interference with various survivin-related transcriptional factors.
Interestingly, our data also demonstrated that decreases at the mRNA level did not translate into decreases in survivin protein level in 17-AAG treated A549 cells. Together with results from the translation inhibition experiment, the protein degradation experiment and the examination of the survivin-related 26S proteasome, the current study strongly indicates that Hsp90 also interferes with survivin expression at the post-transcriptional level. Thus, Hsp90-targeted treatment interferes with the process of survivin gene transcription, protein translation and protein degradation simultaneously. In fact, Hsp90 plays an important role in the assembly and maintenance of the 26S proteasome [20, 36]. The activity of 26S proteasome was shown to be reduced by the addition of the Hsp90 inhibitor, geldanaymicin, in vitro. Reduced proteasomal activity was also shown previously in Hsp90-inhibited multiple myeloma cells . On the other hand, previous studies demonstrated that indomethacin (NASID) and chlamydocin (HDAC inhibitor) enhanced survivin degradation through ubiquitin proteasome machinery in cells [37, 38]. In our study, the use of proteasome inhibitor MG-132 was shown effective in increasing the amount of survivin present in our tested cancer cell lines, indicating that the activity of proteasome was important for survivin regulation. Therefore, the level of activity of proteasome might be one of the determinants of the amount of survivin present in Hsp90-inhibited cancer cells. However, it is hard to determine whether the interference with proteasome plays the most important role in the up-regulation of survivin. Further investigations are needed to determine the relative importance of transcription, translation and proteasome-related protein degradation in different Hsp90-targeted cancer cells. It is also worth noting that both 17-AAG and geldanamycin treatment reduced the amount of survivin presented in HeLa cells and this result was consistent with other studies. In contrast, results of the 3D-culture model revealed that 17-AAG treatment (1 μM) was also able to induce the over-expression of survivin in three dimensional cultured A549, HONE-1 and HT-29 cells (Additional file 1). Thus, the current study indicates that targeting Hsp90 may induce cell line-specific responses in the expression of survivin.
Importantly, results of the current study raise the concern that Hsp90 inhibitors might not function in a way as we previously thought. Indeed, literature reported that 17-AAG promoted formation of osteolytic lesions and bone metastases in murine breast cancer model, even though the drug reduced tumor growth at the orthotopic site . Furthermore, Kayani et al. demonstrated that 17-AAG treatment was able to enhance the expression of Hsp70 in C2C12 muscle fiber cells and the recovery of extensor digitorum longus (EDL) following lengthening contraction-induced damage in animal model . Thus, targeting Hsp90 with small molecular inhibitors may not be able to induce cell death in certain circumstances.