Ins(1,4,5)P3 3-kinase (IP3K) phosphorylates the Ca2+-mobilizing second messenger Ins(1,4,5)P3 to yield the putative second messenger Ins(1,3,4,5)P4. A HeLa cell line was established expressing the rat B isoform of IP3K under the control of an inducible promoter. The IP3KB-transfected cell line possessed 23-fold greater IP3K activity than untransfected cells after induction of IP3KB expression, but only 0.23-fold greater activity when IP3KB expression was not induced. Elevating IP3KB expression significantly reduced levels of Ins(1,4,5)P3 and increased levels of Ins(1,3,4,5)P4 after stimulation of cells with histamine, but had no effect on basal levels. Histamine- and ATP-evoked cytosolic Ca2+ responses were dramatically reduced upon elevation of IP3KB expression. On stimulation with a supramaximal dose of histamine, 67% of cells induced to express IP3KB gave no detectable elevation in cytosolic Ca2+, compared with 3% of uninduced cells. The quantity of Ca2+ within thapsigargin-sensitive and -insensitive stores was unaffected by elevation of IP3KB expression, as was capacitative Ca2+ entry. These data suggest that IP3KB may play a significant role in the regulation of Ins(1,4,5)P3 levels, and consequently in Ca2+ responses following stimulation of cells with Ins(1,4,5)P3-elevating agonists.

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