CELL DENSITY-DEPENDENT REGULATION OF PLC-GAMMA-1 TYROSINE PHOSPHORYLATION AND CATALYTIC ACTIVITY IN AN INTESTINAL-CELL LINE (IEC-6)

Administration of epidermal growth factor (EGF) to rats has been shown to induce both mitogenic and nonmitogenic responses in the intestine. The mechanisms to describe a multiplicity of hormonal responses withi n a single tissue are unclear but likely involve selectivity among rec eptor substrates. A nontransformed rat jejunal crypt intestinal epithe lial cell line (IEC-6) was studied to determine if the regulation of r eceptor tyrosine kinase substrates is affected by cell population phys iology. EGF stimulated a rapid increase in inositol trisphosphate in c onfluent but not subconfluent cells. Similarly, treatment of confluent IEC-6 cells with EGF provoked a significant increase in the hydrolysi s of Ptdlns 4,5-P-2 by immunoisolated PLC gamma 1. The tyrosine phosph orlation state of PLC gamma 1 and the association of PLC gamma 1 with the EGF receptor were increased by EGF in confluent cells only. In con trast, the autophosphorylation state of the EGF receptor and the tyros ine phosphorylation state of another SH2-containing EGF receptor subst rate SHC were increased by EGF regardless of cell density. Western blo t analysis revealed equal protein expression of PLC gamma 1 in conflue nt and subconfluent cells. EGF receptor protein expression and ligand binding capacity were slightly increased in confluent compared to subc onfluent cells. EGF regulation of PLC gamma 1, therefore, is regulated by physiological factors dependent on cell density in IEC-6 cells. (C ) 1995 Wiley-Liss, Inc.