Impaired β-Oxidation and Altered Complex Lipid Fatty Acid Partitioning with Advancing CKD

Abstract

Studies of lipids in CKD, including ESRD, have been limited to measures of conventional lipid profiles. We aimed to systematically identify 17 different lipid classes and associate the abundance thereof with alterations in acylcarnitines, a metric of β-oxidation, across stages of CKD. From the Clinical Phenotyping Resource and Biobank Core (CPROBE) cohort of 1235 adults, we selected a panel of 214 participants: 36 with stage 1 or 2 CKD, 99 with stage 3 CKD, 61 with stage 4 CKD, and 18 with stage 5 CKD. Among participants, 110 were men (51.4%), 64 were black (29.9%), and 150 were white (70.1%), and the mean (SD) age was 60 (16) years old. We measured plasma lipids and acylcarnitines using liquid chromatography-mass spectrometry. Overall, we identified 330 different lipids across 17 different classes. Compared with earlier stages, stage 5 CKD associated with a higher abundance of saturated C16-C20 free fatty acids (FFAs) and long polyunsaturated complex lipids. Long-chain-to-intermediate-chain acylcarnitine ratio, a marker of efficiency of β-oxidation, exhibited a graded decrease from stage 2 to 5 CKD (P<0.001). Additionally, multiple linear regression revealed that the long-chain-to-intermediate-chain acylcarnitine ratio inversely associated with polyunsaturated long complex lipid subclasses and the C16-C20 FFAs but directly associated with short complex lipids with fewer double bonds. We conclude that increased abundance of saturated C16-C20 FFAs coupled with impaired β-oxidation of FFAs and inverse partitioning into complex lipids may be mechanisms underpinning lipid metabolism changes that typify advancing CKD.

Keywords: Acylcarnitines; Free fatty acids; chronic kidney disease; complex lipids; lipids.

Graphical abstract

Figure 1.

Various lipids identified by the LC/MS-based lipidomics platform. (A) Distribution of 17 classes of identified lipids in the cohort. (B) Alteration of relative mean values of FFAs by acyl chain and number of double bonds from stage 2 to 5 CKD. CerP, ceramide-phosphates; CL, cardiolipin; LPC, lysophosphatidylcholine; LPE, lysophosphatidylethanolamine; MAG, monoacylglycerol; PC, phosphatidylcholine; PG, phosphatidylglycerol; PI, phosphatidylinositol; pPC, plasmenyl-phosphatidylcholine; pPE, plasmenyl-phosphatidylethanolamine.

Figure 2.

Reciprocal significant alteration in abundance of FFAs and complex lipids by worsening stages of CKD: (A) TAGs carry three fatty acid acyl chains, whereas DAGs and PEs carry only two fatty acids. (B) Comparison of standardized mean values of TAGs, DAGs, PEs, and FFAs by carbon number and number of double bonds in stages 2–5 CKD. The significance of the interaction term of change in abundance of lipid by carbon number and number of double bonds from stage 2–5 CKD is shown by P values in the diagonal of each lipid class using mixed linear model.

Figure 3.

Impaired β-oxidation by advancing stages of CKD. (A) Illustration of the relationship between FFAs and acylcarnitines during mitochondrial β-oxidation pathway. (B) Comparing standardized mean values of plasma acylcarnitines by carbon number and number of double bonds by CKD stages. The P values refer to significance of alteration of plasma acylcarnitine level by carbon number and numbers of double-bond interaction terms at various stages of CKD. (C) Comparing the different length acylcarnitine ratios by stage of CKD. Values are shown as mean and SEMs.

Figure 4.

Increased abundance of polyunsaturated complex lipids with higher number of double bonds by advancing CKD. Estimated subclasses of complex lipids are defined on the basis of application of PCA data reduction from the list of the entire complex lipids within each lipid class. The significance of the interaction term was estimated using mixed linear model. CL, cardiolipin; pPC, plasmenyl-phosphatidylcholine; pPE, plasmenyl-phosphatidylethanolamine.

Figure 5.

Impaired β-oxidation is associated with higher abundance of polyunsaturated complex lipids with higher number of double bonds in TAG, DAG, and PE class; but is associated with a lower abundance of shorter complex lipids with lower number of double bonds in pPC, CE, SM, and PI class. Independent associates of long (C16–C20)-to-intermediate (C5–C14) acylcarnitine ratio among the high double-bond long and low double-bond short lipids. The subgroups of long and short complex lipids were defined according to unsupervised PCA. Multiple linear regression models showing the partial correlation coefficients (r) adjusted for age, sex, race, short-chain FFAs, and long-chain FFAs were used for the estimations. Decrease and increase in abundance are shown with the down and up arrows, respectively. AC, acylcarnitine; pPC, plasmenyl-phosphatidylcholine.

Figure 6.

Summary of proposed mechanisms of lipid alterations observed in CKD. Advanced CKD is characterized by (A) increased saturated FFA partially due to (B) poor dietary intake of the polyunsaturated fatty acid (PUFA) diet and increased synthesis and decreased catabolism of FFAs. Increased FFA leads to a flurry of adverse outcomes, including (C) autophagy, apoptosis, cell death, and eventually, CKD progression via (D) intermediary mechanisms, including endoplasmic reticulum (ER) stress. (E) Early in the course of disease, adaptive upregulation of β-oxidation tries to ameliorate the adverse effects by metabolizing FFAs faster. (F) Furthermore, mechanisms for utilization and compartmentalization of FFAs into complex lipids activate, which include stearoyl-CoA desaturase 1 (SCD1) and diacylglycerol O-acyltransferase 1 (DGAT1) upregulation, leading to TAG synthesis. (G) With further progression of CKD, impairment of β-oxidation prevails, which leads to further intracellular accumulation of FFAs, creating a vicious cycle for further injury. (H) Short complex lipids consisting of shorter FFAs with less dependence on mitochondrial function become preferred lipids for catabolism, leading to their low abundance in plasma. AMPK, adenosine monophosphate–activated protein kinase; MCP1, monocyte chemotactic protein 1; mTORC1, mammalian target of rapamycin complex 1; PKC, protein kinase C.