Lipid mediators of inhalation exposure-induced pulmonary toxicity and inflammation

Many inhalation exposures induce pulmonary inflammation contributing to disease progression. Inflammatory processes are actively regulated via mediators including bioactive lipids. Bioactive lipids are potent signaling molecules involved in both pro-inflammatory and resolution processes through receptor interactions. The formation and clearance of lipid signaling mediators are controlled by multiple metabolic enzymes. An imbalance of these lipids can result in exacerbated and sustained inflammatory processes which may result in pulmonary damage and disease. Dysregulation of pulmonary bioactive lipids contribute to inflammation and pulmonary toxicity following exposures. For example, inhalation of cigarette smoke induces activation of pro-inflammatory bioactive lipids such as sphingolipids, and ceramides contributing to chronic obstructive pulmonary disease. Additionally, exposure to silver nanoparticles causes dysregulation of inflammatory resolution lipids. As inflammation is a common consequence resulting from inhaled exposures and a component of numerous diseases it represents a broadly applicable target for therapeutic intervention. With new appreciation for bioactive lipids, technological advances to reliably identify and quantify lipids have occurred. In this review, we will summarize, integrate, and discuss findings from recent studies investigating the impact of inhaled exposures on pro-inflammatory and resolution lipids within the lung and their contribution to disease. Throughout the review current knowledge gaps in our understanding of bioactive lipids and their contribution to pulmonary effects of inhaled exposures will be presented. New methods being employed to detect and quantify disruption of pulmonary lipid levels following inhalation exposures will be highlighted. Lastly, we will describe how lipid dysregulation could potentially be addressed by therapeutic strategies to address inflammation.

The Interaction between Intestinal ACSL5 Expression and Gut Microbiota in Mice Fed a High Fat Diet

Objectives

A major contributor to the development of obesity is the excessive intake of calories and fat. Our preliminary data indicate that a critical step for the absorption of dietary fat is the acylation of FFA to acyl-CoA by enterocyte acyl-CoA long-chain synthetase 5 (ACSL5). In previous unpublished studies we demonstrated that mice with intestinal specific deficiency of ACSL5 (ACSL5^int-/-) fed a high-fat diet (HFD) were protected against diet-induced obesity (DIO) secondary to reduced HFD intake. Since the composition of the intestinal microbiome has been demonstrated to modulate DIO and metabolism, the objective of this project is to explore the relative role of reduced HFD intake versus intestinal deficiency of ACSL5 on the composition of the host microbiome.

Methods

The intestinal deficiency of ACSL5 mouse model (ACSL5^int-/-) and floxed littermates ACSL5^loxP/loxP were fed either a low-fat diet (LFD, 4% fat) or a high-fat diet (HFD, 60% fat) for two weeks. Cecal contents were collected for 16S rDNA gene sequencing. For the pair-feeding study, one group of ACSL5^int-/- mice and one group of ACSL5^loxP/loxP mice received a HFD for 16 weeks ad libitum. At the same time, a second group of ACSL5^loxP/loxP mice was pair-fed with the same amount of food ingested by ACSL5^int-/- littermates.

Results

Our studies demonstrate that HFD fed ACSL5^int-/- mice possess a significantly distinct microbiome structure compared to ACSL5^loxP/loxP mice. Taxa analysis revealed that in 8 out of 11 ACSL5^int-/- mice fed a HFD levels of Akkermansia muciniphila were present while it was not detected in any of the 8 ACSL5^loxP/loxP mice. During pair-feeding, we observed that ACSL5^int-/- mice consumed 20% less energy when fed a HFD and were protected against DIO and insulin resistance.

Conclusions

Reduced food intake resulting from intestine-specific deficiency of ACSL5 protects against the development of HFD-induced obesity and is associated with an altered microbial structure that is consistent with an improved metabolic profile. Ongoing studies using metagenomic sequencing will provide specific information on taxonomy at the species level, expression of genes and functional characterization of the microbiome of mice fed varying levels of high fat intake and intestinal ACSL5 deficiency.

Funding Sources

USDA/ARS-8050-51,000-097-02S, 5P30DK046200-29, 5R01DK119337-02, Atkins Foundation.

Abstract 3744: Differences in lipid metabolism between non-metastatic and metastatic breast cancer cells

Breast cancer remains a serious public health concern, as it accounts for 30% of estimated new cancer cases among US women this year. Importantly, metastasis is the primary contributor to breast cancer fatality; therefore, preventing the progression of breast cancer is crucial. One emerging hallmark of cancer is the dysregulation of cellular energetics. Specifically, there is an association between breast cancer aggressiveness and neutral lipid accumulation in cytoplasmic lipid droplets (CLDs). Although this relationship was first identified decades ago, large gaps still exist regarding how breast cancer cells accumulate excess lipids, and how those lipids may contribute to breast cancer metastasis. We hypothesize that the neutral lipids stored in CLDs in metastatic cells can be utilized for energy production to drive migration, a critical step of cancer metastasis. To test this hypothesis, we compared triacylglycerol levels (triacylglycerol assay), CLD accumulation (transmission electron microscopy), mRNA levels of genes involved in lipid metabolism (qPCR), and cell migration (wound healing assay) between non-metastatic MCF10A-ras and metastatic MCF10CA1a human breast cancer cells. Both cell models are derived from the same parental mammary epithelial cell line. Consistent with previous literature, the metastatic cells had significantly more triacylglycerol (110-fold) and CLDs than the non-metastatic cells. In addition, the metastatic cell line had significantly higher mRNA abundance of de novo lipogenic enzymes compared to the non-metastatic cells, including a 3-fold increase in ATP citrate lyase expression (p=0.02) and a 2-fold increase in fatty acid synthase expression (p=0.04), suggesting greater lipid synthetic capability in the metastatic cells. Interestingly, cell migration of the metastatic cell line was reduced by 43% 24 hours after scratch and treatment with etomoxir, an irreversible inhibitor of carnitine palmitate-transferase I (p = 0.005 compared to vehicle control). In contrast, etomoxir had no effect on the rate of migration of the non-metastatic cells compared to vehicle control. Since etomoxir reduces fatty acid oxidation capacity, the results suggest that lipids stored in metastatic cells provide substrate for energy production needed to promote breast cancer cell migration. These results support the hypothesis that accumulation of CLDs in metastatic cells, potentially due to upregulated de novo lipogenesis, drive the progression of breast cancer by providing substrate for fatty acid oxidation to support the key step of the metastatic cascade, migration.

Citation Format: Chaylen Andolino, Josie Asher, Alyssa S. Zembroski, Kimberly Buhman, Dorothy Teegarden. Differences in lipid metabolism between non-metastatic and metastatic breast cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3744.

1α,25-dihydroxyvitamin D inhibits de novo fatty acid synthesis and lipid accumulation in metastatic breast cancer cells through down-regulation of pyruvate carboxylase

Both increased de novo fatty acid synthesis and higher neutral lipid accumulation are a common phenotype observed in aggressive breast cancer cells, making lipid metabolism a promising target for breast cancer prevention. In the present studies, we demonstrate a novel effect of the active metabolite of vitamin D, 1α,25-dihydroxyvitamin D (1,25(OH)₂D) on lipid metabolism in malignant breast epithelial cells. Treatment of MCF10CA1a breast epithelial cells with 1,25(OH)₂D (10 nM) for 5 and 7 days decreased the level of triacylglycerol, the most abundant form of neutral lipids, by 20%(±3.9) and 50%(±5.9), respectively. In addition, 1,25(OH)₂D treatment for 5 days decreased palmitate synthesis from glucose, the major fatty acid synthesized de novo (48%±5.5 relative to vehicle). We have further identified the anaplerotic enzyme pyruvate carboxylase (PC) as a target of 1,25(OH)₂D-mediated regulation and hypothesized that 1,25(OH)₂D regulates breast cancer cell lipid metabolism through inhibition of PC. PC mRNA expression was down-regulated with 1,25(OH)₂D treatment at 2 (73%±6 relative to vehicle) and 5 (56%±8 relative to vehicle) days. Decrease in mRNA abundance corresponded with a decrease in PC protein expression at 5 days of treatment (54%±12 relative to vehicle). Constitutive overexpression of PC in MCF10CA1a cells using a pCMV6-PC plasmid inhibited the effect of 1,25(OH)₂D on both TAG accumulation and de novo palmitate synthesis from glucose. Together, these studies demonstrate a novel mechanism through which 1,25(OH)₂D regulates lipid metabolism in malignant breast epithelial cells.

1α,25-Dihydroxyvitamin D Inhibits the Metastatic Capability of MCF10CA1a and MDA-MB-231 Cells in an In Vitro Model of Breast to Bone Metastasis

Breast cancer metastasis to the bone continues to be a major health problem, with approximately 80% of advanced breast cancer patients expected to develop bone metastasis. Although the problem of bone metastasis persists, current treatment options for metastatic cancer patients are limited. In this study, we investigated the preventive role of the active vitamin D metabolite, 1α,25-dihydroxyvitamin D (1,25(OH)2D), against the metastatic potential of breast cancer cells using a novel three-dimensional model (rMET) recapitulating multiple steps of the bone metastatic process. Treatment of MCF10CA1a and MDA-MB-231 cells inhibited metastasis in the rMET model by 70% (±5.7%) and 21% (±6%), respectively. In addition, 1,25(OH)2D treatment decreased invasiveness (20 ± 11% of vehicle) and decreased the capability of MCF10CA1a cells to survive in the reconstructed bone environment after successful invasion through the basement membrane (69 ± 5% of vehicle). An essential step in metastasis is epithelial-mesenchymal transition (EMT). Treatment of MCF10CA1a cells with 1,25(OH)2D increased gene (2.04 ± 0.28-fold increase) and protein (1.87 ± 0.20-fold increase) expression of E-cadherin. Additionally, 1,25(OH)2D treatment decreased N-cadherin gene expression (42 ± 8% decrease), a marker for EMT. Collectively, the present study suggests that 1,25(OH)2D inhibits breast cancer cell metastatic capability as well as inhibits EMT, an essential step in the metastatic process.

Intestinal lipoprotein assembly in apobec-1-/- mice reveals subtle alterations in triglyceride secretion coupled with a shift to larger lipoproteins

Mammalian enterocytes express apolipoprotein (apo)B-48, which is produced after posttranscriptional RNA editing of the nuclear apoB-100 transcript by the catalytic deaminase apobec-1. Earlier studies in apobec-1-/- mice revealed an apoB-100-only lipoprotein profile but no gross defects in triglyceride absorption. However, subtle defects may have been obscured by the mixed genetic background. In addition, the intrinsic susceptibility to proteolytic degradation of intestinal apoB-100 and apoB-48 has been questioned. Accordingly, we examined triglyceride absorption, intestinal apoB expression, and lipoprotein secretion in apobec-1-/- mice backcrossed into a C57BL/6 background. Inbred apobec-1-/- mice absorb triglyceride normally, yet secrete triglyceride-rich lipoproteins more slowly than wild-type congenic controls. There was comparable induction of apoB synthesis in response to fat feeding in both genotypes, but apoB-100 was preferentially retained and more extensively degraded than apoB-48. By contrast, synthesis, secretion, and content of apo A-IV were indistinguishable in apobec-1-/- and wild-type mice with 100% recovery, suggesting no degradation of this apoprotein in either genotype. Newly secreted lipoproteins from isolated enterocytes of wild-type mice revealed apoB-48 in both high-density lipoproteins and very low-density lipoproteins. By contrast, apobec-1-/- mice secreted apoB-100-containing particles that were almost exclusively in the low and very low-density lipoproteins range with no apoB-100-containing high-density lipoproteins. These studies establish the existence of preferential degradation of intestinal apoB-100 and subtle defects in triglyceride secretion in apobec-1-/- mice, coupled with a shift to the production of larger particles, findings that suggest an important divergence in intestinal lipoprotein assembly pathways with the different isoforms of apoB.

Identification of a novel Arg-->Cys mutation in the LDL receptor that contributes to spontaneous hypercholesterolemia in pigs

We previously carried out genetic and metabolic studies in a partially inbred herd of pigs carrying cholesterol-elevating mutations. Quantitative pedigree analysis indicated that apolipoprotein (apo)B and a second major gene were responsible for the hypercholesterolemia in these animals. In this study, we assessed LDL receptor function by three different methods: ligand blots of liver membranes using beta-very low density lipoprotein (VLDL) as a ligand; low density lipoprotein (LDL)-dependent proliferation of T-lymphocytes; and direct binding of 125I-labeled LDL to cultured skin fibroblasts. All three methods demonstrated that LDL receptor ligands bound with decreased affinity to the LDL receptor in these animals. In skin fibroblasts from the hypercholesterolemic pigs, the Kd of binding was about 4-fold higher than in cells from normal pigs. The cDNA of the pig LDL receptor from normal and hypercholesterolemic pigs was isolated and sequenced. We identified a missense mutation that results in an Arg'Cys substitution at the position corresponding to Arg94 of the human LDL receptor. The mutation is in the third repeat of the ligand binding domain of the receptor. By single-stranded conformational polymorphism (SSCP) analysis, we studied the relationship between LDL receptor genotype and plasma cholesterol phenotype. In contrast to humans, the hypercholesterolemia associated with the LDL receptor mutation in pigs was expressed as a recessive trait. The LDL receptor mutation made a far more significant contribution to hypercholesterolemia than did the apoB mutation, consistent with observations made in human subjects with apoB mutations. Within each genotypic group (mutated apoB or mutated receptor), there was a wide range in plasma cholesterol. As the animals were on a well-controlled low-fat diet, this suggests that there are additional genetic factors that influence the penetrance of cholesterol-elevating mutations.