Speed Limits for Nonvesicular Intracellular Sterol Transport.

Sterol transport between the endoplasmic reticulum (ER) and plasma membrane (PM) occurs by nonvesicular mechanisms requiring sterol transport proteins (STPs). Here we examine the idea that transport is enhanced at membrane contact sites where the ER is closely apposed to the PM. We conclude that sterol desorption from the membrane, rather than STP-mediated diffusion, is rate limiting in the cellular context, so there is no apparent kinetic benefit to having STP-mediated sterol transfer occur at contact sites. Contact sites may instead compartmentalize lipid synthesis or transport machinery, providing opportunities for regulation.

[Steroidogenic acute regulatory protein-related lipid transfer 4 (StarD4) promotes breast cancer cell proliferation and its mechanism].

Oncogene StarD4 had the function of promoting proliferation and metastasis of triple-negative breast cancer (TNBC), but its clinical value and molecular mechanism are unknown. This paper found that StarD4 was highly expressed in cancer tissues of TNBC patients, and higher expression level of StarD4 in TNBC patient resulted in poorer prognosis. Based on transcriptomics of MDA-MB-231 cell model, the results of bioinformatics analysis showed that down-regulated expression level of StarD4 led to overall downregulation of cholesterol-relative genes and significant enrichment of cancer mechanism and pathway. Further analysis and investigation verified that StarD4 might cross-promote the protein stability of receptor ITGA5 through the cholesterol pathway to enhance TNBC progression, which provides guidance for clinical application of TNBC diagnosis and treatment.

SREBP2-STARD4 is involved in synthesis of cholesteryl ester stimulated by mono-butyl phthalate in MLTC-1 cells.

Sterol is synthesized from cholesterol which is from the hydrolysis of stored cholesteryl esters. The process of maintaining cholesterol homeostasis is regulated by SREBP2-STARD4. Lots of researches demonstrated that male steroidogenesis could be interfered by di-n-butyl phthalate (DBP) or monobutyl phthalate (MBP). However, mechanisms of MBP exposure in this process have not been uncovered clearly. The objectiveof this study was to explore roles of SREBP2 and STARD4 in cholesteryl estersynthesis stimulated by MBP in mouse Leydig tumor cells (MLTC-1). MLTC-1 exposedto 10-8, 10-7, 10-6, 10-5 M MBP showed that levels of cholestery ester were increased significantly at 10-7 M MBP. Besides, cholesteryl ester synthesis stimulated by MBP was down-regulate when STARD4 or SREBP2 were inhibited. Activity of SREBP2 binding to the promoter of STARD4 was increased after MBP exposure. This study suggests that MBP can increase cholesteryl ester synthesis through SREBP2-STARD4 signal pathway in MLTC-1 cells.

Structural analysis of human sterol transfer protein STARD4.

The steroidogenic acute regulatory protein (StAR)-related lipid transfer domain-4 (STARD4) is a sterol-binding protein that is involved in cholesterol homeostasis by intracellular sterol transport. In this work, we determined the crystal structures of human STARD4 and its Ω1-loop mutant in apo forms at 1.95 and 1.7 Šresolutions, respectively. The structure of human STARD4 displays a conserved α-helix/ß-grip fold containing a deep hydrophobic pocket. The Ω1-loop which serves as a lid for the hydrophobic pocket has a closed conformation. The shape of the sterol-binding cavity in the closed form is not complementary to accommodate cholesterol, suggesting that a conformational change of the Ω1-loop is essential for sterol binding. The human STARD4 displayed sterol transfer activity between liposomes, and the mutations in the Ω1-loop and the hydrophobic wall abolished the transfer activity. This study confirms the structural conservation of the STARD4 subfamily proteins and the flexibility of the Ω1-loop and helix α4 required for sterol transport.

Role of STARD4 and NPC1 in intracellular sterol transport.

Cholesterol plays an important role in determining the biophysical properties of membranes in mammalian cells, and the concentration of cholesterol in membranes is tightly regulated. Cholesterol moves among membrane organelles by a combination of vesicular and nonvesicular transport pathways, but the details of these transport pathways are not well understood. In this review, we discuss the mechanisms for nonvesicular sterol transport with an emphasis on the role of STARD4, a small, soluble, cytoplasmic sterol transport protein. STARD4 can rapidly equilibrate sterol between membranes, especially membranes with anionic lipid headgroups. We also discuss the sterol transport in late endosomes and lysosomes, which is mediated by a soluble protein, NPC2, and a membrane protein, NPC1. Homozygous mutations in these proteins lead to a lysosomal lipid storage disorder, Niemann-Pick disease type C. Many of the disease-causing mutations in NPC1 are associated with degradation of the mutant NPC1 proteins in the endoplasmic reticulum. Several histone deacetylase inhibitors have been found to rescue the premature degradation of the mutant NPC1 proteins, and one of these is now in a small clinical trial.

Membrane specificity of the human cholesterol transfer protein STARD4.

STARD4 regulates cholesterol homeostasis by transferring cholesterol between the plasma membrane and endoplasmic reticulum. The STARD4 structure features a helix-grip fold surrounding a large hydrophobic cavity holding the sterol. Its access is controlled by a gate formed by the Ω1 and Ω4 loops and the C-terminal α-helix. Little is known about the mechanisms by which STARD4 binds to membranes and extracts/releases cholesterol. All available structures of STARD4 are without a bound sterol and display the same closed conformation of the gate. The cholesterol transfer activity of the mouse STARD4 is enhanced in the presence of anionic lipids, and in particular of phosphatidylinositol biphosphates (PIP2) for which two binding sites were proposed on the mouse STARD4 surface. Yet only one of these sites is conserved in human STARD4. We here report the results of a liposome microarray-based assay and microseconds-long molecular dynamics simulations of human STARD4 with complex lipid bilayers mimicking the composition of the donor and acceptor membranes. We show that the binding of apo form of human STARD4 is sensitive to the presence of PIP2 through two specific binding sites, one of which was not identified on mouse STARD4. We report two novel conformations of the gate in holo-STARD4: a yet-unobserved close conformation and an open conformation of Ω4 shedding light on the opening/closure mechanism needed for cholesterol uptake/release. Overall, the modulation of human STARD4 membrane-binding by lipid composition, and by the presence of the cargo supports the capacity of human STARD4 to achieve directed transfer between specific organelle membranes.

Bioinformatic analysis indicated that STARD4-AS1 might be a novel ferroptosis-related biomarker of oral squamous cell carcinoma.

Background: Oral squamous cell carcinoma (OSCC) stands as the predominant form of oral cancer, marked by a poor prognosis. Ferroptosis, a type of programmed cell death, plays a critical role in the initiation and progression of various cancers. Long non-coding RNAs (lncRNAs) are prominent in modulating cancer development. Nevertheless, the prognostic significance of ferroptosis-related lncRNAs (FRLs) in OSCC remains inadequately explored. This study aims to develop a predictive signature based on FRLs to forecast the prognosis of OSCC patients. Methods: We gathered expression profiles of FRLs along with clinical data from The Cancer Genome Atlas (TCGA) and FerrDb databases. A prognostic model based on 10 FRLs were constructed using Cox regression analyses with LASSO algorithms, and their predictive power was evaluated. Then, the model was used to investigate functional enrichment, immune landscape, m6A genes, somatic variations, and drug response in different risk cohorts of patients. Finally, the expression and function of STARD4-AS1 (steroidogenic acute regulator protein-related lipid transfer domain containing 4-antisense RNA 1), a potential prognostic marker for OSCC screening based on our bioinformatics analysis, were investigated in vitro. Results: We developed a signature comprising 10 FRLs to stratify patients into two risk cohorts according to their calculated risk scores. Patients classified as high-risk exhibited significantly poorer prognoses compared to those in the low-risk cohort. Furthermore, survival analysis, patient risk heat plot, and risk curve verified the accuracy of the signature. The role of this signature in OSCC was well investigated using immune microenvironment, mutational, and gene set enrichment analysis (GSEA). Moreover, seven drugs, including cisplatin and docetaxel, were identified as potential treatments for patients with high-risk cancers. In addition, the knockdown of STARD4-AS1 in OSCC cell lines markedly inhibited cell proliferation and migration and induced ferroptosis. Conclusion: Using this signature may improve overall survival predictions in OSCC, throwing new light on immunotherapies and targeted therapies. Moreover, STARD4-AS1 might regulate the process of ferroptosis and could be used as a novel biomarker of OSCC.

G protein-coupled receptor 35 attenuates nonalcoholic steatohepatitis by reprogramming cholesterol homeostasis in hepatocytes.

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease worldwide. Fat accumulation "sensitizes" the liver to insult and leads to nonalcoholic steatohepatitis (NASH). G protein-coupled receptor 35 (GPR35) is involved in metabolic stresses, but its role in NAFLD is unknown. We report that hepatocyte GPR35 mitigates NASH by regulating hepatic cholesterol homeostasis. Specifically, we found that GPR35 overexpression in hepatocytes protected against high-fat/cholesterol/fructose (HFCF) diet-induced steatohepatitis, whereas loss of GPR35 had the opposite effect. Administration of the GPR35 agonist kynurenic acid (Kyna) suppressed HFCF diet-induced steatohepatitis in mice. Kyna/GPR35 induced expression of StAR-related lipid transfer protein 4 (STARD4) through the ERK1/2 signaling pathway, ultimately resulting in hepatic cholesterol esterification and bile acid synthesis (BAS). The overexpression of STARD4 increased the expression of the BAS rate-limiting enzymes cytochrome P450 family 7 subfamily A member 1 (CYP7A1) and CYP8B1, promoting the conversion of cholesterol to bile acid. The protective effect induced by GPR35 overexpression in hepatocytes disappeared in hepatocyte STARD4-knockdown mice. STARD4 overexpression in hepatocytes reversed the aggravation of HFCF diet-induced steatohepatitis caused by the loss of GPR35 expression in hepatocytes in mice. Our findings indicate that the GPR35-STARD4 axis is a promising therapeutic target for NAFLD.

G protein-coupled receptor 35 attenuates nonalcoholic steatohepatitis by reprogramming cholesterol homeostasis in hepatocytes

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease worldwide. Fat accumulation "sensitizes" the liver to insult and leads to nonalcoholic steatohepatitis (NASH). G protein-coupled receptor 35 (GPR35) is involved in metabolic stresses, but its role in NAFLD is unknown. We report that hepatocyte GPR35 mitigates NASH by regulating hepatic cholesterol homeostasis. Specifically, we found that GPR35 overexpression in hepatocytes protected against high-fat/cholesterol/fructose (HFCF) diet-induced steatohepatitis, whereas loss of GPR35 had the opposite effect. Administration of the GPR35 agonist kynurenic acid (Kyna) suppressed HFCF diet-induced steatohepatitis in mice. Kyna/GPR35 induced expression of StAR-related lipid transfer protein 4 (STARD4) through the ERK1/2 signaling pathway, ultimately resulting in hepatic cholesterol esterification and bile acid synthesis (BAS). The overexpression of STARD4 increased the expression of the BAS rate-limiting enzymes cytochrome P450 family 7 subfamily A member 1 (CYP7A1) and CYP8B1, promoting the conversion of cholesterol to bile acid. The protective effect induced by GPR35 overexpression in hepatocytes disappeared in hepatocyte STARD4-knockdown mice. STARD4 overexpression in hepatocytes reversed the aggravation of HFCF diet-induced steatohepatitis caused by the loss of GPR35 expression in hepatocytes in mice. Our findings indicate that the GPR35-STARD4 axis is a promising therapeutic target for NAFLD.

Steroidogenic acute regulatory protein-related lipid transfer 4 (StarD4) promotes breast cancer cell proliferation and its mechanism / 生物医学工程学杂志

Oncogene StarD4 had the function of promoting proliferation and metastasis of triple-negative breast cancer (TNBC), but its clinical value and molecular mechanism are unknown. This paper found that StarD4 was highly expressed in cancer tissues of TNBC patients, and higher expression level of StarD4 in TNBC patient resulted in poorer prognosis. Based on transcriptomics of MDA-MB-231 cell model, the results of bioinformatics analysis showed that down-regulated expression level of StarD4 led to overall downregulation of cholesterol-relative genes and significant enrichment of cancer mechanism and pathway. Further analysis and investigation verified that StarD4 might cross-promote the protein stability of receptor ITGA5 through the cholesterol pathway to enhance TNBC progression, which provides guidance for clinical application of TNBC diagnosis and treatment.

The StarD4 subfamily of steroidogenic acute regulatory-related lipid transfer (START) domain proteins: new players in cholesterol metabolism.

Cholesterol levels in the body are maintained through the coordinated regulation of its uptake, synthesis, distribution, storage and efflux. However, the way cholesterol is sorted within cells remains poorly defined. The discovery of the newly described StarD4 subfamily, part of the steroidogenic acute regulatory lipid transfer (START) domain family of proteins, affords an opportunity for the study of intracellular cholesterol movement, metabolism and its disorders. The three members of this intracellular subfamily of proteins (StarD4, StarD5 and StarD6) have a similar lipid binding pocket specific for sterols (cholesterol in particular), but differing regulation and localization. The ability to bind and transport cholesterol through a non-vesicular mean suggests that they play a previously unappreciated role in cholesterol homeostasis.

Macrophage mitochondrial damage from StAR transport of 7-hydroperoxycholesterol: implications for oxidative stress-impaired reverse cholesterol transport.

StAR family proteins in vascular macrophages participate in reverse cholesterol transport (RCT). We hypothesize that under pathophysiological oxidative stress, StARs will transport not only cholesterol to macrophage mitochondria, but also pro-oxidant cholesterol hydroperoxides (7-OOHs), thereby impairing early-stage RCT. Upon stimulation with dibutyryl-cAMP, RAW264.7 macrophages exhibited a strong time-dependent induction of mitochondrial StarD1 and plasma membrane ABCA1, which exports cholesterol. 7α-OOH uptake by stimulated RAW cell mitochondria (like cholesterol uptake) was strongly reduced by StarD1 knockdown, consistent with StarD1 involvement. Upon uptake by mitochondria, 7α-OOH (but not redox-inactive 7α-OH) triggered lipid peroxidation and membrane depolarization while reducing ABCA1 upregulation. These findings provide strong initial support for our hypothesis.