Drug-induced Myc-mediated apoptosis of cancer cells is inhibited by stress protein Hsp70.
The Myc oncoprotein serves a dual function by stimulating cells both towards growth and apoptosis. The latter functions are often abrogated during tumor development. The Hsp70 stress protein is a potent anti-apoptotic molecule, but its potential role in protecting cells from Myc-mediated apoptosis has not been investigated. Our results show that activated Myc potentiated apoptosis induced by the cancer drugs etoposide (ETO) and camptothecin (CAMP) in v-Myc-expressing human U-937 monoblastic cells and in Rat1 cells containing a conditionally active Myc/estrogen receptor (MycER) fusion protein. However, both heat shock and ectopic Hsp70 expression protected the cells from Myc-mediated apoptosis after drug treatment in both systems. The increased susceptibility to the anti-tumor drugs by activated Myc was enhanced by siRNA-mediated knockdown of Hsp70 expression in U-937 cells. Addressing the mechanisms by which Myc and Hsp70 promotes and inhibits drug-induced apoptosis, respectively, we found that v-Myc stimulated cytochrome c release and activation of effector caspase-9, -3 and -7, but not of initiator caspase-8. Inhibition of caspase-9 specifically reduced v-Myc-stimulated apoptosis, whereas inhibition of caspase-8 and -3/7 reduced apoptosis both in v-myc-expressing and parental ETO-treated U-937 cells. Interestingly, Myc-stimulated activation of effector caspases was inhibited, but cytochrome c release was not affected by Hsp70 expression, suggesting that Hsp70 interferes with the proapoptotic function of Myc downstream of mitochondria, at the level of caspase-9 and downstream caspases. In conclusion, Hsp70 seems to have key function in inhibition of apoptosis mediated by Myc and may therefore play an important role in Myc-driven oncogenesis.
Afanasyeva
Elena A
EA
Laboratory of Cell Protection Mechanism, Institute of Cytology RAS, St. Petersburg 194064, Russia.
Komarova
Elena Yu
EY
Larsson
Lars-Gunnar
LG
Bahram
Fuad
F
Margulis
Boris A
BA
Guzhova
Irina V
IV
D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D015153:Blotting, Western; D002166:Camptothecin / Q000494:pharmacology; D020169:Caspases / Q000187:drug effects / Q000378:metabolism*; D045304:Cytochromes c / Q000378:metabolism; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004967:Estrogens / Q000378:metabolism*; D005047:Etoposide / Q000494:pharmacology; D015972:Gene Expression Regulation, Neoplastic; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D006801:Humans; D016403:Lymphoma, Large B-Cell, Diffuse / Q000378:metabolism / Q000473:pathology; D015514:Oncogene Proteins, Fusion / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D014178:Translocation, Genetic; D020298:U937 Cells; D015854:Up-Regulation
D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D004967:Estrogens; D018840:HSP70 Heat-Shock Proteins; C489427:MYC protein, human; D015514:Oncogene Proteins, Fusion; D016271:Proto-Oncogene Proteins c-myc; D005047:Etoposide; D045304:Cytochromes c; D020169:Caspases; D002166:Camptothecin