Stress-activated protein kinases are negatively regulated by cell density

Open Access
Publisher Website
Google Scholar
Abstract
Stimulation by UV irradiation, TNFα, as well as PDGF or EGF activates the JNK/SAPK signalling pathway in mouse fibroblasts. This results in the phosphorylation of the N‐terminal domain of c‐Jun, increasing its transactivation potency. Using an antibody that specifically recognizes c‐Jun phosphorylated at Ser63, we show that culture confluency drastically inhibited c‐Jun N‐terminal phosphorylation due to the inhibition of the JNK/SAPK pathway. Transfection experiments demonstrate that the inhibition occurs at the same level as, or upstream of, the small G‐proteins cdc42 and Rac1. In contrast, the classical MAPK pathway was insensitive to confluency. The inhibition of JNK/SAPK activation depended on the integrity of the actin microfilament network. These results were confirmed and extended in monolayer wounding experiments. After PDGF, EGF or UV stimulation, c‐Jun was predominantly phosphorylated in cells bordering the wound, which are the cells that move to occupy the wounded area. Thus, modulation of the stress‐dependent signal cascade by confluency will restrict c‐Jun N‐terminal phosphorylation in response to mitogenic or chemotactic agents to cells that border a wounded area.