Phorbol diester (PDE) tumor promoters affect cells by interacting with specific receptors on the plasma membrane. Cells are known to regulate their receptor populations in response to many external and internal stimuli, including differentiation. The human promyelocytic leukemia cell line HL-60, which can be induced by dimethyl sulfoxide to differentiate into mature granulocytes, was used as a model to examine whether PDE receptors are regulated as cells differentiate in vitro. PDE binding was measured using [20-3H]phorbol 12,13-dibutyrate ([3H]PDBu). Specific binding of [3H]PDBu to the undifferentiated cells was rapid, reversible, and time and concentration related. One class of noncooperative binding sites was found with approximately 3.3 × 105 cells, having a Kd of 27.5 nm. [3H]PDBu could be displaced from the binding sites by a series of biologically active PDEs, but not by inactive ones. The characteristics of [3H]PDBu binding to the differentiated HL-60 cells were almost identical to those of the undifferentiated cells, except that there was an increase in the number of binding sites to 9.1 × 105 cells. Production of reactive oxygen metabolites by the cells, as monitored by chemiluminescence (CL) in response to PDEs, was examined before and after dimethyl sulfoxide differentiation, to determine if the change in receptor density was accompanied by a change in cell function. Only the differentiated HL-60 cells produced a quantifiable CL response when exposed to PDEs. The potency of the PDEs in causing CL generation was the same as that for displacing [3H]PDBu. The correlation between CL generation and affinity for the binding site suggests that the PDE binding sites mediate this effect of the PDEs and, therefore, are receptors. These studies indicate that HL-60 cells regulate their PDE receptors as they differentiate in vitro, with minimal extracellular influence. This may reflect a portion of an internal mechanism to enhance or change response to an endogenous ligand for the PDE receptor as cells mature.

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Supported in part by grants from the United States Environmental Protection Agency (R-808861010), the American Cancer Society, the Virginia Heart Association, and the NIH (ES03572).

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