Effects of a water-soluble antitumor ether phosphonoinositide, D-myo-inositol 4-(hexadecyloxy)-3(S)-methoxybutanephosphonate (C₄-PI), on inositol lipid metabolism in breast epithelial cancer cell lines

We have demonstrated previously that d-myo-inositol 4-(hexadecyloxy)-3(S)-methoxybutanephosphonate (C₄-PI), an isosteric phosphonate analog of phosphatidylinositol developed to inhibit inositol lipid metabolism, was unable to inhibit phosphatidylinositol (PI) 3-kinase activity. We now report the effects of the compound on other aspects of inositol metabolism. We demonstrated that C₄-PI inhibits the activity of purified recombinant PI-phospholipase C-β (PLC-β) at all concentrations tested; it enhanced the activity of PI-PLC-γ and PI-PLC-δ at low concentrations (10 μM), while severely inhibiting their activities at higher concentrations. In the breast cancer cell lines MCF-7 (estrogen receptor positive) and MDA-MB-468 (estrogen receptor negative), C₄-PI had no effect on the uptake of d-myo-inositol but severely inhibited its incorporation into PI. In spite of the drastic decrease in PI synthesis, C₄-PI did not affect the levels of inositol incorporated into phosphatidylinositol 4,5-bisphosphate (PIP₂) in the cells. In vitro assays showed that C₄-PI inhibited PI synthase activity (inhibition of 35% at 50 μM) but had little effect on PI 4-kinase activity (inhibition of 13% at 150 μM). C₄-PI inhibited the proliferation of MCF-7 and MDA-MB-468 cell lines with ic₅₀ values of 12 and 18 μM. Taken together, the results suggest that the accumulation of [³H]inositol in PIP₂ in cells incubated with C₄-PI may be due to the inhibition of PIP₂ hydrolysis in the cells with no effect on its synthesis. The role of these C₄-PI-induced effects in the mechanism of growth inhibition by C₄-PI remains to be established. Copyright (C) 1999 Elsevier Science Inc.