SYK-3BP2 Pathway Activity in Parenchymal and Myeloid Cells Is a Key Pathogenic Factor in Metabolic Steatohepatitis

BACKGROUND & AIMS

Spleen tyrosine kinase (SYK) signaling pathway regulates critical processes in innate immunity, but its role in parenchymal cells remains elusive in chronic liver diseases. We investigate the relative contribution of SYK and its substrate c-Abl Src homology 3 domain-binding protein-2 (3BP2) in both myeloid cells and hepatocytes in the onset of metabolic steatohepatitis.

METHODS

Hepatic SYK-3BP2 pathway was evaluated in mouse models of metabolic-associated fatty liver diseases (MAFLD) and in obese patients with biopsy-proven MAFLD (n = 33). Its role in liver complications was evaluated in Sh3bp2 KO and myeloid-specific Syk KO mice challenged with methionine and choline deficient diet and in homozygous Sh3bp2^KI/KI mice with and without SYK expression in myeloid cells.

RESULTS

Here we report that hepatic expression of 3BP2 and SYK correlated with metabolic steatohepatitis severity in mice. 3BP2 deficiency and SYK deletion in myeloid cells mediated the same protective effects on liver inflammation, injury, and fibrosis priming upon diet-induced steatohepatitis. In primary hepatocytes, the targeting of 3BP2 or SYK strongly decreased the lipopolysaccharide-mediated inflammatory mediator expression and 3BP2-regulated SYK expression. In homozygous Sh3bp2^KI/KI mice, the chronic inflammation mediated by the proteasome-resistant 3BP2 mutant promoted severe hepatitis and liver fibrosis with augmented liver SYK expression. In these mice, the deletion of SYK in myeloid cells was sufficient to prevent these liver lesions. The hepatic expression of SYK is also up-regulated with metabolic steatohepatitis and correlates with liver macrophages in biopsy-proven MAFLD patients.

CONCLUSIONS

Collectively, these data suggest an important role for the SYK-3BP2 pathway in the pathogenesis of chronic liver inflammatory diseases and highlight its targeting in hepatocytes and myeloid cells as a potential strategy to treat metabolic steatohepatitis.

Bone dynamics and inflammation: lessons from rare diseases

The adaptor protein 3BP2 (SH3-domain binding protein 2), which is encoded by the SH3BP2 locus, nucleates a signaling complex comprising ABL, SRC, VAV, and SYK, and facilitates an open active configuration of these proteins, leading to their kinase activation. Gain-of-function missense mutations in the SH3BP2 gene cause cherubism, an autosomal dominant disorder associated with severe craniofacial developmental defects in children. Previous studies have demonstrated that 3BP2 and its degradation pathway regulate bone metabolism, energy metabolism, and inflammation and that dysregulation of the 3BP2 degradation pathway is associated with human disorders. Herein, we discussed lessons from cherubism indicating that 3BP2 studies could elucidate the pathogenesis of bone loss caused by inflammation and identify suitable therapeutic targets.

Timed Regulation of 3BP2 Induction Is Critical for Sustaining CD8+ T Cell Expansion and Differentiation

Successful anti-viral response requires the sustained activation and expansion of CD8⁺ T cells for periods that far exceed the time limit of physical T cell interaction with antigen-presenting cells (APCs). The expanding CD8⁺ T cell pool generates the effector and memory cell populations that provide viral clearance and long-term immunity, respectively. Here, we demonstrate that 3BP2 is recruited in cytoplasmic microclusters and nucleates a signaling complex that facilitates MHC:peptide-independent activation of signaling pathways downstream of the TCR. We show that induction of the adaptor molecule 3BP2 is a sensor of TCR signal strength and is critical for sustaining CD8⁺ T cell proliferation and regulating effector and memory differentiation.

Dimitriou et al. show that the adaptor protein 3BP2 lowers the threshold of T cell activation and that the induction of the 3BP2 signaling module at later time points may serve to recapitulate and prolong the biochemical signals emanating from the TCR required for sustained MHC:peptide-independent T cell proliferation.

Effects of a spleen tyrosine kinase inhibitor on progression of the lupus nephritis in mice

The Fc receptors (FcR) have pivotal roles in the pathogenesis of the autoimmune glomerulonephritis. We therefore investigated the effects of a Syk inhibitor on the progression of lupus nephritis and SH3 domain binding protein 2 and p38MAP kinase signalings in mice. NZB/W F1 mice, a model of lupus nephritis, received a Syk inhibitor R406. Western blotting and immunohistochemistry revealed that R406 treatment significantly delayed the appearance of proteinuria, histologically improved their glomerulosclerosis and inhibited the increased the expression of MCP-1 and TGF-β1 mRNAs and the nephrin and podocin proteins in the kidney. The treatment suppressed the phosphorylation of 3BP2 in white blood cells from the spleen and significantly inhibited the phosphorylation of p38MAPK in the kidney but did not affect expression of neonatal Fc receptor. These findings indicate the important roles and mechanisms of Fcγ receptors I and III in the development of autoimmune glomerulonephritis and suggest the possible application of Syk inhibitors as novel medicines for the glomerulonephritis.

Tyrosine phosphorylation of 3BP2 is indispensable for the interaction with VAV3 in chicken DT40 cells

Adaptor protein c-Abl SH3 domain-binding protein-2 (3BP2) is known to play regulatory roles in immunoreceptor-mediated signal transduction. We have previously demonstrated that Tyr(174), Tyr(183) and Tyr(446) in mouse 3BP2 are predominantly phosphorylated by Syk, and the phosphorylation of Tyr(183) and the Src homology 2 (SH2) domain of mouse 3BP2 are critical for B cell receptor (BCR)-induced activation of nuclear factor of activated T cells (NFAT) in human B cells. In this report, we have shown that Syk, but not Abl family protein-tyrosine kinases, is critical for BCR-mediated tyrosine phosphorylation of 3BP2 in chicken DT40 cells. Mutational analysis showed that Tyr(174), Tyr(183) and Tyr(426) of chicken 3BP2 are the major phosphorylation sites by Syk and the SH2 domain of 3BP2 is critical for tyrosine phosphorylation. In addition, phosphorylation of Tyr(426) is required for the inducible interaction with the SH2 domain of Vav3. Moreover, the expression of the mutant form of 3BP2 in which Tyr(426) was substituted to Phe resulted in the reduction in BCR-mediated Rac1 activation, when compared with the case of wild-type. Altogether, these data suggest that 3BP2 is involved in the activation of Rac1 through the regulation of Vav3 by Syk-dependent phosphorylation of Tyr(426) following BCR stimulation.