Apolipoprotein A-1-derived amyloid in atherosclerotic plaques of the human aorta.

Previous studies have shown that the amyloid localized to the aortic intima may be a biochemical entity different from other forms of localized amyloid. The amyloid fibril protein in one patient studied consisted of an N-terminal fragment of apolipoprotein A-1 (apo A-1). Since this patient was later shown to carry a missense mutation in the apo A-1 gene, leading to a deletion at position 107 of the mature protein, the question remained whether wild-type apo A-1 is amyloidogenic. In autopsy specimens from the thoracic aorta from 69 individuals, intimal atherosclerotic plaque-related amyloid was present in 11 cases (16%) and amyloid outside plaques in 37 cases (54%). The immunoreactivity of amyloid localized to the aortic intima was evaluated with the aid of antisera against N-terminal segments of apo A-1. The amyloid in association with atherosclerotic plaques was positively labelled by immunohistochemistry. The amyloid fibril protein from one patient, previously shown not to carry any mutation in the apo A-1 gene, was purified and shown by amino acid sequence analysis to be of apo A-1 nature. The result shows that wild-type apo A-1 is amyloidogenic and gives rise to a common localized form of amyloid associated with atherosclerosis.

Apolipoprotein A-I-derived amyloid in atherosclerosis. Its association with plasma levels of apolipoprotein A-I and cholesterol.

Wild-type apolipoprotein A-I (apo A-I)-derived amyloid commonly occurs in atherosclerotic plaques. To clarify apo A-I amyloid formation, plasma levels of apo A-I and cholesterol were related to the presence of amyloid in atherosclerotic plaques in 15 patients with peripheral atherosclerosis, subjected to arterial reconstruction. Plasma levels of apo A-I and high-density lipoprotein (HDL) cholesterol were slightly higher in patients with apo A-I-derived amyloid than in those without, but the difference was not significant. Levels of low-density lipoprotein cholesterol and total cholesterol were significantly higher in the group with amyloid. High concentrations of apo A-I in the arterial intima are probably of greater importance to amyloid formation than high plasma levels of the protein. During atherosclerosis, the acute phase reactant serum amyloid A may displace apo A-I from HDL, leading to increased concentration of lipid-free apo A-I in the intima and conformational changes of apo A-I, which make it more fibrillogenic. Some forms of amyloid fibrils have been shown to be cytotoxic. Apo A-I-derived amyloid is possibly a pathogenically important factor in atherosclerosis.

Medin: an integral fragment of aortic smooth muscle cell-produced lactadherin forms the most common human amyloid.

Aortic medial amyloid is a form of localized amyloid that occurs in virtually all individuals older than 60 years. The importance and impact of the amyloid deposits are unknown. In this study we have purified a 5.5-kDa aortic medial amyloid component, by size-exclusion chromatography and RP-HPLC, from three individuals, and we have shown by amino acid sequence analysis that the amyloid is derived from an integral proteolytic fragment of lactadherin. Lactadherin is a 364-aa glycoprotein, previously known to be expressed by mammary epithelial cells as a cell surface protein and secreted as part of the milk fat globule membrane. The multidomain protein has a C-terminal domain showing homology to blood coagulation factors V and VIII. We found that the main constituent of aortic medial amyloid is a 50-aa-long peptide, here called medin, that is positioned within the coagulation factor-like domain of lactadherin. Our result is supported by the specific labeling of aortic medial amyloid in light and electron microscopy with two rabbit antisera raised against two synthetic peptides corresponding to different parts of medin. By using in situ hybridization we have shown that lactadherin is expressed by aortic medial smooth muscle cells. Furthermore, one of the synthetic peptides forms amyloid-like fibrils in vitro. Lactadherin was not previously known to be an amyloid precursor protein or to be expressed in aortic tissue. The structure of lactadherin may implicate an important regulatory function in the aorta.

Extensive intimal apolipoprotein A1-derived amyloid deposits in a patient with an apolipoprotein A1 mutation.

In the aortic intima amyloid deposits are often associated with atherosclerotic plaques. In a recent study of one patient with aortic intimal amyloid the major fibril protein was an N-terminal fragment of apolipoprotein A1 (apoA1) consisting of 69 amino acid residues. In the present study, we have screened the apoA1 gene for mutations in autopsy cases with aortic intimal amyloid immunohistochemically positive for apoA1, using single stranded conformational polymorphism (SSCP) analysis and DNA sequencing. All cases except one had a normal apoA1 gene sequence. One case of exceptionally severe atherosclerosis combined with extensive intimal amyloid deposits showed an apoA1 deletion corresponding to Lys 107. Thus, wild type apoA1 is amyloidogenic but our findings suggest that the expression of a mutant apoA1-form may be associated with enhanced amyloidogenicity.

Apolipoprotein A1-derived amyloid in human aortic atherosclerotic plaques.

Amyloid deposits in the aortic intima are very common in association with atherosclerosis and aging. In the present study, a major fibril protein purified from amyloid present in human atherosclerotic plaques was shown to be a 69-amino acid N-terminal fragment of apolipoprotein AI. Although senile form of localized apolipoprotein AI-derived amyloidosis has recently been documented in pulmonary vessels of dogs, this is the first example of a localized human amyloid derived from this apolipoprotein.

Senile aortic amyloid. Evidence for two distinct forms of localized deposits.

Aortic tissues obtained at autopsy were examined from 84 patients (age, 18-96 years). Amyloid deposits were present in the media in 61 of 63 (97%) of the patients above the age of 50. In addition, intimal amyloid deposits were present in 35% of this group. Intimal amyloid differed from medial amyloid both in its morphologic characteristics and its association with atherosclerosis. An antiserum raised to a low molecular weight protein extracted from amyloid fibrils of the aortic media reacted specifically with medial amyloid but did not react with intimal deposits. Neither type of amyloid reacted with anti-ATTR (Senile systemic amyloid), anti-AANF (isolated atrial amyloid), or antisera to other known forms of amyloid. These findings are consistent with the presence of two separate forms of localized amyloid in the aging aorta.