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Hexamethylenebisacetamide (HMBA)-induced murine erythroleukemia (MEL) differentiation is a multistep process. Commitment is the capacity to express terminal cell division and characteristics of the differentiated phenotype even after the cells are removed from culture with inducer. Culture of MEL cell line 745A-DS19 (DS19) with HMBA causes commitment to terminal differentiation after a latent period of about 10-12 hr. Previous studies have shown that during this latent period, HMBA causes a number of metabolic changes, including modulation in expression of certain protooncogenes. We now report the development of a MEL cell line (designated V3-17) derived from DS19 that is resistant to vincristine and is (i) markedly more sensitive to HMBA, (ii) induced to commitment without a detectable latent period, and (iN) resistant to the effects of phorbol ester and dexamethasone, which are potent inhibitors of HMBAmediated DS19 differentiation. We suggest that this V3-17 MEL cell line may express a factor that circumvents HMBAmediated early events, which prepare the cells for commitment to terminal differentiation.Hexamethylenebisacetamide (HMBA)-mediated murine erythroleukemia (MEL) cell terminal differentiation is a multistep process (1, 2). Upon culture of MEL cell line 745A-DS19 (DS19) (3) with HMBA (4), there is a latent period of -10 to 12 hr during which commitment to terminal differentiation cannot be detected. Commitment is defined as the capacity to express characteristics of the erythroid differentiated phenotype, including loss of proliferative capacity, despite removal of the inducer (5, 6). This early, latent period is followed by a period during which an increasing proportion of the population expresses characteristics of terminal differentiation, including loss of proliferative capacity.During the latent period, the inducer initiates a number of metabolic changes that precede irreversible commitment to differentiation. Among these are alterations in membrane permeability, which involve Na', K+, and Ca2+ flux (7-9); changes in cell volume (10); a transient increase in cyclic AMP concentration (11); a prompt increase in membraneassociated protein kinase C activity (PKC); the appearance in the cytosol of a Ca2 + -and phospholipid-independent form of PKC, presumably generated by proteolytic cleavage of membrane-bound PKC (12); and the modulation in expression of a number of genes, including c-myb, c-myc, c-fos, and p53 (13)(14)(15)(16). Upon more prolonged culture with HMBA, DS19 cells become irreversibly committed (5, 6). Morphological and molecular changes occur that are similar to normal erythroid terminal cell differentiation, including the coordinated expression of genes for a'-and 83mj-globin, for the heme synthetic enzymes, and for erythroid-specific membrane proteins, as well as suppression of DNA replication and rRNA synthesis (1,17,18).In the present studies we describe the development of a MEL cell line derived from DS19 that is resistant to inhibition of cell growth by vincristine and is de...

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Human erythrocytes contain a calpain activator protein with a molecular mass of approx. 40 kDa. The activator is present in association with the plasma membrane and promotes expression of calpain activity at a concentration of Ca2+ close to physiological values. The initial step of the activating mechanism involves association of the activator with calpain, followed by autoproteolytic activation of the proteinase in the presence of 1 microM Ca2+, at a rate identical to that induced by 1 mM Ca2+. In a reconstituted system, the activator binds to erythrocyte membranes, but not to phospholipid vesicles, suggesting the participation of an intrinsic membrane protein(s). In its membrane-associated form the activator selectively binds calpain, thus favouring interaction of the proteinase with the inner surface of plasma membranes. These results further confirm the importance of a natural activator protein in promoting intracellular activation of calpain under physiological conditions through a site-directed mechanism, which explains the high specificity of the proteinase for membrane of cytoskeletal proteins.

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