Molecular and Cellular Biology

Interstrand crosslinks are cytotoxic lesions that inhibit essential processes including replication and transcription. Replication fork reversal occurs in response to interstrand crosslink inducing drug, MMC, but how replication fork reversal promotes repair of interstrand crosslinks is poorly understood. Here, we investigated the role of the RAD54L translocase in interstrand crosslink repair. We found RAD54L is required to promote nascent DNA degradation in FANCD2 and FANCA-depleted cells consistent with a previous study indicating RAD54L promotes replication fork reversal. We further show RAD54L activity is required for formation of radial chromosomes in FANCD2-deficient cells suggesting fork reversal may be required to generate the intermediate undergoing aberrant fusion in FANC-deficient cells. Finally, we demonstrate FANCD2 foci accumulate and DSBs persist in RAD54L-deficient cells indicating RAD54L is required for efficient repair of DSBs. Together, our results indicate RAD54L plays multiple roles in efficient processing and repair of interstrand crosslinks.

Ubiquitin-specific protease 10 (USP10) is a multifunctional deubiquitinating enzyme that primarily regulates cellular stress responses, including the DNA damage response. Here, we show that USP10 is required for homologous recombination (HR)-mediated repair of DNA double-strand breaks (DSBs) and for the maintenance of genomic stability. USP10-depleted cells exhibit spontaneous micronuclei, impaired DSB repair following zeocin and camptothecin treatment, and reduced sister chromatid exchange. These cells are also more sensitive to irradiation and mitomycin C and display increased chromosomal abnormalities after mitomycin C treatment. Persistent RAD51 foci formation in USP10-depleted cells suggests that USP10 functions at a step downstream of RAD51 nucleofilament formation. This function of USP10 in facilitating HR repair depends on deubiquitinase activity but is independent of G3BP1/2 and PABP binding. In addition, a newly identified nucleolar localization signal is required for USP10s function in DSB repair. Together, these findings indicate that USP10 maintains genome integrity by localizing to the nucleolus and facilitating HR-mediated repair of DSBs.

Proper maintenance of gene expression in response to mutations or environmental fluctuations is critical for cell development and survival. Recently, a novel genetic compensation mechanism was described wherein mutant mRNA decay triggers increased transcription of paralogous genes. This effect was reported for several genes, including {beta}-actin (Actb) in mouse embryonic stem cells, where Actb mRNA with a premature termination codon enhances transcription of its paralog, {gamma}-actin (Actg1), and partially rescues cytoskeletal defects. Here we show that, in both mouse and human embryonic stem cells, mutations in the ACTB gene, regardless of mutant mRNA expression, trigger genetic compensation. Furthermore, transgenic expression of mutant ACTB mRNA with a premature stop codon fails to induce genetic compensation. Depletion of the SRF or MRTF-A transcription factors, which are known to increase ACTB transcription in response to low ACTB protein levels, diminishes the genetic compensation response in ACTB mutants. These results suggest that genetic compensation in ACTB mutants is primarily mediated by a transcriptional feedback loop via SRF/MRTF-A, independent of the expression or degradation of mutant ACTB mRNA.

How cells balance growth (cell size) and division (cell number) requires a complex interplay between response to external signals including growth factors, nutrient availability and metabolic cues, along with regulation of the cell cycle. The protein Tuberin (gene TSC2) is a critical regulator of these decisions. In a complex with the protein Hamartin, Tuberin functions as a negative regulator of the Target of Rapamycin (mTOR) pathway, preventing excessive growth under unfavorable conditions. However, how this growth pathway connects to decisions to progress through the G2 phase of the cell cycle and permit cell division is still unclear. In this study, we show that post-translational modification of Tuberin by the Extracellular Signal-Regulated Kinase (ERK) pathway abrogates binding between Tuberin and the mitotic cyclin, Cyclin B1. This causes an increase in mitotic cells, due to an unregulated G2/M transition, increasing the proliferation rate. Our work shows a novel role of Tuberin in cell cycle regulation by growth and mitogenic factors independent of mTOR regulation.

Function of CENPJ/CPAP is essential for centriole duplication and cilia biogenesis. Recently, we showed that CPAP is also an integral Endosomal Sorting Complexes Required for Transport (ESCRT)-0-like protein that recruits ESCRT-I protein TSG101 to early endosome (EE) and positively regulates multi-vesicular body (MVB) formation. Sequential recruitment of the ESCRT protein complexes and AAA+ ATPase VPS4B to EE facilitates MVB biogenesis. VPS4B is critical for ESCRT-III disassembly/recycling and contributes to membrane fission in several cellular processes. Here, we report that CPAP is critical for the protein stability and EE localization of VPS4B, and this function is independent from its role as an ESCRT-0. Other VPS4B-dependent cellular processes such as exosome release, cytokinesis, and retroviral budding are also compromised under CPAP deficiency. Interaction with CPAP prevents the proteasome degradation of VPS4B. The stability and EE localization of VPS4B can be attributed to two different C-terminal domains in CPAP. Overall, these observations provide evidence that CPAP is critical for VPS4B function and suggest that distinct pools of CPAP may be involved in its ESCRT-0 and VPS4B stabilization roles.

Intrinsically disordered regions (IDRs) of proteins are thought to be inherently sensitive to proteolysis and considered one of the key components constituting an efficient degron. Here we report that IDRs can also suppress protein degradation. Our recent study showed that yeast ribosomal proteins, while posttranslationally stable, are subject to cotranslational protein degradation (CTPD). In mapping the degron responsible for CTPD of ribosomal protein Rpl8A, we found that its N-terminal IDR suppresses CTPD, whereas the adjacent structured domain acts as a degron. We further assessed the N-terminal IDRs of 9 other yeast proteins and found that they all inhibit CTPD. These results suggest that suppression of CTPD is likely a generic function of N-terminal IDRs. Moreover, we showed that the N-terminal IDR of human ribosomal protein hRpl7A also functions as a stabilizer against CTPD in human cells. When transplanted to the N-terminus of cystic fibrosis transmembrane conductance regulator (CFTR), the N-terminal IDR of hRpl7A reduces CTPD of CFTR by more than 80%. Thus, the stabilizer function of N-terminal IDRs is conserved from yeast to human. Using mass spectrometry, we demonstrated that HSP70 chaperone proteins Ssa and Ssb bind to the N-terminal IDR of Rpl8A. These data suggest that N-terminal IDRs may inhibit CTPD through recruiting HSP70 chaperone proteins to nascent chains, thereby facilitating cotranslational folding. Our study unveils a new role for IDRs in suppressing CTPD.

The ISWI chromatin remodeler regulates nucleosome spacing using one of two ATPase subunits Snf2h (Smarca5) and Snf2l (Smarca1). While Snf2h stable knockout (KO) is known to markedly reduce genomic binding of CTCF, an architectural protein organizing the 3D genome, ISWIs role in regulating genomic binding and function of lineage-determining transcription factors (LDTFs) during cell fate transition remains largely unclear. Using conditional KO mice and derived cells, we show Snf2h and Snf2l are partially redundant and are required for embryonic development of muscle and adipose tissue as well as myogenesis and adipogenesis in culture. Stable KO of ISWI impairs LDTF-stimulated cell differentiation and disrupts de novo binding of the myogenic LDTF MyoD and the cBAF chromatin remodeler. Surprisingly, acute depletion of ISWI leaves de novo MyoD binding landscape largely intact while disrupting MyoD-dependent recruitment of cBAF and CTCF, with minimal effects on constitutive genomic binding of cBAF and CTCF. Together, our findings identify ISWI as an important mediator connecting LDTF binding to cBAF recruitment and chromatin organization during cell fate transition. Bullet points- ISWI ATPases Snf2h and Snf2l are partially redundant and essential for muscle and adipose development - ISWI is required for MyoD, C/EBP, and PPAR{gamma}-driven cell fate transition - Stable KO of ISWI disrupts genomic binding of MyoD, while acute depletion does not - Acute ISWI deletion disrupts MyoD-dependent, but not constitutive, genomic binding of cBAF and CTCF

Elevated DNA replication stress is a common feature of cancer cells, rendering them dependent on ATR/Chk1 signaling, which controls the DNA replication stress checkpoint, for survival. Although the activation of ATR/Chk1 signaling is well-established, how this signal is maintained until replication stress is fully resolved is less understood. Here, we investigated the roles of RAD9-HUS1-RAD1 interacting nuclear orphan 1 (RHNO1) in cancer progression and its involvement in maintaining ATR/Chk1 signaling during the DNA replication stress response. We demonstrate that depletion of RHNO1 significantly inhibits cancer cell proliferation in vitro and tumor growth in vivo. Mechanistically, we show that, while RHNO1 is dispensable for initial ATR/Chk1 signal activation, it is upregulated and stabilized following replication stress. RHNO1 is required to sustain ATR/Chk1 signaling, prevent premature checkpoint collapse, and suppress genomic instability. Under basal conditions, RHNO1 is rapidly degraded by the proteasome, however, RHNO1 is phosphorylated and stabilized following DNA replication stress. Stabilization of RHNO1 is mediated by ATR/Chk1 signaling which promotes RHNO1 localization to stressed replication forks marked by phosphorylated RPA32. Together, our data reveals a novel positive feedback loop wherein ATR/Chk1 signaling activation stabilizes RHNO1, which, in turn, is required to sustain the signal and thus the replication stress response. This work identifies RHNO1 as a key component in the cellular replication stress response and highlights its potential as a therapeutic target for tumor cells reliant on ATR/Chk1 signaling.

The nonsense-mediated decay pathway (NMD) is an RNA quality control mechanism that regulates the stability of target RNAs. We previously identified the ER-localized SEC13 protein as a novel NMD factor in C. elegans and in HeLa cells; raising the possibility that it could be involved in regulating the stability of mRNAs translated at the ER. SEC13 is a component of several cellular complexes, including the COPII vesicle coat, the nuclear pore complex (NPC) and the nutrient sensing GATOR2 complex. Here, we show that SEC13 interacts with core NMD factors and using a newly developed dual-color fluorescent NMD sensor in U2OS cells, we assessed SEC13 NMD activity, at a single-cell level. Transcriptomic profiling revealed that unlike the previously described ER-NMD factor, NBAS, SEC13 co-regulates the stability of substrates translated both in the cytoplasm and at the ER. We also show that SEC13 function in NMD is largely independent of its function in other cellular complexes. Altogether, these results show that SEC13 is a bona fide NMD factor in mammalian cells. Finally, we utilized an ER stress-activated indicator (ERAI) in U2OS cells to demonstrate that SEC13, together with canonical NMD factors, has a role in the regulation of the unfolded protein response (UPR) at the ER. Thus, the moonlighting functions of SEC13 include a role in NMD pathway and the regulation of ER stress.

Telomerase maintains chromosome ends by extending telomeric DNA, yet how recruited telomerase becomes productively engaged remains poorly understood. Recent studies showed that Replication Protein A (RPA) stimulates telomerase in humans and budding yeast through interactions with TERT and TPP1 orthologs, suggesting a direct role in activation. Here, we provide genetic and structural modeling evidence for a ternary RPA-Trt1TERT-Tpz1TPP1 complex that promotes telomere extension while suppressing recombination in fission yeast. Guided by results from genetic screen, followed by AlphaFold3 modeling and systematic mutagenesis of RPA, Trt1, and Tpz1, we identify four key interfaces supporting telomerase function: Ssb1RPA1-Trt1, Ssb2RPA2-Trt1, Ssb2RPA2-Tpz1, and the TEL-patch-mediated Trt1-Tpz1 interaction. Notably, Tpz1-R81, previously assigned as the TEL patch, instead contacts Ssb2 in the complex. Epistasis and suppressor analyses indicate distinct contributions of the RPA-Trt1 and RPA-Tpz1 interfaces to telomerase activation. Comparative analysis using AlphaFold3 further suggests that these interactions are likely conserved in budding yeast and humans. Together, these findings support a model in which RPA serves as an architectural component that coordinates TERT and TPP1-like factors to enable productive telomerase engagement. Author SummaryTelomeres are specialized structures at chromosome ends that must be maintained to preserve genome stability. Telomerase extends telomeric DNA, but how it becomes fully active after being recruited to telomeres remains poorly understood. In this study, we use fission yeast to examine the role of the conserved single-stranded DNA-binding protein Replication Protein A (RPA) in this process. We find that RPA forms a functional complex with the telomerase catalytic subunit (TERT) and the shelterin protein Tpz1 (a homolog of human TPP1). Genetic and structural analyses identify multiple interactions within this complex that are required for efficient telomere extension. Disrupting these interactions allows telomerase to be recruited but prevents productive telomere elongation. Our results also suggest that similar mechanisms may operate in other organisms, including budding yeast and humans. These findings provide insight into how telomerase activity is regulated at chromosome ends.

PUF60 is a splicing factor related to the polypyrimidine-tract binding protein U2AF2. PUF60 is deleted in developmental disorders such as Verheij syndrome and amplified in approximately 8% of cancers. Thus, both increases and decreases in PUF60 expression can have profound physiological effects. However, little is known about how changes in PUF60 expression impact global splicing patterns. Here, we created a model system of CRISPRa/i in mouse stem cells (mESCs) to transcriptionally upregulate or downregulate Puf60. Our results uncovered extensive transcriptional, post-transcriptional, and post-translational regulation of Puf60 protein expression. We observed that Puf60 protein levels in normal mESCs drop dramatically at a critical cell density, leading to cell death. Puf60 is very essential in stem cells, and its repression causes cell death and impacts specific splicing events, including its own splicing autoregulation, providing valuable insights into the functional consequences of PUF60 dysregulation. Analysis of phosphoprotein data revealed phosphorylation of threonine at the N-terminus of PUF60. Our results showed that mutating threonine to glutamate downregulates the protein and alters its localization. Thus, our study reveals a novel regulatory mechanism of Puf60 phosphorylation that mediates its function and may be related to its frequent overexpression in cancer cells.

Tumor progression is driven by cancer cell fitness, defined as the capacity of malignant cells to maintain growth, adapt to stress and withstand therapy Cellular fitness is fundamentally governed by nucleolar processes, which act as central regulators by integrating RNA processing with ribosome biogenesis to support protein synthesis and stress adaptation. The nuclear protein CYCLON, containing a large intrinsically disordered region (IDR) could be implicated in mediating biomolecular condensates and regulatory plasticity, which are key elements of nucleolar biology. CYCLON also emerged as a candidate regulator of cancer cell fitness, as it is frequently overexpressed across tumor types. Inducible silencing and cell biology approaches have shown that CYCLON maintains nucleolar integrity, controls nucleoli size and number, nucleolin and Ki-67 distribution and prevents nucleolar stress. CYCLON contributes to ribosome biogenesis by binding ribosomal RNA (rRNA) and regulating 32S and 21S pre-rRNA processing, ultimately influencing ribosomal subunit production and global protein synthesis. Its depletion impairs proliferation and clonogenic capacity by prolonging both interphase and mitosis, leading to slowed cell cycle progression. The impact of CYCLON on cellular fitness has been consistently observed across cancer models, reinforcing its essential role in the regulation of nucleolar biology.

Molecular chaperones of the Hsp70 family play essential roles in maintaining proteostasis, particularly under conditions of cellular stress. Posttranslational modifications of Hsp70, collectively termed the chaperone code, are emerging as critical regulators of chaperone function, yet their mechanistic contributions remain incompletely understood. Here, we investigate the functional significance of a conserved phosphorylation site in Hsp70, corresponding to serine 326 in yeast Ssa1 and serine 329 in human HSPA8. We demonstrate that DNA double-strand break stress increases cellular reliance on Hsp70 activity in yeast, highlighting a role for chaperones in the DNA damage response. Loss of Ssa1 serine 326 phosphorylation impairs multiple Hsp70-dependent functions, including prion sequestration and glucocorticoid receptor maturation, indicating that this modification is required for optimal chaperone activity. Extending these findings to human cells, we show that the homologous HSPA8 serine 329 residue is necessary for efficient clearance of polyglutamine aggregates. Mechanistically, a phospho-deficient HSPA8-S329A mutant exhibits reduced client-binding capacity. Together, our findings identify a conserved phosphorylation event that enhances Hsp70 function by promoting client engagement, providing new insight into how the chaperone code regulates proteostasis across species.

MicroRNAs (miRNAs) are small non-coding RNAs that play critical roles in regulating cellular senescence and aging. Our recent studies identified a conserved C. elegans miRNA cluster (miR-229/64/65/66) that is required for normal adult lifespan, with overexpression significantly extending longevity. Notably, cel-miR-229 is evolutionarily conserved in humans, with hsa-miR-425 sharing an identical seed sequence. Here, we investigated the role of miR-425 in mammalian cellular senescence. We found that miR-425 expression is markedly reduced in pharmacologically induced senescence in human lung cancer cells. Restoration of miR-425 expression attenuates senescence and suppresses the expression of senescence-associated secretory phenotype (SASP) cytokines following senescence induction. We further observed that miR-425 levels decline during replicative senescence, whereas stable overexpression in WI-38 fibroblasts delays senescence accumulation and preserves proliferative capacity. Mechanistically, miR-425 suppresses TGF-{beta} signaling, leading to reduced expression of the cyclin-dependent kinase inhibitor p21/CDKN1A and increased phosphorylation of the retinoblastoma (RB) protein, thereby promoting cell-cycle progression. We further identify PPP2CB, the catalytic subunit of protein phosphatase 2A (PP2A), as a direct target of miR-425. PPP2CB expression is downregulated in miR-425-5p overexpressing cells, even under senescence induction. Knockdown of PPP2CB using siRNA phenocopies the effects of miR-425 overexpression, reducing senescence, enhancing proliferative potential, and increasing RB phosphorylation. Collectively, our findings identify miR-425 as a conserved regulator of cellular senescence that acts through upregulation of RB phosphorylation. These results establish a novel miR-425-PPP2CB-RB regulatory axis controlling proliferation and senescence and suggest miR-425 as a potential therapeutic target for mitigating senescence to promote extended health span.

Cellular senescence is a tumor-suppressive program characterized by irreversible growth arrest; however, senescent cells can also promote inflammation and alter the tumor microenvironment through the senescence-associated secretory phenotype (SASP). Although SASP induction is regulated by pathways such as p38/NF-{kappa}B/I{kappa}B{zeta}, the mechanisms that restrain excessive or persistent SASP remain largely unknown. Here, we investigated the role of the Ets family transcription factor EHF in SASP regulation during cellular senescence. In IMR-90 human fibroblasts undergoing oncogene-induced senescence, EHF expression was upregulated after the onset of canonical senescence phenotypes. EHF knockdown did not substantially affect senescence establishment but increased SASP-related gene expression. Conversely, overexpression of full-length EHF suppressed SASP-related gene induction during senescence, whereas an ETS-domain-deficient EHF mutant failed to do so, suggesting that this EHF-mediated SASP suppression requires its DNA-binding domain. Furthermore, knockdown of NFKBIZ, which encodes I{kappa}B{zeta} and is induced downstream of NF-{kappa}B signaling, reduced EHF expression during senescence; however, NFKBIZ overexpression increased EHF and SASP-related gene expression. These results link EHF induction to the p38/NF-{kappa}B/I{kappa}B{zeta} inflammatory axis and support a model in which the inflammatory pathway that induces SASP also engages EHF as a negative regulator of SASP. Finally, conditioned medium from senescent cells promoted HCT116 cancer cell migration, and this activity showed a further increase after EHF knockdown. These findings suggest that EHF suppresses senescence-associated inflammatory responses and may function as a senomorphic effector that attenuates SASP-related inflammation without substantially affecting senescence establishment.

HSP90/R2TP is an essential quaternary chaperone composed of RPAP3, PIH1D1 and the RUVBL1/RUVBL2 AAA+ ATPases. These enzymes are also part of the INO80, SRCAP and TIP60 complexes, but the relationship between these chromatin remodelers and R2TP remains unclear. Here, we performed systematic analyses of the R2TP-specific subunits RPAP3 and PIH1D1. We validated 115 interaction partners and found that many were sensitive to HSP90 or R2TP inhibition. In yeast, epistatic screens revealed functional interactions with Ino80, Swr1 (SRCAP) and NuA4 (TIP60). Consistently, human RPAP3 physically interacted with subunits of INO80, SRCAP and TIP60 and was required for the formation of these complexes. More specifically, RPAP3 enabled the co-translational association of RUVBL1/RUVBL2 with the motor subunit of these chromatin remodelers. In vitro, the client-binding domain of RUVBL1/RUVBL2 modulated their interaction with RPAP3, suggesting that client subunits displace RPAP3 from nascent complexes. Thus, R2TP is an early chaperone of TIP60, SRCAP and INO80, which leaves RUVBL1/RUVBL2 as resident scaffolding subunits.

The inositol pyrophosphate 5-InsP7, composed of an inositol ring substituted with five monophosphates and one diphosphate, modulates diverse cellular functions by protein pyrophosphorylation, during which its {beta}-phosphate moiety is transferred to a pre-phosphorylated serine residue on the target protein. In mammals, the synthesis of 5-InsP7 from its precursor InsP6 is catalyzed by a family of enzymes called IP6Ks. We report that during recovery from genotoxic stress, cells lacking the IP6K isoform IP6K1 exhibit prolonged persistence of DNA damage foci marked by the homologous recombination repair protein RAD51. Expression of catalytically active but not inactive IP6K1 reverses this defect, implying that 5-InsP7 supports the dissolution of RAD51 foci. Upon DNA damage, we observe an increase in IP6K1 activity, contingent on its phosphorylation by the protein kinases CK2 and CDK1. IP6K1 is recruited to sites of DNA damage, and interacts with RAD51, CDK1, and the C-terminal domain (CTD) of BRCA2. Disruption of binding between RAD51 and BRCA2-CTD is known to support the disassembly of RAD51 foci. We show that 5-InsP7 can pyrophosphorylate RAD51, and that the presence of 5-InsP7 diminishes RAD51 binding to BRCA2-CTD. Our findings provide a mechanism by which 5-InsP7 synthesized by IP6K1 facilitates the removal of RAD51 from sites of DNA repair. Summary statementInositol hexakisphosphate kinase 1, an enzyme that catalyses the synthesis of the inositol pyrophosphate 5-InsP7, localises to DNA double strand breaks, and engages in interactions with proteins involved in homologous recombination (HR)-mediated DNA repair. 5-InsP7 disrupts the interaction between RAD51 and the C-terminus of BRCA2, promoting dislodgement of RAD51 from DNA damage sites post-repair.

The tight regulation of iron homeostasis is of great importance for cellular health. An increase in intracellular iron levels results in the formation of free radicals, which damages macromolecules and membranes, eventually resulting in cell death by Ferroptosis. Recently, we showed that patients with mutations in VPS41 display a severe neurodegenerative phenotype with iron deposition in the brain. VPS41 is well known as subunit of the HOPS complex required for fusion of late endosomes and autophagosomes with lysosomes. However, VPS41 has also been identified as inhibitor of Ferroptosis and regulator of redox homeostasis. How VPS41 exerts these functions and if these are dependent on the HOPS complex is unknown. Here we show that depletion of VPS41 results in increased intracellular iron levels, ROS formation and mitochondrial fission. Our findings indicate an important role for VPS41 in the regulation of iron homeostasis and mitochondrial fission and suggest Ferroptosis as a possible cause for neurodegeneration in VPS41 patients.

The mitochondrial membrane protein phosphoglycerate mutase 5 (PGAM5) is a protein of interest in the complex transition from hepatic steatosis to hepatocellular carcinoma. PGAM5 is a serine/threonine/histidine phosphatase that plays a role in mitochondrial biogenesis, mitophagy, and multiple cell death pathways. Increased expression of PGAM5 in hepatocellular carcinoma is correlated with reduced patient survival. In this study, we demonstrate that loss of PGAM5 alters the bioenergetic landscape of liver cancer by promoting mitochondrial oxidant injury and suppressing the glycerophospholipid and lysophospholipid pathways, leading to accumulation of the bioactive phospholipid lysophosphatidylcholine. Additionally, PGAM5 deletion downregulates fatty acid biosynthesis, resulting in reduced cellular diacylglycerol concentrations through two probable mechanisms: attenuated long chain fatty acid uptake and suppressed de novo synthesis. These findings underscore the broad impact of a single phosphatase on mitochondrial function and provide a rationale for therapeutically targeting PGAM5 to disrupt lipid metabolism in hepatocellular carcinoma.

Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) is characterized and initiated by the excessive accumulation of triacylglycerols (TG) and cholesteryl esters (CE) in the liver. Hepatic TG and CE synthesis, lipolysis and transport are tightly regulated by nutritional status, and disruption of this homeostasis contributes to MASLD pathogenesis. We have found that an endoplasmic reticulum-localized arylacetamide deacetylase (AADAC) catalyzes hepatic TG/CE turnover, and suppresses SREBP- and LXR-regulated lipogenesis and fatty acid esterification. Consequently, AADAC deficiency in mice leads to increased hepatic lipid synthesis, exacerbated steatosis, and impaired whole-body metabolism during Western-type diet feeding. These findings implicate AADAC as an important regulator of hepatic neutral lipid metabolism, linking endoplasmic reticulum cholesteryl ester hydrolysis as a modulator of lipid synthesis, and suggest its potential role in limiting MASLD pathogenesis under conditions of chronic overnutrition.