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

We have demonstrated previously that D-myo-inositol 4-(hexadecyloxy)-3(S)-m ethoxybutanephosphonate (C-4-PI), an isosteric phosphonate analog of phosph atidylinositol developed to inhibit inositol lipid metabolism was unable to inhibit phosphatidylinositol (PI) 3-kinase activity. We now report the eff ects of the compound on other aspects of inositol metabolism. We demonstrat ed that C-4-PI inhibits the activity of purified recombinant PI-phospholipa se C-beta (PLC-beta) at all concentrations tested; it enhanced the activity of PI-PLC-gamma and PI-PLC-delta at low concentrations (10 mu M), while se verely inhibiting their activities at higher concentrations. In the breast cancer cell lines MCF-7 (estrogen receptor positive) and MDA-MB-468 (estrog en receptor negative), C-4-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, C-4-PI did not affect the levels of inositol inco rporated into phosphatidylinositol 4,5-bisphosphate (PIP2) in the cells. In vitro, assays showed that C-4-PI inhibited PI synthase activity (inhibitio n of 35% at 50 mu M) but had little effect on PI 4-kinase activity (inhibit ion of 13% at 150 mu M). C-4-PI inhibited the proliferation of MCF-7 and MD A-MB-468 cell lines with IC50 values of 12 and 18 mu M. Taken together, the results suggest that the accumulation of [H-3]inositol in PIP2 in cells in cubated with C-4-PI may be due to the inhibition of PIP, hydrolysis in the cells with no effect on its synthesis. The role of these C-4-PI-induced eff ects in the mechanism of growth inhibition by C-4-PI remains to be establis hed. (C) 1999 Elsevier Science Inc.