Transcriptional regulation and increased functional expression of the inositol trisphosphate receptor in retinoic acid-treated HL-60 cells

The inositol trisphosphate (InsP₃) receptor is an essential regulator of intracellular calcium in many cells including chemoattractant- and cytokine-stimulated neutrophils and differentiated promyelocytic leukemic (HL-60) cells. We examined the expression and function of the InsP₃ receptor and the transcriptional regulation of the InsP₃ receptor gene in HL-60 cells and in HL-60 cells treated for 1-5 days with 1 μM retinoic acid. Radioligand binding studies using membranes from control and retinoic acid-treated HL-60 cells showed that the B_max of InsP₃ receptor increased progressively from 0.24 to 0.69 pmol/mg protein during 5 days retinoic acid treatment with no change in K_D (19 nM). During this period, maximal InsP₃-stimulated Ca²⁺ mobilization increased 2-3-fold. InsP₃ receptor mRNA was present at low levels in HL-60 cells but was increased significantly after treatment with retinoic acid, reaching maximal levels of approximately 4-fold greater than untreated cells after 4 days treatment with retinoic acid. Nuclear run-on assays indicated that the elevated steady state level of InsP₃ receptor mRNA in retinoic acid-treated HL-60 cells was primarily the result of enhanced transcription of the InsP₃ receptor gene. Furthermore, the transcriptional enhancing effect of retinoic acid was seen in the presence of cycloheximide, suggesting that the InsP₃ receptor gene is directly regulated by retinoic acid. The studies also demonstrate that the InsP₃ receptor mRNA is rapidly degraded in HL-60 cells by a mechanism that also requires protein synthesis.