19-nor-1alpha,25-dihydroxyvitamin D(2) (paricalcitol): effects on clonal proliferation, differentiation, and apoptosis in human leukemic cell lines.

Purpose: 19-Nor-1alpha,25-dihydroxyvitamin D(2) (paricalcitol) is an analogue of 1,25(OH)(2)D(3) with reduced calcemic effects that is approved for the suppression of parathyroid hormone in chronic renal failure. Paricalcitol has recently been reported to have anticancer activity in prostate cancer. In order to explore paricalcitol as a potential agent against leukemia, we tested its effects on HL-60 and U937 leukemia cell lines.

Methods: We studied cellular differentiation via expression of CD11b and CD14 surface antigens using flow cytometry, and via the nitroblue tetrazolium (NBT) assay. Cell cycle was analyzed using propidium iodide staining. Apoptosis was assessed with the annexin V assay. Cellular proliferation was determined via colony inhibition on semisolid medium.

Results: Paricalcitol induced the maturation of HL-60 and U937 cells, as shown by increased expression of CD11b differentiation surface antigen. CD14 showed increased expression in HL-60 but not in U937 cells. After exposure to paricalcitol at 10(-8) M for 72 h, the ability of HL-60 cells to reduce NBT was markedly increased. Conversely, U937 cells were unchanged. Paricalcitol inhibited colony formation of both HL-60 and U937 cell lines in semisolid medium after a 10-day incubation (estimated IC(50) of 3x10(-8) M in HL-60 cells and 4x10(-8) M in U937 cells). Paricalcitol at 10(-8) M and 10(-7) M caused a significant dose- and time-dependent increase of apoptosis in HL-60 cells ( P<0.05). In both HL-60 and U937 cells, exposure to 10(-7) M paricalcitol for 72 h increased the number of cells in G(0)/G(1) phase, and decreased the number of cells in S phase.

Conclusions: Paricalcitol inhibits colony formation, induces maturation and causes cell cycle arrest in HL-60 and U937 cells. Additionally, paricalcitol induces apoptosis in HL-60 cells. These findings support the further evaluation of paricalcitol as an antileukemia agent.