**Background:** Acute hepatic steatosis is a well-known phenomenon during early liver regeneration, peaking at 12–24 hours post-hepatectomy with triglyceride content increasing three- to four-fold. While lipid accumulation has been considered essential for regeneration, the mechanisms by which lipids signal to coordinate epigenetic changes and cell proliferation have remained unclear. Chronic steatosis, in contrast, impairs liver regeneration. This study aimed to identify unknown regulators linking acute steatosis to chromatin remodeling during regeneration.
**Methods:** The authors performed an in vivo CRISPR/Cas9 screen using a lentiviral library of 9,094 sgRNAs targeting 1,514 genes (epigenetic regulators, kinases, transcription factors) in Cas9^LSL+/+^Fah^-/-^ mice. Hepatocytes expressing Fah and sgRNAs repopulated the liver after NTBC withdrawal, and enriched sgRNAs were identified by deep sequencing. MIER1 was validated using AAV8-delivered Mier1 sgRNA in Cas9^LSL+/+^ mice subjected to two-thirds partial hepatectomy (PHx). Liver regeneration was assessed by liver/body weight ratio, Ki-67 immunohistochemistry, and immunoblotting for Cyclin D1 and PCNA. RNA-seq, ATAC-seq, and ChIP-seq (FLAG-tagged MIER1) were performed. Lipidomic analysis (10 biological replicates per time point) quantified lipid species before and 24 h after PHx. Primary hepatocytes were treated with free fatty acids (palmitic acid, stearic acid, oleic acid, linoleic acid). Translational control was assessed via puromycin incorporation, EIF2S1 phosphorylation, and polysome profiling with RNA-seq. Adipose-specific Lipe (HSL) knockout mice (Lipe-AKO), chronic HFD-fed mice (8 weeks), and aging mice (~12 months) were used to model chronic steatosis.
**Key Results:** The CRISPR screen identified MIER1 as a top candidate. MIER1 depletion significantly accelerated liver regeneration: liver/body weight ratio was increased at 36 h (control ~2.5% vs. Mier1 sgRNA ~3.5%, P<0.0001), Ki-67+ cells at 48 h were ~15% vs. ~35% (P<0.0001), and Cyclin D1 and PCNA were markedly elevated. MIER1 overexpression delayed regeneration (liver/body weight ratio at 48 h: control ~3.5% vs. MIER1-FLAG ~2.5%, P<0.01). ChIP-seq identified 9,310 MIER1-bound sites enriched near cell cycle genes. ATAC-seq showed that MIER1 depletion increased chromatin accessibility at 24 h post-PHx at 2,221 peaks overlapping MIER1 binding sites, but not in quiescent liver. MIER1 protein decreased at 24–36 h post-PHx (P<0.01 vs. 0 h). Lipidomics revealed marked increases in triacylglycerols, diacylglycerols, cholesteryl esters, and phosphatidylethanolamine at 24 h. Palmitic acid (PA) and stearic acid (SA) treatment of primary hepatocytes reduced MIER1 protein without changing Mier1 mRNA. PA increased EIF2S1-S51 phosphorylation and decreased global protein synthesis. In vivo, p-EIF2S1-S51 transiently increased at 24 h post-PHx (P<0.01 vs. 0 h). Polysome profiling showed a 25.6% reduction in polysome-associated transcripts at 24 h, with Mier1 transcripts in polysomes reduced by 32.1% (P<0.05). ISRIB treatment blocked MIER1 downregulation. Lipe-AKO animals had ~50% less liver triglyceride at 24 h, no EIF2S1 phosphorylation or MIER1 reduction, and impaired regeneration (liver/body weight at 36 h: control ~3.2% vs. Lipe-AKO ~2.4%, P<0.01); MIER1 depletion rescued regeneration. Acute HFD pretreatment accelerated MIER1 reduction (evident at 12 h) and improved regeneration. In chronic HFD and aging mice, MIER1 and p-EIF2S1 remained constant during regeneration, and PA failed to reduce MIER1 in primary hepatocytes; MIER1 depletion significantly improved liver regeneration in both models (e.g., liver/body weight at 48 h in aging: ~2.8% vs. aging+Mier1 sgRNA ~3.8%, P<0.01).
**Clinical Implications:** This study reveals a novel signaling function of acute hepatic steatosis—beyond providing metabolic substrates—that translationally controls MIER1 via the integrated stress response to promote liver regeneration. The finding that chronic steatosis (from HFD or aging) desensitizes hepatocytes to this signal, but that MIER1 depletion can rescue regeneration, identifies MIER1 as a potential therapeutic target for improving liver regeneration in patients with fatty liver disease or elderly individuals undergoing liver resection or transplantation. The human liver organoid data suggest a conserved mechanism, though further human studies are warranted.