Previously we have found that stationary Ehrlich ascites carcinoma (EAC) cells in vivo accumulated heat shock proteins (HSPs) and became resistant to necrotic death induced by prolonged energy deprivation of hyperthermia. Here we report that apoptotic death induced by nutrient starvation, transient ATP depletion, heat shock and a microtubule-disrupting drug, vinblastine, was also suppressed in stationary EAC cells comparing with exponential cells. When exponential (sensitive) cells were subjected to short-term heating with recovery to accumulate inducible form of HSP70, they also became resistant to all of the employed apoptosis-inducing exposures, and an inhibitor of cytosolic protein synthesis, cycloheximide, prevented acquisition of the resistance. It is suggested that in vivo accumulation of HSPs in stationary tumor cells can be endogenous protective device against apoptotic death induced by starvation or some anticancer treatments.

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Tumor adaptation to chronic energy starvation in vivo was studied on Ehrlich ascites carcinoma (EAC) cells. EAC cells were isolated from mice and incubated in a glucose-free medium containing blocators of mitochondrial ATP generation (rotenone, 2,4-dinitrophenol, or oligomycin). ATP level in the treated cells decreased to 3-4% of the initial during 30 min of the incubation. The aggregation of cytoskeletal proteins, blebbing, and necrotic death within 2-3 h were observed in ATP-depleted EAC which were isolated and treated in the exponential phase of growth (5 days after inoculation), whereas stationary EAC (8 days after inoculation) were considerably more resistant to ATP depletion, and actin aggregation as well as bleb formation were suppressed in these cells despite the ATP loss. In contrast to the exponentially growing cells, thermotolerance and unexpected expression of inducible HSP68 and HSP27 as well as an elevated level of HSP90 were found in stationary EAC. Since the stationary cells had decreased content of ATP, ATP/ADP ratio, and energy charge, we suggest that this energy dysbalance may be conducive to HSP induction within the ascites tumor in vivo, and, at the same time, EAC cells with elevated content of HSPs acquire resistance to chronic energy starvation occurring in late stages of the tumor growth.

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