TNF-like weak inducer of apoptosis (TWEAK) is an inflammatory cytokine that activates the Fn14 receptor. Both TWEAK an Fn14 are constitutively expressed in the kidney. TWEAK has been shown to modulate several biological responses such as inflammation, proliferation, differentiation and apoptosis that contribute to kidney injury. However, TWEAK role in fibrosis and TWEAK-activated intracellular signalling pathways remain poorly understood. We have tested the hypothesis that TWEAK can be a potent inducer of renal fibrosis by increasing TGF-β1 expression, (a well known switch in fibrosis process) through cGMP-dependent protein kinase I (PKGI) downregulation. We show that in human mesangial cells, TWEAK increased TGF-β1 expression and activity, leading to higher levels of the extracellular matrix protein fibronectin and decreased cGMP-dependent protein kinase I (PKGI) expression and activity via Ras pathway. PKGI activation with 8-BrcGMP, Ras inactivation with dominant negative Ras or Ras pathway inhibition with the Erk1/2 inhibitor PD98059, resulted in prevention of TWEAK-induced TGF-β1 upregulation. In vivo, exogenous administration of TWEAK to wild-type mice downregulated kidney PKGI and increased kidney TGF-β1 expression. These effects were blunted in H-Ras knockout mice. Together, these data demonstrate for the first time, the key role of PKGI in the TGF-β1 induction by TWEAK in kidney cells.