*Purpose: The purpose of our study was to evaluate the mechanistic role of Spleen Tyrosine Kinase (SYK) signaling on the fibrosis – hepatocarcinogenesis axis and specifically determine its modulatory role in this process in each cellular population including liver parenchymal cells, hepatic stellate cells (HSCs), myeloid cells, B lymphocytes, and T lymphocytes.

*Methods: 12 week-old female C57BL/6J, Tlr4^-/-, Syk^fl/fl, Alb^Cre, Cd4^Cre, LyzM^Cre, Lrat^Cre, and Cd19^Cre mice were treated with thrice weekly injections of thioacetamide (TAA) (250 mg/kg). To induce HCC, 2-week-old male mice were administered with a single dose of DEN (15 mg/kg; Sigma-Aldrich, Saint Louis, MO) via i.p. injection followed by bi-weekly injections of CCl₄ (0.2 ml/kg; Sigma-Aldrich) starting at 8 weeks of age, and mice were sacrificed 24 weeks later. In select experiments, mice were treated with thrice weekly with Piceatannol (20mg/kg; Selleckchem; Houston, TX), PRT062607 (15mg/kg; Selleckchem; Houston, TX), or corn oil via oral gavage for 12 weeks. Hepatic stellate cells, and non-parenchymal immune cells were isolated from the liver and were immunophenotyped using flow cytometry and RNA-seq.

*Results: We discovered high SYK expression in the parenchymal hepatocyte, hepatic stellate cell (HSC), and the inflammatory compartments in the fibrotic liver. We postulated that targeting SYK would mitigate hepatic fibrosis and oncogenic progression. We found that inhibition of SYK with the selective small molecule inhibitors Piceatannol and PRT062607 markedly protected against toxin-induced hepatic fibrosis, associated hepatocellular injury and intra-hepatic inflammation, and hepatocarcinogenesis. SYK inhibition resulted in increased intra-tumoral expression of the p16 and p53 but decreased expression of Bcl-xL and SMAD4. Further, hepatic expression of genes regulating angiogenesis, apoptosis, cell cycle regulation, and cellular senescence were affected by targeting SYK. We found that SYK inhibition mitigated both HSC trans-differentiation and acquisition of an inflammatory phenotype in T cells, B cells, and myeloid cells. However, in vivo experiments employing selective targeted deletion of SYK indicated that only SYK deletion in the myeloid compartment was sufficient to confer protection against fibrogenic progression. Targeting SYK promoted myeloid cell differentiation into hepato-protective TNFα^lowCD206^hi phenotype downregulating mTOR, IL-8 signaling and oxidative phosphorylation.

*Conclusions: Collectively, these data suggest that SYK is an attractive target for experimental therapeutics in treating hepatic fibrosis and oncogenesis.

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