The 3-phosphoinositide pathway as a potential target of anti-cancer therapy

Abstract

The major mediator of the 3-phosphoinositide signal transduction pathway is phosphatidylinositol (PtdIns) 3-kinase (Kapellar and Cantley, 1994). The study of the 3-phosphoinositide pathway has been facilitated by the existence of potent irreversible inhibitors of p110 PtdIns 3-kinase, such as wortmannin (WM) (Powis et. al, 1994). Anti-metabolites of the 3-phosphoinositides generated in this pathway, such as 1D-3-Deoxyphosphatidylinositol (3'Deoxy-PtdIns), are also useful tools. 3'Deoxy-PtdIns is an analog of PtdIns which, although it can be phosphorylated at the 4 or 4 and 5 positions of its inositol ring, is an anti-metabolite of 3-phosphoinositide signaling molecules (Kozikowski et al., 1995). In this study WM and 3'Deoxy-PtdIns were used to determine whether the 3-phosphoinositide pathway was a useful target for the development of a anti-cancer therapy. We found that WM and 3'Deoxy-PtdIns both inhibited the growth of murine C3H and human MCF-7 mammary tumors in vivo, however WM did not inhibit the growth of human UACC2150 tumor. The ability of WM to inhibit C3H tumor growth was not related to inhibition of tumor total PtdIns 3-kinase activity. The existence of the WM-insensitive PtdIns 3-kinase activity observed in these tumors was confirmed in C3H and MCF-7 cell culture lysates, solid tumors and tissue homogenates. In addition to being resistant to inhibition by WM, MCF-7 cell lysate total PtdIns 3-kinase activity was also resistant to five known p110 PtdIns 3-kinase inhibitors. Human normal colon mucosa, colon tumors and placenta also contained WM-insensitive populations of PtdIns 3-kinase activity. Human placental homogenate was chosen as the source for the purification of WM-insensitive Ptdlns 3-kinase because it contained a profile of PtdIns 3-kinase activity which was similar to that of MCF-7 cells. Purification of WM-insensitive PtdIns 3-kinase from human placenta did not result in the identification of PtdIns 3-kinase related proteins. Although WM appeared to inhibit tumor growth by a non-PtdIns 3-kinase dependent mechanism, the results of this study confirmed that the 3-phosphoinositide signal transduction pathway was involved in the growth of mammary tumors. Because of its predominance in solid tumors and normal tissues, the WM-insensitive PtdIns 3-kinase, once identified, may be a suitable target for anti-cancer drug development.