Vitamin B12 and folate decrease inflammation and fibrosis in NASH by preventing syntaxin 17 homocysteinylation

BACKGROUND & AIMS

Several recent clinical studies have shown that serum homocysteine (Hcy) levels are positively correlated, while vitamin B₁₂ (B₁₂) and folate levels are negative correlated, with non-alcoholic steatohepatitis (NASH) severity. However, it is not known whether hyperhomocysteinemia (HHcy) plays a pathogenic role in NASH.

METHODS

We examined the effects of HHcy on NASH progression, metabolism, and autophagy in dietary and genetic mouse models, patients, and primates. We employed vitamin B₁₂ (B₁₂) and folate (Fol) to reverse NASH features in mice and cell culture.

RESULTS

Serum Hcy correlated with hepatic inflammation and fibrosis in NASH. Elevated hepatic Hcy induced and exacerbated NASH. Gene expression of hepatic Hcy-metabolizing enzymes was downregulated in NASH. Surprisingly, we found increased homocysteinylation (Hcy-lation) and ubiquitination of multiple hepatic proteins in NASH including the key autophagosome/lysosome fusion protein, Syntaxin 17 (Stx17). This protein was Hcy-lated and ubiquitinated, and its degradation led to a block in autophagy. Genetic manipulation of Stx17 revealed its critical role in regulating autophagy, inflammation and fibrosis during HHcy. Remarkably, dietary B₁₂/Fol, which promotes enzymatic conversion of Hcy to methionine, decreased HHcy and hepatic Hcy-lated protein levels, restored Stx17 expression and autophagy, stimulated β -oxidation of fatty acids, and improved hepatic histology in mice with pre-established NASH.

CONCLUSIONS

HHcy plays a key role in the pathogenesis of NASH via Stx17 homocysteinylation. B₁₂/folate also may represent a novel first-line therapy for NASH.

LAY SUMMARY

The incidence of non-alcoholic steatohepatitis, for which there are no approved pharmacological therapies, is increasing, posing a significant healthcare challenge. Herein, based on studies in mice, primates and humans, we found that dietary supplementation with vitamin B₁₂ and folate could have therapeutic potential for the prevention or treatment of non-alcoholic steatohepatitis.

Purification and characterization of sequential cobalamin-dependent radical SAM methylases ThnK and TokK in carbapenem β-lactam antibiotic biosynthesis

ThnK and TokK are cobalamin-dependent radical S-adenosylmethionine enzymes that catalyze sequential methylations of a common carbapenem biosynthetic intermediate. ThnK was an early characterized member of the subfamily of cobalamin-dependent radical S-adenosylmethionine enzymes. Since initial publication of the ThnK function, the field has progressed, and we have made methodological strides in the expression and purification of this enzyme and its ortholog TokK. An optimized protocol for obtaining the enzymes in pure and active form has enabled us to characterize their reactions and gain greater insight into the kinetic behavior of the sequential methylations they catalyze. We share here the methods and strategy that we have developed through our study of these enzymes.

Application of adaptive neuro-fuzzy inference systems (ANFIS) to delineate estradiol, glutathione and homocysteine interactions

The rationale of the current study was to elucidate the contributing factors for the gender-based differences in total plasma homocysteine levels. A total of 413 subjects comprising of 293 men and 120 women were enrolled for the study. Chemiluminescence technology for vitamin B₁₂, folate and total plasma homocysteine; ELISA for estradiol and 8-oxo-2-deoxyguanosine; Ellman's method for total glutathione; and PCR-RFLP analysis for the detection of methylene tetrahydrofolate reductase (MTHFR) C677T polymorphism were employed. No statistically significant differences were observed between the men and women in the distribution of age (p = 0.82), vitamin B₁₂ (p = 0.23), folate (p = 0.36) and MTHFR C677T polymorphism (p = 0.35). However, the total plasma homocysteine levels were higher in men compared to women (28.4 ± 17.9 vs. 20.6 ± 13.6 μmol/L, p < 0.0001). In order to explain this gender differences in homocysteine, adaptive neuro-fuzzy inference systems (ANFIS) were developed to understand trivariate interactions among estradiol, glutathione and homocysteine. In the presence of adequate estradiol levels, inverse association was observed between glutathione and homocysteine. This association is lost when estradiol levels were inadequate. Estradiol was found to quench homocysteine mediated oxidative DNA damage. Irrespective of gender, combined deficiency of vitamin B₁₂ and folate showed positive association with hyperhomocysteinemia and vice versa. Homocysteine reduction in response to vitamin status varied according to gender with men responding to folate and women responding to B₁₂. To conclude, gender-differences in homocysteine are attributable estradiol mediated lowering of homocysteine that prevents inactivation of glutathione mediated oxidative defense in women.

Alanine scanning mutagenesis identifies an asparagine-arginine-lysine triad essential to assembly of the shell of the Pdu microcompartment

Bacterial microcompartments (MCPs) are the simplest organelles known. They function to enhance metabolic pathways by confining several related enzymes inside an all-protein envelope called the shell. In this study, we investigated the factors that govern MCP assembly by performing scanning mutagenesis on the surface residues of PduA, a major shell protein of the MCP used for 1,2-propanediol degradation. Biochemical, genetic, and structural analysis of 20 mutants allowed us to determine that PduA K26, N29, and R79 are crucial residues that stabilize the shell of the 1,2-propanediol MCP. In addition, we identify two PduA mutants (K37A and K55A) that impair MCP function most likely by altering the permeability of its protein shell. These are the first studies to examine the phenotypic effects of shell protein structural mutations in an MCP system. The findings reported here may be applicable to engineering protein containers with improved stability for biotechnology applications.