The extracellular chaperone haptoglobin prevents serum fatty acid-promoted amyloid fibril formation of 2-microglobulin, resistance to lysosomal degradation, and cytotoxicity

Abstract

Fibril formation of β2-microglobulin and associated inflammation occur in patients on long term dialysis. We show that the plasma protein, haptoglobin prevents the fatty acids-promoted de novo fibril formation of β2-microglobulin even at sub-stoichiometric concentration. The fibrils are cytotoxic and haptoglobin abolishes the cytotoxicity by preventing fibril formation. Haptoglobin does not alleviate the cytotoxicity of preformed fibrils. Fibrillar β2-microglobulin is resistant to lysosomal degradation. However, the species of β2-microglobulin populated in the presence of haptoglobin is susceptible to degradation. We observed that haptoglobin interacts with oligomeric prefibrillar species of β2-microglobulin but not with monomeric or fibrillar β2-microglobulin which may underlie the molecular mechanism. Bis-ANS cross-linking to haptoglobin significantly compromises its chaperone activity, suggesting the involvement of hydrophobic surfaces. Haptoglobin is an acute phase protein whose level increases several-fold during inflammation where local acidosis can occur. Our data shows that haptoglobin prevents fibril formation of β2-microglobulin under conditions of physiological acidosis (between pH 5.5 and 6.5), but with relatively decreased efficiency. However, compromise in its chaperone activity under these conditions is more than compensated by its increased level of expression under inflammation. Erythrolysis is known to release hemoglobin into the plasma. Haptoglobin forms a 1:1 (M/M) complex with hemoglobin. This complex, like haptoglobin, interacts with the prefibrillar species of β2-microglobulin, preventing its fibril formation and the associated cytotoxicity and resistance to intracellular degradation. Thus, our study demonstrates that haptoglobin is a potential extracellular chaperone for β2-microglobulin even in moderately acidic conditions relevant during inflammation, with promising therapeutic implications in β2-microglobulin amyloid-related diseases.