The complexities of hepatic SLC7A11 to promote MASLD mediated by nonessential amino acids

This study is led by Professor Junxia Min / Fudi Wang's research team from Zhejiang University School of Medicine. Metabolic dysfunction–associated steatotic liver disease (MASLD) has brought out heavy health burden in the worldwide. However, relatively few FDA approved treatments are currently available. Developing new therapeutic targets and strategies are urgent. Nonessential amino acid (NEAA) metabolism has been linked to a variety of metabolic diseases. The SLC7A11 gene encodes xCT, the light chain subunit of the glutamate-cystine antiporter system X_c^-. Cystine imported by SLC7A11 is reduced to cysteine, which is used to synthesize GSH (glutathione) and then maintain redox homeostasis. However, the function of SLC7A11 in the progression of MASLD remains unclear. Firstly, immunohistochemistry analysis of liver samples from patients with MASLD and control subjects showed that SLC7A11 expression was correlated with disease grade, whereas SLC7A11 expression varied widely among the liver sections. Theses results suggest that SLC7A11 may play a complex functional role in the progression of MASLD. To study this, researchers used a global Slc7a11 knockout (Slc7a11^-/-) mouse and found aggravated hepatic steatosis and inflammation, as well as decreased cysteine levels and GSH/GSSG ratios in MCD fed Slc7a11^-/- mice compared to control mice. Specific ferroptosis inhibitor ferrostatin-1 (Fer-1) treated mice had reduced hepatic steatosis and malondialdehyde (MDA) levels compared to vehicle-treated mice. These results indicate that the global loss of Slc7a11 exacerbates the progression of MASLD via a mechanism involving cysteine and GSH deficiency–induced ferroptosis.

To further study the putative role of SLC7A11 in MASLD, researchers generated a transgenic mouse line in which Slc7a11 was overexpressed in hepatocytes (referred to hereafter as Slc7a11-LTG mice). The progression of MASLD was promoted in MCD-fed, HFD-fed and Western diet+CCl₄ treated Slc7a11-LTG mice compared to control mice. Slc7a11-LTG-LPKO mice obtained by crossing Slc7a11-LTG mice and liver-specific Pten knockout (LPKO) mice also comfirmed this finding. Primary hepatocytes of Slc7a11-LTG mice had lower levels of glutamate and higher levels of cystine due to enhanced activity of SLC7A11. Notably, cysteine levels and GSH/GSSG ratio were lower in primary hepatocytes of Slc7a11-LTG mice, due to reduced NADPH/NADP⁺. These results were not consistent with the classical function of SLC7A11 and thereby raised researchers’ interest. Fer-1 mitigated the disease progression and inhibited ferroptosis in MASLD models of Slc7a11-LTG mice, suggesting ferroptosis played a role in the process.

To figure out the underlying mechanism, researchers performed non-targeted metabolomics and transcriptomics analysis and found that dysregulated amino acid pathways were significantly enriched. Targeted metabolomics and function assay indicated that serine provided protection against fatty acids-induced lipid accumulation in Slc7a11-LTG hepatocytes. Metabolic flux analysis suggested that decrease in cytosolic glutamate in Slc7a11-LTG hepatocytes underlies the reduction in serine production in these cells. Serine supplementation rescued the MASLD phenotype and ferroptosis in MCD-fed Slc7a11-LTG mice or HFD-fed Slc7a11-LTG mice.These results suggest that overexpressing hepatic Slc7a11 leads to glutamate—and in turn, serine—deficiency, thereby promoting the progression of MASLD.

Serine can be synthesized through transsulfuration pathway. Cystathionine-β-synthase (CBS) and cystathionine γ-lyase (CTH) are the key enzymes. Researchers found that serine significantly reduced lipid ROS levels in hepatocytes of MCD-fed Slc7a11-LTG mice, and blocking CTH with PAG prevented the ability of serine to provide this protection against ferroptosis. Serine is the upstream metabolite in the transsulfuration pathway, and ferroptosis is downstream of the transsulfuration pathway. Therefore, serine deficiency in hepatocytes from Slc7a11-LTG mice impairs the transsulfuration pathway, thereby promoting ferroptosis and accelerating the progression of MASLD. Besides, serum serine/glutamate ratio was significantly lower in these preclinical murine models, suggesting that it might serve as a prognostic biomarker for MASLD in patients.This notion is supported by the meta-analysis of clinical data reported for MASLD patients.

This article reported that dysregulated NEAAs including serine, cysteine and glutamate  mediated by SLC7A11 promoted MASLD progression. Serine supplementation and/or blocking ferroptosis may serve as novel strategies for the prevention and/or treatment of MASLD. Serum serine/glutamate ratio reflects disease progression and may serve as a valuable, minimally invasive prognostic biomarker for patients with MASLD.

---

---