Cell density?dependent regulation of PLC?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 within a single tissue are unclear but likely involve selectivity among receptor substrates. A nontransformed rat jejunal crypt intestinal epithelial cell line (IEC-6) was studied to determine if the regulation of receptor tyrosine kinase substrates is affected by cell population physiology. EGF stimulated a rapid increase in inositol trisphosphate in confluent but not subconfluent cells. Similarly, treatment of confluent IEC-6 cells with EGF provoked a significant increase in the hydrolysis of PtdIns 4,5-P2 by immunoisolated PLC gamma 1. The tyrosine phosphorylation 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 contrast, the autophosphorylation state of the EGF receptor and the tyrosine phosphorylation state of another SH2-containing EGF receptor substrate SHC were increased by EGF regardless of cell density. Western blot analysis revealed equal protein expression of PLC gamma 1 in confluent and subconfluent cells. EGF receptor protein expression and ligand binding capacity were slightly increased in confluent compared to subconfluent cells. EGF regulation of PLC gamma 1, therefore, is regulated by physiological factors dependent on cell density in IEC-6 cells.