EFFECTS OF PROTEIN-TYROSINE-PHOSPHATASE INHIBITORS ON EGF-DEPENDENT AND INSULIN-DEPENDENT MAMMARY EPITHELIAL-CELL GROWTH

Epidermal growth factor (EGF)- and insulin-dependent mammary epithelia l cell mitogenesis is mediated by specific tyrosine kinase receptors, Receptor tyrosine kinase activity is highly regulated in normal cells, whereas amplification of intracellular protein tyrosine phosphorylati on is associated with abnormal growth and/or neoplastic transformation , Since protein tyrosine phosphatases (PTPs) are involved in regulatin g receptor tyrosine kinase signaling, studies were conducted to determ ine the effects of the PTP inhibitors, vanadate and pervanadate, on mi togen-receptor signal transduction and cell growth. Mammary epithelial cells isolated from midpregnant BALB/c mice were grown within collage n gels and maintained on serum-free media, Treatment with 2-8 mu M van adate or pervanadate greatly increased intracellular protein tyrosine phosphorylation, However, in the presence of optimal mitogenic stimula tion (10 ng/ml EGF and 10 mu g/ml insulin), these treatments induced a slight, but significant decrease in cell growth, In contrast, these t reatments significantly increased mammary epithelial cell growth, albe it less than optimally, under submitogenic culture conditions (500 pg/ ml EGF and 10 mu g/ml insulin), Neither vanadate nor pervanadate was f ound to mimic the mitogenic actions of EGF and/or insulin in these cel ls, The growth-stimulatory effects of PTP inhibitors in submitogenic c onditions appear to result from enhanced receptor tyrosine kinase mito genic signaling, whereas PTP inhibitor attenuation of optimal cell gro wth may be due to the suppression of PTP activity associated with cell cycle progression. In addition, treatment with PTP inhibitors was not found to stimulate anchorage-independent growth, as determined by the inability of single cells to form colonies in soft agarose, In conclu sion, these data demonstrate that optimal mitogen-dependent mammary ep ithelial cell growth requires both receptor tyrosine kinase and PTP ac tivity, Furthermore, PTP inhibitor-induced amplification of receptor t yrosine kinase mitogenic signaling is not in itself sufficient to indu ce enhanced cell growth or phenotypic expression of neoplastic transfo rmation.