MIF and JAB1 enhance TGF-beta signaling in transdifferentiating hepatic stellate cells

Hepatic fibrosis is a wound healing process mainly characterized by accumulation of extracellular matrix in response to liver injury. As a consequence of chronic tissue damage hepatic stellate cells undergo a process of transdifferentiation into myofibroblasts. This process is promoted by TGF-beta, a key mediator of fibrosis. The intracellular effectors of TGF-beta signaling, the Smad2/3 proteins, are activated by receptors and translocate as a complex with Smad4, into the nucleus, where they regulate transcription. Macrophage migration inhibitory factor (MIF) is an inflammatory cytokine that broadly controls cell behavior such as proliferation, apoptosis, and inflammatory gene expression. MIF is an intracellular binding partner of JAB1 and has been shown to modulate cell proliferation, cell cycle progression, and the AP1 pathway through JAB1/CSN5. JAB1, a transcriptional coactivator and component of the COP9 signalosome, can modulate TGF-beta signaling by promoting the degradation of both Smad4 and Smad7. In order to elucidate the role of JAB1 and MIF in TGF-beta signaling various adenoviral constructs for overexpression and/or kockdown of JAB1, MIF, and Smad4/7 were generated and effects on TGF-beta signaling were tested in infected transdifferentiating hepatic stellate cells (HSC). Overexpression of MIF and JAB1 in HSC resulted in a marked activation of TGF-beta-mediated signaling as assessed by a CAGA-luc reporter. Both MIF and JAB1 were able to restore Smad7-induced inhibition of the TGF-beta pathway. In addition, overexpression of JAB1 promoted phosphorylation of Smad2. Noteworthy, knockdown of MIF and/or JAB1 by adenoviral shRNAi constructs strongly reduced the expression of alpha smooth muscle actin in transdifferentiating HSC. In conclusion, our results indicate that MIF and JAB1 potentiate transforming growth factor beta effects in transdifferentiating HSC in a cooperative manner. Their regulatory effects may have a major impact on transdifferentiation of HSC and potentially on the progression of liver fibrosis.