Modulation of transforming growth factor beta response and signaling during transdifferentiation of rat hepatic stellate cells to myofibroblasts

Activation of hepatic stellate cells (HSCs) is the key step in liver fibrog enesis, Increased transforming growth factor beta (TGF-beta) expression and extracellular matrix production in patients with hepatic fibrosis and expe rimental models of liver fibrogenesis support implication of TGF-beta in th e pathogenesis of this disease. However, a causative role for TGF-beta duri ng transdifferentiation of HSCs has not been delineated in molecular detail . Using a rat cell culture model of HSC transdifferentiation, we analyzed T GF-beta signal transduction and identified changes between stellate cells a nd their transdifferentiated phenotype. Fully transdifferentiated myofibrob lasts, opposed to HSCs, were not inhibited in proliferation activity on tre atment with TGF-beta 1 Furthermore, stimulation of alpha 2 (I) collagen and Smad7 messenger RNA (mRNA) expression by TGF-beta 1 Was achieved in stella te cells but not in myofibroblasts. Northern and Western blot analyses indi cated significant expression of TGF-beta receptors I and II in both cell ty pes. In contrast, [I-125]-TGF-beta 1 receptor affinity labeling displayed s trongly reduced types I, II, and III receptor presentation at the cell surf ace of myofibroblasts. Moreover, myofibroblasts did not display DNA-binding SMAD proteins in electrophoretic mobility shift assays with a CAGA box. Th ese data indicate that stellate cells are responsive to TGF-beta 1 treatmen t and transduce a signal that may play an important role in liver fibrogene sis. Myofibroblasts display decreased availability of surface receptors for TGF-beta, which could be based on autocrine stimulation. However, lack of activated SMAD complexes with DNA-binding activity and absence of (alpha 2 (I) collagen transcription inhibition by latency-associated peptide (LAP)/a nti-TGF-beta antibody raise the possibility of TGF-beta signaling independe nt receptor down-regulation in myofibroblasts.