Involvement of LH3 and GLT25D1 for glucosyl-galactosyl-hydroxylation on non-collagen-like domain of FGL1

Molecular structure and enzymatic mechanism of the human collagen hydroxylysine galactosyltransferase GLT25D1/COLGALT1

During collagen biosynthesis, lysine residues undergo extensive post-translational modifications through the alternate action of two distinct metal ion-dependent enzyme families (i.e., LH/PLODs and GLT25D/COLGALT), ultimately producing the highly conserved α-(1,2)-glucosyl-β-(1,O)-galactosyl-5-hydroxylysine pattern. Malfunctions in these enzymes are linked to developmental pathologies and extracellular matrix alterations associated to enhanced aggressiveness of solid tumors. Here, we characterized human GLT25D1/COLGALT1, revealing an elongated head-to-head homodimeric assembly. Each monomer encompasses two domains (named GT1 and GT2), both unexpectedly capable of binding metal ion cofactors and UDP-α-galactose donor substrates, resulting in four candidate catalytic sites per dimer. We identify the catalytic site in GT2, featuring an unusual Glu-Asp-Asp motif critical for Mn2+ binding, ruling out direct catalytic roles for the GT1 domain, but showing that in this domain the unexpectedly bound Ca2+ and UDP-α-galactose cofactors are critical for folding stability. Dimerization, albeit not essential for GLT25D1/COLGALT1 activity, provides a critical molecular contact site for multi-enzyme assembly interactions with partner multifunctional LH/PLOD lysyl hydroxylase-glycosyltransferase enzymes.

The structural basis for the human procollagen lysine hydroxylation and dual-glycosylation

The proper assembly and maturation of collagens necessitate the orchestrated hydroxylation and glycosylation of multiple lysyl residues in procollagen chains. Dysfunctions in this multistep modification process can lead to severe collagen-associated diseases. To elucidate the coordination of lysyl processing activities, we determine the cryo-EM structures of the enzyme complex formed by LH3/PLOD3 and GLT25D1/ColGalT1, designated as the KOGG complex. Our structural analysis reveals a tetrameric complex comprising dimeric LH3/PLOD3s and GLT25D1/ColGalT1s, assembled with interactions involving the N-terminal loop of GLT25D1/ColGalT1 bridging another GLT25D1/ColGalT1 and LH3/PLOD3. We further elucidate the spatial configuration of the hydroxylase, galactosyltransferase, and glucosyltransferase sites within the KOGG complex, along with the key residues involved in substrate binding at these enzymatic sites. Intriguingly, we identify a high-order oligomeric pattern characterized by the formation of a fiber-like KOGG polymer assembled through the repetitive incorporation of KOGG tetramers as the biological unit.

Fibrinogen-like 1: A hepatokine linking liver physiology to hematology

A recent study identified the critical contribution of the hepatokine FGL1 to the regulation of iron metabolism during the recovery from anemia. FGL1 is secreted by hepatocytes in response to hypoxia to sequester BMP ligands and repress the transcription of the iron-regulatory hormone hepcidin. This process ensures the proper supply of iron to the bone marrow for new red blood cell synthesis and the restoration of physiological oxygen levels. FGL1 may therefore contribute to the recovery from common anemias and cause iron overload in chronic anemias with ineffective erythropoiesis, such as ß-thalassemia, dyserythropoietic anemia, and myelodysplastic syndromes. However, FGL1 has also been described as a regulator of hepatocyte proliferation, glucose homeostasis, and insulin signaling, as well as a mediator of liver steatosis and immune evasion. Chronic exposure to elevated levels of FGL1 during anemia may therefore have systemic metabolic effects besides iron regulation and erythropoiesis. Here, we are providing an overview of the proposed functions of FGL1 in physiology and pathophysiology.

Role of Fibrinogen-like Protein 1 in Tumor Recurrence Following Hepatectomy

Partial hepatectomy is a first-line treatment for hepatocellular carcinoma. Within 2 weeks following partial hepatectomy, specific molecular pathways are activated to promote liver regeneration. Nevertheless, residual microtumors may also exploit these pathways to reappear and metastasize. Therapeutically targeting molecules that are differentially regulated between normal cells and malignancies, such as fibrinogen-like protein 1 (FGL1), appears to be an effective approach. The potential functions of FGL1 in both regenerative and malignant cells are discussed within the ambit of this review. While FGL1 is normally elevated in regenerative hepatocytes, it is normally downregulated in malignant cells. Hepatectomy does indeed upregulate FGL1 by increasing the release of transcription factors that promote FGL1, including HNF-1α and STAT3, and inflammatory effectors, such as TGF-β and IL6. This, in turn, stimulates certain proliferative pathways, including EGFR/Src/ERK. Hepatectomy alters the phase transition of highly differentiated hepatocytes from G0 to G1, thereby transforming susceptible cells into cancerous ones. Activation of the PI3K/Akt/mTOR pathway by FGL1 allele loss on chromosome 8, a tumor suppressor area, may also cause hepatocellular carcinoma. Interestingly, FGL1 is specifically expressed in the liver via HNF-1α histone acetylase activity, which triggers lipid metabolic reprogramming in malignancies. FGL1 might also be involved in other carcinogenesis processes such as hypoxia, epithelial-mesenchymal transition, immunosuppression, and sorafenib-mediated drug resistance. This study highlights a research gap in these disciplines and the necessity for additional research on FGL1 function in the described processes.

The Effect of the NFκB-USP9X-Cx43 Axis on the Dynamic Balance of Bone Formation/Degradation during Ossification of the Posterior Longitudinal Ligament of the Cervical Spine

Connexin 43- (Cx43-) mediated nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) signaling has been found involved in the ossification of the posterior longitudinal ligament (OPLL). However, the underlying mechanism how OPLL is regulated has not been elucidated. In the present study, primary ligament fibroblast were isolated; immunoprecipitation (IP) and liquid chromatography-mass spectrometry (LC-MS) assays were applied to identify potential binding proteins of Cx43. Protein interaction was then confirmed by co-IP assay. Alkaline phosphatase (ALP) activity and alizarin red staining were used to evaluate ossification. Luciferase reporter assay and chromatin immunoprecipitation (ChIP) assay were employed to assess the binding between NF-κB p65 and target gene. Lipoxygenase inhibitor (5,8,11-eicosatriynoic acid, EPA) was applied to induce endoplasmic reticulum (ER) stress, and 4-phenylbutyrate (4-PBA) was used as an ER-stress inhibitor. Expression of USP9X, Cx43, and nuclei p65 in ligaments from patients and controls was detected by Western blotting. The results showed that ubiquitin-specific protease 9 X-chromosome (USP9X), a deubiquitylating enzyme, was a candidate of Cx43 binding proteins, and USP9X inhibited Cx43 ubiquitination. In vitro experiments showed that USP9X promoted ossification of primary ligament fibroblasts and nuclear translocation of NF-κB p65 by regulating Cx43 expression. Moreover, NF-κB can bind to the USP9X promoter to promote its transcription. When ER stress was inhibited by 4-PBA, USP9X levels, NF-κB nuclei translocation, and ALP activity were decreased. Reverse results were obtained when ER stress was induced by EPA. PDTC, an NF-κB inhibitor, could abolish the effects of EPA. Furthermore, USP9X, Cx43, and nuclei p65 were significantly upregulated in ligaments from OPLL patients than non-OPLL controls. USP9X was positively correlated with CX43 and nuclei p65 in OPLL samples. Overall, the findings suggest that the ER stress–NFκB-USP9X-Cx43 signaling pathway plays an important role in OPLL progression.