A putative regulatory circuit involving EGFR. (a) Protein expression levels in skin fibroblasts 2 hours after treatment with serum or individual growth factors. Total protein extracts from treated cells were loaded equally on a gel followed by western blot analysis using EGFR, HRAS and ERBB2 antibodies. (b) mRNA expression patterns for CBL, ERBB2, HRAS, SPRY2 and EGFR across all growth stimulations in foreskin fibroblasts. EGFR mRNA is more strongly induced by serum compared to the GFs, but the induction of EGFR protein is only modest in response to serum. Its negative regulator, CBL, is more strongly induced at the RNA level in response to EGF, concordant with the strong down-regulation of EGFR protein in EGF treated cells.