Up-regulation of Ca2+ influx mediated by store-operated channels in HL60 cells induced to differentiate by 1α,25-Dihydroxyvitamin D3

The physiologically active form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 (1,25D3), induces promyelocytic HL60 cells to differentiate towards monocyte-like cells. During this differentiation increased cytosolic calcium (Cai2+) and expression of surface receptors for chemotactic factors "prime" the cell for the activation of monocyte functions and the triggering of the respiratory burst pathway. We examined whether the Ca2+ influx mediated by store-operated channels (SOC) contributed to the increased Cai2+ following exposure of HL60 cells to 10(-7) M 1,25D3. Cells treated with 1,25D3 for 72 hr demonstrated a rapid transient rise in Cai2+ followed by a second, phasic, increase in Cai2+ in response to the purinergic agonist ATP. This second Cai2+ transient was blocked by Ni2+, SKF 96365, or withdrawal of extracellular Ca2+. In cells suspended in Ca(2+)-free medium, peak changes (delta) in [Ca2+]i elicited by ATP-induced Ca2+ mobilization occurred with similar EC50 values in differentiated and vehicle (EtOH)-treated cells; however, peak [Ca2+]i was reduced by 55% in 1,25D3-treated cells. Decreased Ca2+ mobilization was associated with a 25-35% reduction in intracellular Ca2+ stores (determined with ionomycin). 1,25D3-treated cells exposed to ATP or thapsigargin (Tg) in Ca(2+)-free medium for 3 min with subsequent addition of 1 mM Ca2+ exhibited a respective 80% or 120% stimulation in peak [Ca2+]i compared to EtOH-treated cells. Enhanced Ca2+ influx mediated by SOC was also seen in these cells as an increase in the rate of Mn2+ entry after exposure to ATP or Tg. At 96 hr after addition of 1,25D3, when differentiated phenotype was established, basal Ca2+i and Ca2+ entry mediated by SOC returned to control values, but Ca2+ store size remained reduced. Up-regulation of Ca2+ influx via the SOC pathway during 1,25D3-induced differentiation may contribute to the functional properties of the maturing monocyte, or to the resetting of molecular programs responsible for the changing phenotype.