Signal transduction in confluent C3H 10T1/2 cells. The role of focal adhesion kinase

The activities of extracellular signal-regulated kinases (ERK1/ERK2) is req uired for proliferation of several types of cells. The performed analysis s howed stimulation of ERK's by fetal calf serum (FCS) or fibronectin in the C3H 10T1/2 cell cultures at logarithmic phase of growth. The ERKs activity was not stimulated in confluent cells. This could not be accounted for a pa rtial down regulation of ERK since its level was stable in both types of ce lls regardless of their density and kind of stimulation. Searching for ERK upstream elements we studied the integrin receptor gene transcript by RT-PC R and focal adhesion kinase (FAK) by Western blotting and phosphorylation a ssays. It was found that FCS and fibronectin stimulated phosphorylating activity o f FAK in the cells at the logarithmic phase of growth, but were inefficient in the confluent cells. FAK RT-PCR showed the presence of alpha5 and beta1 integrin transcripts, and p125(FAK) was at the same level regardless of th e type of stimulation. These data indicate that the ability of FAK to be activated plays an import ant role in ERK regulation and, in consequence in proliferation and growth inhibition during confluence.