Biphenyl 2,3′,4,5′,6-pentakisphosphate, a novel inositol polyphosphate surrogate, modulates Ca2+ responses in rat hepatocytes

Benzene polyphosphates containing phosphate groups on one ring are Ins( 1,4,5) P-3 5-phosphatase inhibitors when evaluated against type-I Ins( 1,4,5) P-3 5- phosphatase. A novel biphenyl derivative, biphenyl 2,3', 4,5', 6- pentakisphosphate, with five phosphate groups on two rings was synthesized: It inhibited the activity of two inositol 5- phosphatases: type I and SHIP2 with Ins( 1,3,4,5) P-4 as substrate. The inhibition was competitive with respect to the substrate. IC50 value measured in rat hepatocytes, which contains the native Ins( 1,4,5) P-3 5- phosphatase, was in the micromolar range at 1.0 mu M Ins( 1,4,5) P-3 as substrate. Biphenyl 2,3', 4,5', 6- pentakisphosphate did not affect the activity of Ins( 1,4,5) P-3 3- kinase A in the 5 - 100 mu M range. Surprisingly, experimental evidence supports an effect of biphenyl 2,3', 4,5', 6- pentakisphosphate at the level of the Ins( 1,4,5) P-3 receptor. Finally, when injected into rat hepatocytes, the analog affected the frequency of Ca2+ oscillations in a positive or negative way depending on its concentration. At very high concentrations of the analog, Ca2+ oscillations were even suppressed. These data were interpreted as a dual effect of the biphenyl 2,3', 4,5', 6- pentakisphosphate on cytosolic [ Ca2+] increases: an activation effect through an increase in Ins( 1,4,5) P-3 level via Ins( 1,4,5) P-3 5- phosphatase inhibition and an inhibitory effect, which was exerted directly on the Ins( 1,4,5) P-3 receptor. Thus, our data show for the first time that the frequency of Ca2+ oscillations in response to a Ca2+- mobilizing agonist can be controlled by inhibitors of type-I Ins( 1,4,5) P-3 5- phosphatase.