Regulation of myc gene expression in HL-60 leukaemia cells by a vitamin D metabolite

Abstract

HL-60, a cell line established from a patient with promyelocytic leukaemia, responds to a variety of inducing agents by ceasing division and acquiring some of the characteristics of either granulocytes or monocytes^1–4. Among the agents so far tested, only a comparative few occur naturally in vertebrates and would appear to have significant clinical potential in the treatment of leukaemic patients. One of the most promising of these is the dihydroxymetabolite of vitamin D₃, 1,25(OH)₂D₃. This compound circulates in normal man and has a major role in calcium homeostasis⁵. Moreover, it has recently been reported that 1,25(OH)₂D₃ increases the survival time of mice injected with myeloid leukaemia cells⁶. We⁷ and McCarthy et al.⁸ have previously shown that HL-60 cells respond to near physiological levels of 1,25(OH)₂D₃ by rapidly acquiring a number of monocyte-like features. Here we document that these phenotypic changes are preceded by a marked decrement in the expression of the c-myc oncogene. In fact, the diminution in the level of c-myc mRNA parallels the dose dependency and metabolite specificity shown by the various other indicators of phenotypic change. In addition, we demonstrate that removal of vitamin D₃, after the onset of maturational change, results in the reappearance of elevated myc mRNA levels. We believe this to be the first demonstration of a sequential relationship between the application of an exogenous inducing agent, a reduction in myc mRNA levels and the development of characteristics associated with normal cell maturation.