Apolipoprotein A-I in glucose metabolism and amyloidosis
The role of Apolipoprotein A-I (ApoA-I), the main protein component of HDL, in cholesterol transport and metabolism is well known and has been studied for more than four decades. More recently, ApoA-I protein has been shown to also have a positive role in glucose control by both stimulation of glucose uptake by muscles and by increasing glucose-stimulated pancreatic insulin secretion.
Two of the four papers included in this thesis are focused on the role of ApoA-I in glucose control.
In paper I, we discovered that pre-incubation of beta cells and isolated murine islets with ApoA-I augmented glucose stimulated insulin secretion. To dissect the cellular mechanisms of action, we used a variety of functional and microscopic approaches. We concluded that ApoA-I’s positive action on beta cells involves ApoA-I internalization into beta cells, Pdx1 nuclear translocation, and increased levels of proinsulin processing enzymes. Altogether, these events lead to an increased number of insulin granules.
In paper II, we addressed the impact of hyperglycemia on the function of ApoA-I in glucose control. Prolonged hyperglycemia in poorly controlled diabetes leads to an increase in reactive glucose metabolites that covalently modify proteins, including ApoA-I, by non-enzymatic glycation reaction. To investigate the impact of ApoA-I glycation on its functionality, we chemically glycated ApoA-I with two different metabolites and performed structural and functional studies. We concluded that site-specific, covalent modifications of ApoA-I alter the protein structure, reduce the lipid-binding capability as well as the ability to catalyze cholesterol efflux from macrophages. Glycation modifications eliminated the ApoA-I stimulatory effect on the in vivo and in vitro glucose clearance. Altogether, it was concluded that glycation modification of ApoA-I impairs the ApoA-I protein functionality in lipid and glucose metabolism.
The two remaining papers included in this thesis are focused on another aspect of ApoA-I, its ability to aggregate in insoluble fibrils causing a disease known as ApoA-I related amyloidosis. So far, more than twenty known human amyloidogenic variants of the APOA1 gene have been found to lead to progressive accumulation of ApoA-I protein in vital organs, causing their dysfunction and failure. ApoA-I amyloidogenic mutations are associated with low ApoA-I and HDL-cholesterol plasma levels, however, subjects affected by ApoA-I-related amyloidosis do not show a higher risk of cardiovascular diseases.
In paper IV , we investigated the structural features, the lipid-binding properties and the functionality of four ApoA-I amyloidogenic variants. We found that these variants are characterized by a higher efficiency at catalyzing cholesterol efflux from macrophages. This finding can at least in part explain why the carriers of ApoA-I amyloidogenic variants do not have a higher risk of developing cardiovascular diseases despite lower levels of HDL-cholesterol. To further expand on these observations, in paper III, we examined the clinical plasma samples obtained from patients carrying two of the variants previously investigated in vitro and from matched control individuals. Patients displayed a unique HDL profile with a higher content of the smaller HDL particles was observed in samples from carriers as compared to controls. In line with previous observations, the HDL from the carriers had an improved cholesterol efflux capacity. Structural analysis revealed that ApoA-i variants in 8.4 nm HDL particles showed an increased protein dynamics in close proximity to the region of the mutations. This region-specific increased protein flexibility may contribute to improved functionality of the ApoA-I variants in catalyzing cholesterol efflux.