Modulation of the Intracellular Ca²⁺‐dependent Proteolytic System is Critically Correlated with the Kinetics of Differentiation of Murine Erythroleukemia Cells

Calpain has been identified as the intracellular proteinase that catalyzes the selective down‐regulation of protein kinase C (PKC) isoforms, occurring in the early stages of commitment to terminal erythroid differentiation of murine erythroleukemia (MEL) cells induced by hexamethylenebisacetamide. This conclusion has been reached through direct experiments performed with two MEL cell clones, one characterized by a high and the other by a low rate of differentiation. In both cell types, introduction of an anti‐calpain antibody resulted in a significant delay in the onset of down‐regulation of PKC isoforms, and in an increase in the latent period that precedes differentiation. Both cell lines also displayed reduced rates of PKC decay and accumulation of mature erythroid cells. Furthermore, in the fast‐responding clone, calpastatin, the natural calpain‐inhibitor protein, was found to be almost completely absent, resulting in activation and expression of proteolytic activity of calpain even at micromolar concentrations of Ca²⁺, a condition not sufficient to trigger calpain activation in the slowly responding clone which contains high levels of calpastatin. The fast‐responding MEL cell clone, enriched with calpastatin, displayed a lower rate of cell differentiation, with a kinetics almost identical to that observed following introduction of the anti‐calpain antibody. It is proposed that Ca²⁺‐dependent proteolysis plays a crucial role for the progress of MEL cell differentiation through the specific degradation of PKC isozymes.