Specific increase in calcium-activated neutral protease with low calcium sensitivity (m-calpain) in proerythroblastic K562 cell line cells induced to differentiation by phorbol 12-myristate 13-acetate.

Changes in the level of calcium-activated neutral proteases (calpains) in K562 cells induced to differentiate by phorbol 12-myristate 13-acetate (PMA) were examined by an immunohistochemical technique and Western blot analysis. A remarkable increase in m-calpain (high-Ca(2+)-requiring form) level was detected after PMA-treatment, while there was no significant difference in mu-calpain (low-Ca(2+)-requiring form) level between PMA-treated and untreated K562 cells. To confirm whether the increase in m-calpain is specific to PMA-induced differentiation, we examined changes in calpain in K562 cells cultured in serum-free medium and in synchronized cells. The results indicate that the increase has no relation to growth arrest or to cell cycle. PMA-treated cells exhibited increased nonspecific esterase activity, suggesting monocytic differentiation. Immunoelectron microscopic study showed the reactions of dense deposits with monoclonal anti-m-calpain antibody on cell membranes, on membranes of coated vesicles, and on rough endoplasmic reticulum of K562 cells after 26 h of PMA treatment.