Inhibition and recovery of macromolecular synthesis, membrane transport, and lysosomal function following exposure of cultured cells to hyperthermia

Iodine-125 epidermal growth factor (EGF) was used as a molecular probe to investigate the effects of hyperthermia on its binding, internalization, and degradation in rat fibroblasts. The decreased affinity of the receptor for EGF showed no evidence of recovery 12 hr after exposure to 45°C for 30 min. Receptor-mediated endocytosis of EGF was refractory to heating at 45°C for 30 min; there was no evidence of delayed effects of heating for at least 14 hr. In contrast, degradation of internalized EGF (a lysosomal function) was sensitive to heating and showed only partial recovery up to 14 hr after exposure to 45°C; the internalized EGF was incompletely degraded to products larger than iodotyrosine. Synthesis of protein and RNA, as determined by incorporation of [³H]leucine and [³H]uridine into acid-insoluble material, was decreased to very low values after heating to 45°C for 30 min. Only incorporation of [³H]leucine showed any recovery over the 14 hr following heating. In contrast, transport of [³H]leucine and [³H]uridine were only minimally affected by the heat treatment. It is postulated that the refractivity of transport processes to elevated temperature, coupled with the sensitivity of the lysosomal degradation system, might contribute to heat-induced cell toxicity and death.