title: Rac1 promotes TGF-beta-stimulated mesangial cell type I collagen expression through a PI3K/Akt-dependent mechanism.
abstract: Transforming growth factor (TGF)-beta is a central mediator in the progression of glomerulosclerosis, leading to accumulation of aberrant extracellular matrix proteins and inappropriate expression of smooth muscle alpha-actin in the kidney. Previously, we reported that disrupting the cytoskeleton diminished TGF-beta-stimulated type I collagen accumulation in human mesangial cells. As cytoskeletal signaling molecules, including the Rho-family GTPases, have been implicated in fibrogenesis, we sought to determine the respective roles of RhoA and Rac1 in HMC collagen I expression. TGF-beta1 activated both RhoA and Rac1 within 5 min of treatment, and this activation was dependent on the kinase activity of the type I TGF-beta receptor. TGF-beta1-stimulated induction of type I collagen mRNA expression and promoter activity was diminished by inhibiting Rac1 activity and was increased by a constitutively active Rac1 mutant, whereas inhibiting RhoA activity had no such effect. Rac1 activation required phosphatidylinositol-3-kinase (PI3K) activity. Furthermore, the PI3K antagonist, LY294002, reduced TGF-beta1-stimulated COL1A2 promoter activity and Rac1 activation. It also partially blocked active Rac1-stimulated collagen promoter activity, suggesting that PI3K activity contributes to both TGF-beta activation of Rac1 and signal propagation downstream of Rac1. Thus, while both Rac1 and RhoA are rapidly activated in response to TGF-beta1 in human mesangial cells, only Rac1 activation enhances events that contribute to mesangial cell collagen expression, through a positive feedback loop involving PI3K.
authors.affiliation: Division of Kidney Diseases, Department of Pediatrics, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA. s-hubchak@northwestern.edu
authors.forename: Susan C
authors.initials: SC
authors.lastname: Hubchak
authors.forename: Erin E
authors.initials: EE
authors.lastname: Sparks
authors.forename: Tomoko
authors.initials: T
authors.lastname: Hayashida
authors.forename: H William
authors.initials: HW
authors.lastname: Schnaper
mesh_terms: D000199:Actins / Q000032:analysis / Q000378:metabolism; D015152:Blotting, Northern; D024042:Collagen Type I / Q000096:biosynthesis*; D005355:Fibrosis; D006801:Humans; D007668:Kidney / Q000473:pathology; D007672:Kidney Cortex / Q000166:cytology; D050527:Mesangial Cells / Q000187:drug effects / Q000378:metabolism*; D051058:Oncogene Protein v-akt / Q000502:physiology*; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D010957:Plasmids / Q000235:genetics; D012313:RNA / Q000096:biosynthesis / Q000302:isolation & purification; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D020830:rac1 GTP-Binding Protein / Q000502:physiology*; D020741:rho GTP-Binding Proteins / Q000378:metabolism; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D000199:Actins; D024042:Collagen Type I; C107953:RAC1 protein, human; D016212:Transforming Growth Factor beta; D012313:RNA; D019869:Phosphatidylinositol 3-Kinases; D051058:Oncogene Protein v-akt; D020830:rac1 GTP-Binding Protein; D020741:rho GTP-Binding Proteins; D020742:rhoA GTP-Binding Protein