Dibutyryl cAMP enhances the effect of 1,25-dihydroxyvitamin D3 on a human promyelocytic leukemia cell, HL-60, at both the receptor and the postreceptor steps.

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) induces differentiation of a human promyelocytic leukemia cell line, HL-60, into monocytes/macrophages, and 25-hydroxyvitamin D3- and 1,25-(OH)2D3-24-hydroxylase activities in HL-60 mitochondria via a steroid-hormone receptor mechanism. Dibutyryl cyclic adenosine monophosphate (dbcAMP), a granulocyte inducer, significantly augmented the differentiation-inducing effect of 1,25-(OH)2D3 along the monocyte/macrophage pathway. Furthermore, dbcAMP significantly potentiated the effect of 1,25-(OH)2D3 on HL-60 cells to hydroxylate 1,25-(OH)2[26,27-3H]D3 to form 1,24,25-(OH)3[26,27-3H]D3. DbcAMP seemed to augment the effect of 1,25-(OH)2D3 in part through upregulation of the 1,25-(OH)2D3 receptor, because 10(-7) M dbcAMP increased 1,25-(OH)2D3 receptor levels approximately 2.3-fold, which was similar to a 1.9-fold augmentation by the same concentrations of dbcAMP of 1,25-(OH)2D3-induced cell characteristics to hydroxylate C-24 of 1,25-(OH)2[26,27-3H]D3. However, dbcAMP is also known to enhance HL-60 cell differentiation caused by other differentiation inducers. We have established another HL-60 clone which acquires resistance to 1,25-(OH)2D3 in the induction of cell differentiation by a defect at the postreceptor step, as reflected by resistance to other differentiation inducers, such as retinoic acid and dimethyl sulfoxide. Even in this resistant clone, dbcAMP significantly enhanced the differentiation-inducing effect of 1,25-(OH)2D3. Of interest, this clone showed resistance to dbcAMP in the induction of cell differentiation. Furthermore, we have demonstrated that intracellular cAMP levels were significantly lower in uremic serum-treated cells than in cells treated with normal human serum and that a significant positive correlation was found between intracellular cAMP levels and 1,25-(OH)2D3-induced cell differentiation. These data indicated that the intracellular cAMP level is one of the major determinants of 1,25-(OH)2D3-induced HL-60 cell differentiation and that dbcAMP could enhance the effects of 1,25-(OH)2D3 on HL-60 cells not only by increasing 1,25-(OH)2D3 receptor levels but also at the postreceptor step.