Proteolysis and cell death

Cell death can occur by a number of different mechanisms, some of which are associated with significant protein degradation. L-cells in monolayer culture were labelled with 14C-leucine and 3H-thymidine and placed in a cold chase medium for each experiment. General cellular necrosis, induced by either repeated freeze-thawing or NaCN, rapidly disrupted cellular structure, but produced only small amounts of acid soluble 14C over a period of 24 h. Selective cell death was produced by adding 5 mM thymidine to growing tissue cultures. With such treatment, cell death became manifest only after a 24 h lag period and progressively more intense in the next 48 h. The process was characterized first by cellular enlargement and protein accumulation, followed by the formation and release of eosinophilic hyaline bodies from cell cytoplasm. These bodies were often found adjacent to the plasma membranes of viable cells, as well as within cellular vacuoles. Biochemical studies indicated that proteolysis was significantly increased; this proteolysis, but not cell death, was prevented by NH4Cl. Cell death, formation of eosinophilic hyaline bodies, and proteolysis were prevented by cycloheximide. Cell cultures that manifest cell turnover, i.e. selective cell death, apoptosis-type, will show increased amounts of vacuolar proteolysis, which may be confused with accelerated protein turnover occurring in the vacuolar system.