The Extracellular Chaperone Haptoglobin Prevents Serum Fatty Acid-promoted Amyloid Fibril Formation of β2-Microglobulin, Resistance to Lysosomal Degradation, and Cytotoxicity

Fibril formation of (2)-microglobulin and associated inflammation occur in patients on long term dialysis. We show that the plasma protein haptoglobin prevents the fatty acid-promoted de novo fibril formation of (2)-microglobulin even at substoichiometric concentration. The fibrils are cytotoxic, and haptoglobin abolishes the cytotoxicity by preventing fibril formation. Haptoglobin does not alleviate the cytotoxicity of preformed fibrils. Fibrillar (2)-microglobulin is resistant to lysosomal degradation. However, the species of (2)-microglobulin populated in the presence of haptoglobin is susceptible to degradation. We observed that haptoglobin interacts with oligomeric prefibrillar species of (2)-microglobulin but not with monomeric or fibrillar (2)-microglobulin that may underlie the molecular mechanism. 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. Haptoglobin is an acute phase protein whose level increases severalfold during inflammation, where local acidosis can occur. Our data show that haptoglobin prevents fibril formation of (2)-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 (mol/mol) complex with hemoglobin. This complex, like haptoglobin, interacts with the prefibrillar species of (2)-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 (2)-microglobulin even in moderately acidic conditions relevant during inflammation, with promising therapeutic implications in (2)-microglobulin amyloid-related diseases.