Metabolic dysfunction-associated steatotic liver disease (MASLD), which affects about 30% of the population worldwide, is driven in part by excessive hepatic de novo lipogenesis (the production of fatty acids within the liver). In mice, the minor intron splicing factors ZRSR1 and ZRSR2 can influence hepatic lipid metabolism by controlling the processing of Insig transcripts. Whether these proteins also regulate metabolism independently of their canonical splicing function has remained unclear.
A new study published in Life Metabolism by Professor Xu-Yun Zhao and colleagues at Shanghai Jiao Tong University School of Medicine identifies a previously unrecognized, splicing-independent role for ZRSR1 in restraining hepatic lipogenesis (Figure 1). In mice with high-fat diet-induced obesity and steatosis, hepatic ZRSR1 expression decreased, whereas ZRSR2 expression and global minor intron splicing were largely preserved. This separation enabled the researchers to examine ZRSR1 functions beyond its canonical role in the minor spliceosome. Analysis of a public human liver RNA-sequencing dataset likewise found no global minor intron retention in steatotic samples, but the study did not establish the same ZRSR1 mechanism in humans.
Using mouse models with liver-specific Zrsr1 overexpression or disruption, the researchers found that increased hepatic Zrsr1 reduced high-fat diet-induced weight gain, liver triglyceride accumulation, steatosis, and insulin resistance, whereas Zrsr1 disruption generally produced the opposite effects. Neither manipulation caused global minor intron retention, supporting a splicing-independent metabolic role for ZRSR1.
RNA sequencing and molecular analyses pointed to the LXR-SREBP1c lipogenic pathway. Zrsr1 overexpression lowered Srebp1c and downstream lipogenic genes, including Fasn, Scd1, and Dgat2. ZRSR1 localized to the nucleus and interacted with LXRα through its second zinc finger and RNA-binding domains. Rather than destabilizing LXRα or preventing LXRα-RXRα heterodimer formation, ZRSR1 reduced LXRα occupancy at target promoters, including Srebp1c, as shown by CUT&Tag. This limited Srebp1c transcription and de novo lipogenesis.
In AML12 cells and primary mouse hepatocytes, Zrsr1 overexpression blunted LXR agonist-induced lipogenic gene expression. In AML12 cells, it also reduced the incorporation of radiolabeled acetate into fatty acids, a direct readout of de novo lipogenesis. Restoring Srebp1c expression largely rescued the lipogenic gene program, supporting SREBP1c as a major downstream mediator of ZRSR1.
Together with the authors’ earlier work, the findings support a stage-specific model of MASLD progression. During early steatosis, selective ZRSR1 downregulation may increase LXR-SREBP1c-dependent transcription without causing detectable minor intron retention. At later MASH and fibrotic stages, more extensive loss of both ZRSR1 and ZRSR2 may additionally promote minor intron retention in Insig genes and SREBP1c cleavage. These two mechanisms could contribute sequentially to escalating lipogenesis as disease advances.
These findings broaden the known functions of minor-spliceosome proteins and reveal a new regulatory layer in fatty liver biology. The evidence is preclinical and does not establish ZRSR1 as a therapeutic target. Human ZRSR1 is a pseudogene, whereas ZRSR2 is the principal human minor intron splicing factor. Further work must determine whether an analogous splicing-independent mechanism operates in human MASLD.
DOI
10.1093/lifemeta/loag020