Separate roles for H-Ras and Rac in signaling by transforming growth factor (TGF)-beta - H-Ras is essential for activation of MAP kinase, partially required for transcriptional activation by TGF-beta, but not required for signaling of growth suppression by TGF-beta

The signaling components located downstream of the transforming growth fact or (TGF)-beta receptor are poorly understood. We constructed adenoviral vec tors expressing a dominant-negative form of either H-Ras (AdCARasY57) or Ra c (AdCARacN17), and used them to examine the roles of H-Ras and Pac in TGF- beta signaling using arterial endothelial cells in primary culture, and sev eral established cells including a mink lung epithelial cell line (MvlLu). The rapid activation of p42/44 MAP kinase (MAPK) by TGF-beta 1 was eliminat ed completely, and transcriptional activation by TGF-beta 1 of the plasmino gen activator inhibitor-1 gene was reduced by 50% in both endothelial cells and MvlLu when they were infected with AdCARasY57. However, the antiprolif erative effect of TGF-beta, as assessed by the induction of the mRNA for Cd k4/6-specific inhibitor p15(INK4B) and by DNA synthesis, was not affected i n AdCARasY57-infected cells. A MAPK kinase (MEK)1/2 inhibitor, U0126 also a bolished MAPK activation and partially inhibited transcriptional activation by TGF-beta, suggesting that MAPK may be partially involved in this pathwa y. MAPK activation, transcriptional activation and growth suppression by TG F-P were all unaffected in cells infected with AdCARacN17, although the DNA synthesis elicited by serum mitogens was suppressed completely in the infe cted cells. Our data indicate that H-Ras is essential for mitogen-activated protein kinase activation, partly required for transcriptional activation by TGF-beta, but not critically involved in the signaling that exerts the a ntiproliferative effect of TGF-beta. The results also suggest that Rac may not serve as an essential molecule in signaling by TGF-beta in the cells te sted.