Assaying Low-Density-Lipoprotein (LDL) Uptake into Cells

Shaping immune landscape of colorectal cancer by cholesterol metabolites

Cancer immunotherapies have achieved unprecedented success in clinic, but they remain largely ineffective in some major types of cancer, such as colorectal cancer with microsatellite stability (MSS CRC). It is therefore important to study tumor microenvironment of resistant cancers for developing new intervention strategies. In this study, we identify a metabolic cue that determines the unique immune landscape of MSS CRC. Through secretion of distal cholesterol precursors, which directly activate RORγt, MSS CRC cells can polarize T cells toward Th17 cells that have well-characterized pro-tumor functions in colorectal cancer. Analysis of large human cancer cohorts revealed an asynchronous pattern of the cholesterol biosynthesis in MSS CRC, which is responsible for the abnormal accumulation of distal cholesterol precursors. Inhibiting the cholesterol biosynthesis enzyme Cyp51, by pharmacological or genetic interventions, reduced the levels of intratumoral distal cholesterol precursors and suppressed tumor progression through a Th17-modulation mechanism in preclinical MSS CRC models. Our study therefore reveals a novel mechanism of cancer-immune interaction and an intervention strategy for the difficult-to-treat MSS CRC.

Cold shock domain-containing protein E1 is a posttranscriptional regulator of the LDL receptor

The low-density lipoprotein receptor (LDLR) controls cellular delivery of cholesterol and clears LDL from the bloodstream, protecting against atherosclerotic heart disease, the leading cause of death in the United States. We therefore sought to identify regulators of the LDLR beyond the targets of current therapies and known causes of familial hypercholesterolemia. We found that cold shock domain-containing protein E1 (CSDE1) enhanced hepatic LDLR messenger RNA (mRNA) decay via its 3' untranslated region and regulated atherogenic lipoproteins in vivo. Using parallel phenotypic genome-wide CRISPR interference screens in a tissue culture model, we identified 40 specific regulators of the LDLR that were not previously identified by observational human genetic studies. Among these, we demonstrated that, in HepG2 cells, CSDE1 regulated the LDLR at least as strongly as statins and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors. In addition, we showed that hepatic gene silencing of Csde1 treated diet-induced dyslipidemia in mice to a similar degree as Pcsk9 silencing. These results suggest the therapeutic potential of targeting CSDE1 to manipulate the posttranscriptional regulation of the LDLR mRNA for the prevention of cardiovascular disease. Our approach of modeling a clinically relevant phenotype in a forward genetic screen, followed by mechanistic pharmacologic dissection and in vivo validation, may serve as a generalizable template for the identification of therapeutic targets in other human disease states.

Involvement of LDL and ox-LDL in Cancer Development and Its Therapeutical Potential

Lipid metabolism disorder is related to an increased risk of tumorigenesis and is involved in the rapid growth of cancer cells as well as the formation of metastatic lesions. Epidemiological studies have demonstrated that low-density lipoprotein (LDL) and oxidized low-density lipoprotein (ox-LDL) are closely associated with breast cancer, colorectal cancer, pancreatic cancer, and other malignancies, suggesting that LDL and ox-LDL play important roles during the occurrence and development of cancers. LDL can deliver cholesterol into cancer cells after binding to LDL receptor (LDLR). Activation of PI3K/Akt/mTOR signaling pathway induces transcription of the sterol regulatory element-binding proteins (SREBPs), which subsequently promotes cholesterol uptake and synthesis to meet the demand of cancer cells. Ox-LDL binds to the lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) and cluster of differentiation 36 (CD36) to induce mutations, resulting in inflammation, cell proliferation, and metastasis of cancer. Classic lipid-lowering drugs, statins, have been shown to reduce LDL levels in certain types of cancer. As LDL and ox-LDL play complicated roles in cancers, the potential therapeutic effect of targeting lipid metabolism in cancer therapy warrants more investigation.

Aqueous extracts of lingonberry and blackberry leaves identified by high-content screening beneficially act on cholesterol metabolism

Decreasing circulating low-density lipoprotein (LDL) cholesterol levels leads to decreased risk of cardiovascular diseases. Natural compounds are capable of lowering LDL-cholesterol even on top of lifestyle modification or medication. To identify novel plant-derived compounds to lower plasma LDL cholesterol levels, we performed high-content screening based on the transcriptional activation of the promoter of the LDL receptor (LDLR). The identified hits were thoroughly validated in human hepatic cell lines in terms of increasing LDLR mRNA and protein levels, lowering cellular cholesterol levels and increasing cellular LDL uptake. By means of this incremental validation process in vitro, aqueous extracts prepared from leaves of lingonberries (Vaccinium vitis-idaea) as well as blackberries (Rubus fruticosus) were found to have effects comparable to lovastatin, a prototypic cholesterol-lowering drug. When applied in vivo in mice, both extracts induced subtle increases in hepatic LDLR expression. In addition, a significant increase in high-density lipoprotein (HDL) cholesterol was observed. Taken together, aqueous extracts from lingonberry or blackberry leaves were identified and characterized as strong candidates to provide cardiovascular protection.

Design of 18 nm Doxorubicin-Loaded 3-Helix Micelles: Cellular Uptake and Cytotoxicity in Patient-Derived GBM6 Cells

The fate of nanocarrier materials at the cellular level constitutes a critical checkpoint in the development of effective nanomedicines, determining whether tissue level accumulation results in therapeutic benefit. The cytotoxicity and cell internalization of ∼18 nm 3-helix micelle (3HM) loaded with doxorubicin (DOX) were analyzed in patient-derived glioblastoma (GBM) cells in vitro. The half-maximal inhibitory concentration (IC₅₀) of 3HM-DOX increased to 6.2 μg/mL from <0.5 μg/mL for free DOX in patient-derived GBM6 cells, to 15.0 μg/mL from 6.5 μg/mL in U87MG cells, and to 21.5 μg/mL from ∼0.5 μg/mL in LN229 cells. Modeling analysis of previous 3HM biodistribution results predicts that these cytotoxic concentrations are achievable with intravenous injection in rodent GBM models. 3HM-DOX formulations were internalized intact and underwent intracellular trafficking distinct from free DOX. 3HM was quantified to have an internalization half-life of 12.6 h in GBM6 cells, significantly longer than that reported for some liposome and polymer systems. 3HM was found to traffic through active endocytic processes, with clathrin-mediated endocytosis being the most involved of the pathways studied. Inhibition studies suggest substantial involvement of receptor recognition in 3HM uptake. As the 3HM surface is PEG-ylated with no targeting functionalities, protein corona-cell surface interactions, such as the apolipoprotein-low-density lipoprotein receptor, are expected to initiate internalization. The present work gives insights into the cytotoxicity, pharmacodynamics, and cellular interactions of 3HM and 3HM-DOX relevant for ongoing preclinical studies. This work also contributes to efforts to develop predictive mathematical models tracking the accumulation and biodistribution kinetics at a systemic level.

Haploid genetic screens identify SPRING/C12ORF49 as a determinant of SREBP signaling and cholesterol metabolism

The sterol-regulatory element binding proteins (SREBP) are central transcriptional regulators of lipid metabolism. Using haploid genetic screens we identify the SREBP Regulating Gene (SPRING/C12ORF49) as a determinant of the SREBP pathway. SPRING is a glycosylated Golgi-resident membrane protein and its ablation in Hap1 cells, Hepa1-6 hepatoma cells, and primary murine hepatocytes reduces SREBP signaling. In mice, Spring deletion is embryonic lethal yet silencing of hepatic Spring expression also attenuates the SREBP response. Mechanistically, attenuated SREBP signaling in SPRING^KO cells results from reduced SREBP cleavage-activating protein (SCAP) and its mislocalization to the Golgi irrespective of the cellular sterol status. Consistent with limited functional SCAP in SPRING^KO cells, reintroducing SCAP restores SREBP-dependent signaling and function. Moreover, in line with the role of SREBP in tumor growth, a wide range of tumor cell lines display dependency on SPRING expression. In conclusion, we identify SPRING as a previously unrecognized modulator of SREBP signaling.

PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux

The absence of functional peripheral myelin protein 22 (PMP22) is associated with shortened lifespan in rodents and severe peripheral nerve myelin abnormalities in several species including humans. Schwann cells and nerves from PMP22 knock-out (KO) mice show deranged cholesterol distribution and aberrant lipid raft morphology, supporting an unrecognized role for PMP22 in cellular lipid metabolism. To examine the mechanisms underlying these abnormalities, we studied Schwann cells and nerves from male and female PMP22 KO mice. Whole-cell current-clamp recordings in cultured Schwann cells revealed increased membrane capacitance and decreased membrane resistance in the absence of PMP22, which was consistent with a reduction in membrane cholesterol. Nerves from PMP22-deficient mice contained abnormal lipid droplets, with both mRNA and protein levels of apolipoprotein E (apoE) and ATP-binding cassette transporter A1 (ABCA1) being highly upregulated. Despite the upregulation of ABCA1 and apoE, the absence of PMP22 resulted in reduced localization of the transporter to the cell membrane and diminished secretion of apoE. The absence of PMP22 also impaired ABCA1-mediated cholesterol efflux capacity. In nerves from ABCA1 KO mice, the expression of PMP22 was significantly elevated and the subcellular processing of the overproduced protein was aberrant. In wild-type samples, double immunolabeling identified overlapping distribution of PMP22 and ABCA1 at the Schwann cell plasma membrane and the two proteins were coimmunoprecipitated from Schwann cell and nerve lysates. Together, these results reveal a novel role for PMP22 in regulating lipid metabolism and cholesterol trafficking through functional interaction with the cholesterol efflux regulatory protein ABCA1.SIGNIFICANCE STATEMENT Understanding the subcellular events that underlie abnormal myelin formation in hereditary neuropathies is critical for advancing therapy development. Peripheral myelin protein 22 (PMP22) is an essential peripheral myelin protein because its genetic abnormalities account for ∼80% of hereditary neuropathies. Here, we demonstrate that in the absence of PMP22, the cellular and electrophysiological properties of the Schwann cells' plasma membrane are altered and cholesterol trafficking and lipid homeostasis are perturbed. The molecular mechanisms for these abnormalities involve a functional interplay among PMP22, cholesterol, apolipoprotein E, and the major cholesterol-efflux transporter protein ATP-binding cassette transporter A1 (ABCA1). These findings establish a critical role for PMP22 in the maintenance of cholesterol homeostasis in Schwann cells.