( 12) has shown that α 2-macroglobulin effectively suppresses the amyloid fibril formation of β2m. The details of plasma factors that suppress or inhibit the fibril formation are not completely understood. Fatty acids, lysophospholipids ( 1, 7, 8), glycosaminoglycan, and proteoglycan are among the factors that are known to promote the fibril formation of β2m ( 9– 11). Therefore, understanding the factors (especially from plasma) that modulate the amyloid fibril formation of β2m is important in the context of β2m amyloidosis. A critical balance of hydrophobic and hydrophilic interactions as modulated by environmental factors, for example anionic co-solutes, is important for the amyloid fibril formation of β2m ( 6). Interestingly, amyloid fibril deposition did not occur in transgenic mice expressing elevated levels of human β2m either alone or when injected with the fibril seed ( 5). Eventually, fibrillar deposition of β2m occurs in the joints of patients undergoing dialysis for extended periods of time and leads to acute inflammation and tissue destruction by infiltrating macrophages, a pathological condition termed dialysis-related amyloidosis ( 4). However, defective homeostasis due to failure of kidney function and its inability to flow through a dialysis membrane leads to its accumulation in the blood. Its turnover depends on the rate of its degradation in the kidney. Β 2-Microglobulin (β2m), 4 a component of the type I major histocompatibility complex, is present in low concentrations in circulating blood. Therefore, an understanding of factors in the extracellular environment that modulate amyloidogenicity of proteins is important and can be exploited for therapeutic development. Among such diseases, Alzheimer disease, prion disease, systemic amyloidosis, Finnish type amyloidosis, and dialysis-related amyloidosis involve extracellular amyloid deposition of the misfolded proteins/polypeptides that are characteristic of the disease ( 2, 3). Protein conformational diseases involve misfolding of proteins leading to either loss of crucial function of the protein or toxic gain of function of the misfolded species ( 1). Thus, our study demonstrates that haptoglobin is a potential extracellular chaperone for β 2-microglobulin even in moderately acidic conditions relevant during inflammation, with promising therapeutic implications in β 2-microglobulin amyloid-related diseases. This complex, like haptoglobin, interacts with the prefibrillar species of β 2-microglobulin, preventing its fibril formation and the associated cytotoxicity and resistance to intracellular degradation. Haptoglobin forms a 1:1 (mol/mol) complex with hemoglobin. Erythrolysis is known to release hemoglobin into the plasma. However, compromise in its chaperone activity under these conditions is more than compensated by its increased level of expression under inflammation. Our data show that haptoglobin prevents fibril formation of β 2-microglobulin under conditions of physiological acidosis (between pH 5.5 and 6.5) but with relatively decreased efficiency. Haptoglobin is an acute phase protein whose level increases severalfold during inflammation, where local acidosis can occur. 1,1′-Bis(4-anilino)naphthalene-5,5′-disulfonic acid cross-linking to haptoglobin significantly compromises its chaperone activity, suggesting the involvement of hydrophobic surfaces. We observed that haptoglobin interacts with oligomeric prefibrillar species of β 2-microglobulin but not with monomeric or fibrillar β 2-microglobulin that may underlie the molecular mechanism. However, the species of β 2-microglobulin populated in the presence of haptoglobin is susceptible to degradation. Fibrillar β 2-microglobulin is resistant to lysosomal degradation. Haptoglobin does not alleviate the cytotoxicity of preformed fibrils. The fibrils are cytotoxic, and haptoglobin abolishes the cytotoxicity by preventing fibril formation. We show that the plasma protein haptoglobin prevents the fatty acid-promoted de novo fibril formation of β 2-microglobulin even at substoichiometric concentration. Fibril formation of β 2-microglobulin and associated inflammation occur in patients on long term dialysis.
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