Regulation of thrombin-induced stress fibre formation in Swiss 3T3 cells by the 70-kDa S6 kinase

The signal transduction systems that mediate growth factor receptor-induced cellular shape change have not been fully elucidated, but are known to involve alterations in the state of actin filaments, termed stress fibres. It now appears from several studies that the GTP-binding protein, Rho, is involved. However, the mechanisms by which Rho is activated, and what effecters Rho in turn stimulates are largely matters of conjecture. The present work shows that thrombin is an effective stimulant of stress fibre formation in Swiss 3T3 cells. In addition, we show the 70 kDa form of S6 kinase (p70(s6k)) to colocalise with stress fibres in both unstimulated and thrombin-activated cells. Coincident with the thrombin-induced formation of stress fibres is the elevated association p70(s6k) With the fibres. Pretreatment of cells with rapamycin, to inhibit p70(s6k) activation, inhibits thrombin-induced stress fibre formation and the associated presence of p70(s6k) On the fibres, supporting a role for p70(s6k) in thrombin-stimulated stress fibre formation. Thrombin is also shown to stimulate p70(s6k) activity and that this is inhibited by rapamycin. Thus, the data presented show that thrombin activates stress fibre formation through stimulation of p70(s6k) via a non-G(i) pathway. (C) 1997 Academic Press.