Heat shock protein-70kDa (Hsp70) is a member of molecular chaperone family, involved in the proper folding of various proteins. Hsp70 is important for tumor cell survival and is also reported to be involved in enhancing the drug resistance of various cancer types. Hsp70 controls apoptosis both upstream and downstream of the mitochondria by regulating the mitochondrial membrane permeabilization (MMP) and apoptosome formation respectively. In the present study, we have elucidated the role of Hsp70 in Gambogic acid (GA) induced apoptosis in bladder cancer cells. We observed that functional inhibition of Hsp70 by Pifithrin-μ switches GA induced caspase dependent (apoptotic) cell death to caspase independent cell death. However, this cell death was not essentially necrotic in nature, as shown by the observations like intact plasma membranes, cytochrome-c release and no significant effect on nuclear condensation/fragmentation. Inhibition of Hsp70 by Pifithrin-μ shows differential effect on MMP. GA induced MMP and cytochrome-c release was inhibited by Pifithrin-μ at 12h but enhanced at 24h. Pifithrin-μ also reverted back GA inhibited autophagy which resulted in the degradation of accumulated ubiquitinated proteins. Our results demonstrate that Hsp70 plays an important role in GA induced apoptosis by regulating caspase activation. Therefore, inhibition of Hsp70 may hamper with the caspase dependent apoptotic pathways induced by most anti-cancer drugs and reduce their efficacy. However, the combination therapy with Pifithrin-μ may be particularly useful in targeting apoptotic resistant cancer cells as Pifithrin-μ may initiate alternative cell death program in these resistant cells.
Biochimica et biophysica acta. 2016 Jul 07 [Epub ahead of print]
Mohammad Ishaq, Rani Ojha, Kapil Sharma, Gaurav Sharma, Shrawan K Singh, Sekhar Majumdar
Division of Cell Biology and Immunology, CSIR-Institute of Microbial Technology (IMTECH), Chandigarh 160 036, India., Department of Urology, Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh 160 012, India., Division of Cell Biology and Immunology, CSIR-Institute of Microbial Technology (IMTECH), Chandigarh 160 036, India., Division of Cell Biology and Immunology, CSIR-Institute of Microbial Technology (IMTECH), Chandigarh 160 036, India., Department of Urology, Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh 160 012, India., Division of Cell Biology and Immunology, CSIR-Institute of Microbial Technology (IMTECH), Chandigarh 160 036, India. Electronic address: .