Tyrphostins inhibit epidermal growth factor (EGF)-receptor tyrosine kinase activity in living cells and EGF-stimulated cell proliferation

Synthetic compounds called tyrphostins were examined for their effects on cells which are mitogenically responsive to epidermal growth factor (EGF). We studied in detail the effects of two tyrphostins on EGF binding, tyrosine phosphorylation in intact cells, EGF-receptor internalization, and mitogenesis. These compounds inhibited EGF-stimulated 3H]thymidine incorporation in a specific manner and the degree of selectivity varied. Both compounds inhibited EGF-stimulated receptor autophosphorylation and tyrosine phosphorylation of endogenous substrates in intact cells at doses that correlated with the IC50 for 3H] thymidine incorporation. These results are consistent with the notion that tyrosine phosphorylation is a crucial signal in transduction of the mitogenic message delivered by EGF. The compound RG50864 demonstrated specificity at inhibiting EGF-stimulated cell growth compared with stimulation with either platelet-derived growth factor or serum. For both compounds RG50864 and RG50810, long term exposure (16 h) of cells to tyrphostins was required for optimal inhibition because of the instability and slow action of these compounds. Tyrphostins did not alter cell surface display of EGF-receptor, EGF binding or EGF-induced internalization, degradation, and down-regulation of EGF receptors. These novel synthetic inhibitors, specific for EGF-receptor kinase, offer a new method to inhibit EGF-stimulated cell proliferation which may be useful in treating specific pathological conditions involving cellular proliferation, including different types of cancers.