Tamoxifen and vanadate synergize in causing accumulation of polyphosphoinositides in GH4C1 membranes

Tamoxifen caused a dramatic stimulation of phosphatidylinositol kinase and phosphatidylinositol-phosphate (PIP) kinase activity in GH4C1 membrane preparation with an ED50 of 20 microM. Vanadate ions alone did not appreciably elevate the amount of polyphosphoinositides; however, when added together with tamoxifen it synergistically enhanced the formation of PIP and phosphatidylinositol-bisphosphate (PIP2). Vanadate caused the inhibition of phosphomonoesterase activity in the membranes that converts PIP2 to PIP and PIP to phosphatidylinositol. The synergism between tamoxifen and vanadate thus results from tamoxifen-induced stimulation of the phosphoinositide kinase reaction and vanadate inhibition of the backward phosphomonoesterase reaction. Tamoxifen had no effect on phosphomonoesterase activity. With optimal concentrations of the drugs, PIP was increased from 8.3 to 75%, and PIP2 was augmented from 0.36 to 8.5% of the total membrane phosphoinositides. Tamoxifen and vanadate are thus useful tools for the investigation of phosphoinositides metabolism.