Transition to tetraploidy in 1,25-dihydroxyvitamin D3-resistant HL60 cells is preceded by reduced growth factor dependence and constitutive up-regulation of Sp1 and AP-1 transcription factors.

Increased ploidy is an ominous event in the progression of human malignancies. It is usually associated with an increased growth rate of the neoplastic cells and a generally more autonomous and aggressive biological behavior. However, it has not been established whether the more rapid growth rate and growth factor independence are consequences of the polyploid, karyotypically increasingly aberrant nature of these cells or whether the accelerated, more autonomous growth contributes to polyploidization. In this study, we have examined a recently described (H. J. Wajchman et al., Exp. Cell Res., 224: 312-322, 1996) series of sublines of HL60 cells with increasing resistance to the monocytic differentiation-inducing steroid hormone 1,25-dihydroxyvitamin D3 (1,25D3) and found that growth factor independence, shown by reduced requirement for serum supplementation of the medium and the ability to grow at low seeding densities, precedes polyploidization of these cultures. The growth factor independence was found to be accompanied by constitutive changes in the DNA binding pattern of the ubiquitous transcription factor Sp1, characteristic of an exposure to 1,25D3. Similar changes in the pattern of AP-1 binding were also observed in the 1,25D3-resistant HL60 sublines, but the intensity of the DNA binding by AP-1 was increased only in sublines with resistance to 1,25D3 but still near-diploid. The data suggest that the culture of HL60 cells in the presence of 1,25D3 results in constitutive up-regulation of growth-related machinery that reduces the need for growth factors and cytokines and demonstrate that this increased growth potential precedes polyploidization of the culture populations.