Apolipoprotein AV: gene expression,physiological role in lipid metabolism and clinical relevance

The apolipoprotein APOA5 gene, a member of the gene cluster on chromosome 11q23 that includes APOA1, APOC3 and APOA4, has gained considerable interest as it encodes ApoAV, a key determinant of circulating levels of potentially atherogenic triglyceride-rich lipoproteins (TRL). Indeed, strong associations between genetic variants of the APOA5 gene sequence and elevated triglyceride (TG) levels have been established. This apolipoprotein may potentiate lipolysis of TRL through facilitation of lipoprotein interaction with lipoprotein lipase. In addition, ApoAV may enhance clearance of remnant lipoproteins by mediating their interaction with the LDL receptor-related protein (LRP)1. The implication of ApoAV in intravascular TRL metabolism is further supported by studies that have demonstrated upregulation of APOA5 gene expression by nuclear receptors (PPARá, FXR and HNF4á) and hormones (thyroxine) involved in hypotriglyceridemic pathways. APOA4 expression may equally be modulated by nutritional status and, more specifically, by stimulation of lipogenesis through transcriptional regulation mediated by insulin and SREBP-1c. However, despite the fact that studies in mice have clearly revealed that plasma levels of ApoAV are inversely correlated with plasma TG levels, the relationship between ApoAV and metabolism of TRL remains controversial in man. Indeed, positive correlations between ApoAV and TG levels have recently been observed in patients with hypertriglyceridemia and Type 2 diabetes. The question as to whether ApoAV is a key determinant of TG levels in humans therefore remains conjectural.