Spontaneous Mn2+ entry is specifically inhibited by calyculin A in human leukemic HL-60 cells

The effects of calyculin A and other agents which enhance protein Ser/Thr phosphorylation, on the cytosolic free Ca2+ concentration ([Ca2+]i) and spontaneous Mn2+ entry were investigated in fura-2-loaded human leukemic HL-60 cells. Calyculin A (30 nM), a specific inhibitor of protein Ser/Thr phosphatase (PP) 1 and 2A, significantly decreased [Ca2+]i. By contrast, another structurally unrelated inhibitor of PP1 and 2A, okadaic acid (1 microM), caused a slight elevation in [Ca2+]i. Forskolin (30 microM), which could enhance protein kinase A activity by raising cAMP concentration, also caused a rise in [Ca2+]i. Phorbol myristate acetate (PMA, 300 nM), an activator of protein kinase C, did not have a significant effect on [Ca2+]i. Spontaneous entry of Mn2+ (a surrogate ion for Ca2+) was strongly inhibited by calyculin A, but not okadaic acid, forskolin or phorbol myristate acetate. Such inhibition was not significantly affected by staurosporine (300 nM), a non-specific inhibitor of protein Ser/Thr kinases. Our results suggest that calyculin A inhibited a plasmalemmal leak pathway to Mn2+ (and Ca2+), probably leading to a decrease in [Ca2+]i. Inhibition of spontaneous Mn2+ entry by calyculin A may depend on a specific protein phosphorylation pattern induced by staurosporine-insensitive protein kinase(s).