Evidence for a role for protein kinase C in the modulation of bombesin-activated cellular signalling in human breast cancer cells.
The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) is a potent activator of protein kinase C (PKC) and is known to affect a variety of biochemical processes in human breast cancer cells. In the present study we have employed MCF-7 cells to investigate the effects of TPA on inositol lipid signalling, the putative pathway leading to PKC activation and intracellular Ca2+ mobilization. Phosphoinositide hydrolysis in MCF-7 cells was stimulated by bombesin (BN) as evidenced by increases in both inositol phosphate production and cytidine diphosphate diacylglycerol (CDP-DG) accumulation. Pretreatment of MCF-7 cells with TPA caused attenuation of both these BN-induced responses. This inhibitory action of TPA on inositol phosphate production was mimicked by diacylglycerol analogues and was reversed by staurosporine, H-7 and tamoxifen, all known inhibitors of PKC. Furthermore, putative down-regulation of PKC by prolonged TPA pretreatment also reversed the inhibitory action of TPA and enhanced BN-induced phosphoinositide hydrolysis. TPA also inhibited BN-induced increases in cytosolic Ca2+ concentration ([Ca2+]i) and caused a dose-dependent inhibition of epidermal growth factor (EGF) binding in MCF-7 cells. However, EGF receptor occupancy was unaffected by BN. These data support an inhibitory role for PKC in the regulation of phosphoinositide hydrolysis and [Ca2+]i in breast cancer cells and provide a potential mechanism for feedback regulation of this signalling pathway in these cells.
Patel
K V
KV
Unit of Metabolic Medicine, Clinical Endocrinology and Chemical Pathology, St. Mary's Hospital Medical School, London, UK.
Schrey
M P
MP
D001839:Bombesin / Q000037:antagonists & inhibitors* / Q000378:metabolism; D001943:Breast Neoplasms / Q000201:enzymology*; D002118:Calcium / Q000378:metabolism; D003567:Cytidine Diphosphate Diglycerides / Q000378:metabolism*; D015536:Down-Regulation / Q000187:drug effects; D006801:Humans; D006868:Hydrolysis / Q000187:drug effects; D010716:Phosphatidylinositols / Q000378:metabolism; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D003567:Cytidine Diphosphate Diglycerides; D010716:Phosphatidylinositols; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D001839:Bombesin; D002118:Calcium