Stimulation of hepatic inositol 1,4,5-trisphosphate kinase activity by Ca2+-dependent and -independent mechanisms

Abstract

A cytosolic fraction derived from rat hepatocytes was used to investigate the regulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] kinase, the enzyme which converts Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. The activity was doubled by raising the free Ca2+ concentration of the assay medium from 0.1 microM to 1.0 microM. A 5 min preincubation of the hepatocytes with 100 microM-dibutyryl cyclic AMP (db.cAMP) plus 100 nM-tetradecanoylphorbol acetate (TPA) resulted in a 40% increase in Ins(1,4,5)P3 kinase activity when subsequently assayed at 0.1 microM-Ca2+. This effect was smaller at [Ca2+] greater than 0.5 microM, and absent at 1.0 microM-Ca2+. Similar results were obtained after preincubation with 100 microM-db.cAMP plus 300 nM-vasopressin (20% increase at 0.1 microM-Ca2+; no effect at 1.0 microM-Ca2+). Preincubation with vasopressin, db.cAMP or TPA alone did not alter Ins(1,4,5)P3 kinase activity. It is proposed that these results, together with recent evidence implicating Ins(1,3,4,5)P4 in the control of Ca2+ influx, could be relevant to earlier findings that hepatic Ca2+ uptake is synergistically stimulated by cyclic AMP analogues and vasopressin.

Okadaic acid inhibits angiotensin II stimulation of Ins(1,4,5)P3 and calcium signalling in rat hepatocytes

“…3A). Interestingly, rat liver Ins(1,4,5)P~ 3-kinase has previously been reported to be stimulated following activation of protein kinase C and cAMP-dependent protein kinase [22]. Since OKA is able to increase the phosphorylation state of many of the same phosphoproteins as seen following the activation of such kinases [16], OKA treatment might also be expected to activate this enzyme.…”

“…[22]. Briefly, hepatocytes were treated ~vith 300 aM OKA for 13 rain at 37°C, rapidly pelleted, and the supernatant aspirated.…”

Inhibitors of protein kinase C prolong the falling phase of each free-calcium transient in a hormone-stimulated hepatocyte

“…A similar argument suggests to us that, in the hepatocyte, negative feedback from PKC, through activation of InsP3 kinase [16] or phosphatase [25], cannot be responsible for controlling the falling phase of free Ca unless each receptor type has, somehow, control of separate pools of these InsP3-metabolizing enzymes, within the same individual cell. It is unlikely that PKC activation of these InsP,-metabolizing enzymes is important, in view of the lack of effect of pretreatment with phorbol ester on the activity of both these enzymes in rat hepatocytes [26] and several other cell types [17,[27][28][29]. Likewise, PKC-mediated activation of the plasmalemmal Ca pump [30] would be unlikely to generate receptor-specified time courses to the free-Ca spikes, unless different sub-sets of pumps responded (in the same individual cell) to activation of different receptors.…”

Sensory transduction in eukaryotes

“…The kinase that phosphorylates Ins(1,4,5)P3 at the 3-position is present in a soluble form in virtually all mammalian cells [229,258,2591 and is activated in most if not all tissues by Ca2+/calmodulin [260,261]. Moreover, Ins(1 ,4,5)P3 3-kinase and 5-phosphatase activities may be stimulated by agents that activate protein kinases A and C, respectively [262,2631. Furthermore, Ins(1,3,4,5)P4 is a potent inhibitor of the Ins(1,4,5)P3 5-phosphatase [257].…”