Effect of N-glycosylation on secretion, degradation and lipoprotein distribution of human serum amyloid A4.

Abstract

Serum amyloid A (SAA) is a family of apolipoproteins predominantly synthesized and secreted by the liver. Human SAA4 is constitutively expressed and contains an N-glycosylation site that is not present in other SAA subtypes. SAA4 proteins are not fully glycosylated, resulting in the presence of both glycosylated and non-glycosylated forms in human plasma. The efficiency of N-glycosylation in SAA4 is known to be influenced by some reasons such as genetic polymorphism and metabolic disorders. However, the specific role of N-glycosylation in SAA4 remains largely unexplored. This study aimed to investigate how N-glycosylation affects the secretion, degradation, and lipoprotein distribution of SAA4. Initially, we designed and constructed an SAA4 plasmid vector to compare with the expression pattern of endogenous SAA4. The exogenous SAA4 was partially N-glycosylated, analogous to endogenous SAA4 in human hepatocellular carcinoma cells. Subsequently, we created a non-glycosylated mutant by replacing asparagine 76 with glutamine. Immunoblotting assays showed that the disruption of N-glycans did not affect the secretion and degradation of SAA4. Furthermore, we analyzed the lipoprotein profiles of SAA4 in the conditioned medium derived from transfected cells. The results revealed that non-glycosylated mutant SAA4 exhibited a distinct lipoprotein distribution compared to wild-type SAA4. Our findings suggest that N-glycosylation may be a key regulator of the distribution of SAA4 in lipoproteins, shedding light on the previously unknown physiological activities of human SAA4.