Characteristics of lipid metabolism including serum apolipoprotein M levels in patients with primary nephrotic syndrome

BACKGROUND: Apolipoprotein M (apoM) is a 26-kD apolipoprotein that is mainly expressed in specific cell types, such as human liver parenchymal cells and kidney proximal renal tubular epithelial cells. ApoM can regulate the formation of pre-β-HDL and the reverse cholesterol transport and thus plays an important role in the metabolism of lipids and lipoproteins, meaning that it can affect the development of lipid metabolism disorders. Significantly elevated serum apoM levels are detected in patients with hyperlipidemia. However, few studies have shown how apoM is expressed in primary nephrotic syndrome (PNS), which is often accompanied with hyperlipidemia, and the underlying mechanism is poorly understood. This study was aimed at examining the apoM levels in patients with PNS and at determining the effects of PNS on serum apoM levels in these patients. METHODS: This study included patients with hyperlipidemia (n = 37), the PNS with hyperlipidemia group (n = 62), PNS without hyperlipidemia group (n = 33), and healthy controls (n = 73). The age and body–mass index (BMI) matched among the groups of participants. Their serum apoM concentrations were measured by an enzyme-linked immunosorbent assay. Serum levels of conventional lipids and renal function indices were assessed using an automatic biochemical analyzer. The data were analyzed by means of Pearson’s correlation coefficient (continuous variables) or Student’s t test (mean differences). RESULTS: The average serum apoM concentrations were higher in the hyperlipidemia group (61.1 ± 23.2 mg/L, P = 0.004) than in the healthy controls (31.6 ± 18.92 mg/L). The serum apoM concentrations were lower in the PNS with hyperlipidemia group (25.1 ± 16.31 mg/L, P = 0.007) and in the PNS without hyperlipidemia group (21.00 ± 17.62 mg/L, P = 0.003) than in the healthy controls. The serum apoM concentrations in the PNS with hyperlipidemia group did not differ significantly from those in the PNS without hyperlipidemia group (P = 0.083). Moreover, serum apoM levels positively correlated with serum high-density lipoprotein cholesterol (HDL-C) and apoA1 levels and negatively correlated with proteinuria in PNS patients (r = 0.458, P = 0.003; r = 0.254, P = 0.022; r = −0.414, P = 0.028). CONCLUSION: Serum apoM concentrations are higher in patients with hyperlipidemia than in healthy controls. Low serum apoM levels in patients with PNS are likely caused by PNS.