AbstractNormal rat kidney NRK cells grown in the presence of epidermal growth factor (EGF) or platelet‐derived growth factor (PDGF) have a normal phenotype and undergo density‐dependent growth inhibition, whereas in the presence of multiple growth factors, density arrest is lost and the cells become phenotypically transformed. We studied the influence of the protein tyrosine phosphatease (PTPase) inhibitor sodium orthovanadate on the mitogenic stimulation of NRK cells by growth factors and on transformation‐linked properties as loss of density‐dependent growth inhibition and anchorage‐independent growth. The fraction of cells in serum‐deprived monolayer cultures that is induced to proliferate upon mitogenic stimulation by EGF or PDGF is only slightly enhanced upon addition of low concentrations (25–50 μM) of vanadate. Addition of vanadate per se induces proliferation of only a very limited amount of cells, but results in a shift of the dose‐response curves for other growth factors to lower concentrations. Vanadate added in combination with EGF or PDGF is able to mimic the effect of transforming growth factor β (TGFβ) in inducing phenotypic transformation. In monolayer cultures density‐dependent growth inhibition is lost and anchorage‐independent proliferation is observed on dishes coated with poly(2‐hydroxy‐ethyl methacrylate) (polyHEMA). The extent of these changes is similar to that induced by TGFβ. However, the morphology of the obtained colonies in polyHEMA‐coated dishes is quite different. Cells transformed by TGFβ in the presence of EGF form rather amorphous colonies, whereas in the presence of orthovanadate colonies are formed that tend to fall apart in loose cells. The effect of vanadate on cell transformation is dependent on the growth factor conditions in a bimodal way. When a suboptimal dose of growth factor(s) is used, 25 μM vanadate is very effective in preventing density‐induced growth inhibition and stimulating anchorage‐independent proliferation. However, the same concentration of vandate is inhibitory when cells are maximally stimulated and antagonizes the transforming effect of TGFβ added in combination with other growth factors. It is hypothesized that vanadate acts on a set of different protein tyrosine phosphatases. Some of these are positive and others negative regulato
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