Transcriptional Regulation and Increased Functional Expression

The inositol trisphosphate (Inspa) receptor is an essential regulator of intracellular calcium in many cells including chemoattractantand 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 PM retinoic acid. Radioligand binding studies using membranes from control and retinoic acid-treated HL-60 cells showed that the B,,, of InsP3 receptor increased progressively from 0.24 to 0.69 pmol/mg protein during 6 days retinoic acid treatment with no change in KO (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 InsP, receptor gene. Furthermore, the transcriptional enhancing effect of retinoic acid was seen in the presence of cycloheximide, suggesting that the InsPa 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.