HL60 cells differentiate to monocytes or neutrophils in response to 1 alpha,25(OH)2-vitamin D3 (D3) and retinoids respectively. D3 and retinoid actions converge since their receptors (VDR, RAR) heterodimerise with a common partner, RXR, which also interacts with thyroid hormone (T3) receptors (T3R). HL60 cells were treated with combinations of D3 and retinoids to induce differentiation and to investigate whether increased VDR or RAR expression correlated with monocyte or neutrophil differentiation and whether altered receptor concentrations affected DNA-binding specificity. As assessed by Western blotting, VDR and RXR expression was unchanged in monocytes relative to controls but levels of RAR and T3R were reduced. In contrast, only VDR expression was reduced in neutrophils. T3 did not promote differentiation or influence its induction by D3 or retinoids and did not affect expression of any receptor. Gel mobility-shift analysis revealed that nuclear extracts from undifferentiated cells, monocytes and neutrophils interacted differently with VDRE-, RARE- and RXRE-binding sites. Monocyte nuclear protein/DNA complexes contain readily detectable VDR and RXR whereas neutrophil complexes contain RAR and RXR. Thus hormone-induced changes in receptor stoichiometry favour either VDR/RXR or RAR/RXR heterodimerisation and correlate with hormone-induced differentiation of HL60 cells to monocytes or neutrophils respectively.

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