Besides its effect in inhibiting proliferation and inducing differentiation of HL-60 to monocyte-like cells, 1,25-dihydroxyvitamin D3 also causes a rise in intracellular pH (pHi) from 7.17 ± 0.02 to 7.3 ± 0.05, as measured by the fluorescence of 2′,7′-bis(carboxyethyl)-5-(6)-carboxyfluorescein. The effect of 1,25-dihydroxyvitamin D3 on pHi is dose dependent in a parallel manner to its effect on the proliferation and differentiation processes. The elevation of pHi by 1,25-dihydroxyvitamin D3 is gradual but precedes the differentiation process. A significant increase in pHi was obtained after 16 h of incubation with the hormone and reached its maximum level after 48 h. pHi then dropped back to its initial level, which is also similar to that of peripheral normal blood monocytes.

A rise in pHi was not observed during incubation of HL-60 cells with 24,25-dihydroxyvitamin D3, a metabolite which does not promote their differentiation. In contrast, other agents such as phorbol 12-myristate 13-acetate known to induce differentiation in this cell line do cause an increase in pHi. Moreover, alkalinization of HL-60 cells by NH4Cl causes induction of differentiation to monocytes.

The results suggest that a rise in pHi plays a role in regulating the molecular mechanism of the inhibition of proliferation and the induction of differentiation in HL-60 cells.

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