Expression profile and functional activity of Platelet-derived growth factor-C and -D in bile duct-ligated rat livers and transdifferentiating hepatic stellate cells

Background/Aims: We analyzed the expression of the novel identified platelet-derived growth factor-C and -D (PDGF-C, PDGF-D) in an experimental bile duct-ligated rat model and compared them to those of PDGF-A, PDGF-B and their receptors, and assessed the biological function of these novel cytokines in cultured hepatic stellate cells (HSC).

Methods: The mRNA for PDGF-A, -B, -C, and –D, as well as for PDGF receptor-α and -β chains (PDGFRα and PDGFRβ) in normal and fibrotic rat livers, was assessed quantitatively by real time PCR. The protein level of PDGF-D was quantified by immunoblotting and immunohistochemistry. Phosphorylation of PDGF receptors and activation of downstream PDGF-signaling components (ERK1/2, JNK, p38 MAPK, PKB/Akt) was analysed by immunoprecipitation and Western blot analysis.

Results: Upon bile duct ligature, the mRNA expression of all PDGF isoforms and receptors was upregulated and the PDGF-A, -B and -D expression was significantly higher than the increase in PDGF-C. Additionally, we demonstrated that the expression of PDGF-D and PDGFRβ also increased significantly at the protein level. Immunostaining of liver sections revealed that PDGF-D was localized along the fibrotic septa of the portal areas comparable to PDGF-B and its cognate PDGFRβ. We found that PDGF-D potently activates and stimulates the PDGFRβ-driven pathway in HSC, as evidenced by PDGFRβ autophosphorylation and subsequent activation of the downstream signaling molecules ERK1/2, JNK-, p38 MAPK, and PKB/Akt. In contrast, the biological activity and potency of PDGF-C was lower and comparable to the PDGF-A isoform. Immunohistochemical staining revealed a strong activity of phosphor-Tyr in periportal fibroblasts and perisinusoidal proliferative HSC, both expressing PDGFRβ. The staining was notably overlapping alongside the positive staining for PDGF-B and -D. Furthermore, PDGF-D exerted mitogenic effects comparable to PDGF-B in both cultured HSC and myofibroblasts, while PDGF-A and -C had only marginal mitogenic capacity. The fibrogenic effects of PDGF-D in HSC were comparable to PDGF-B, the latter showing the highest effect-potential for extracellular matrix production.

Conclusions: PDGF-D possesses potential pathogenetic properties on activation and matrix remodeling in HSC. Therefore, this cytokine must be added to the ever-increasing list of molecular targets suitable for therapeutic interventions.

PDGF - bile-duct ligation - cellular activation - hepatic stellate cells - liver fibrosis - myofibroblast - transdifferentiation