De novo synthesis, uptake and proteolytic processing of lipocalin‐type prostaglandin D synthase, β‐trace, in the kidneys

Lipocalin‐type prostaglandin D synthase (L‐PGDS) is a multifunctional protein that produces prostaglandin D 2 and binds and transports various lipophilic substances after secretion into various body fluids as β‐trace. L‐PGDS has been proposed to be a useful diagnostic marker for renal injury associated with diabetes or hypertension, because the urinary and plasma concentrations are increased in patients with these diseases. However, it remains unclear whether urinary L‐PGDS is synthesized de novo in the kidney or taken up from the blood circulation. In crude extracts of monkey kidney and human urine, we found L‐PGDS with its original N‐terminal sequence starting from Ala23 after the signal sequence, and also its N‐terminal‐truncated products starting from Gln31 and Phe34. In situ hybridization and immunohistochemical staining with monoclonal antibody 5C11, which recognized the amino‐terminal Ala23–Val28 loop of L‐PGDS, revealed that both the mRNA and the intact form of L‐PGDS were localized in the cells of Henle’s loop and the glomeruli of the kidney, indicating that L‐PGDS is synthesized de novo in these tissues. However, truncated forms of L‐PGDS were found in the lysosomes of tubular cells, as visualized by immunostaining with 10A5, another monoclonal antibody, which recognized the three‐turn α‐helix between Arg156 and Thr173. These results suggest that L‐PGDS is taken up by tubular cells and actively degraded within their lysosomes to produce the N‐terminal‐truncated form. Structured digital abstract MINT‐7266187 : L‐PGDS (uniprotkb: P41222 ) and Cathepsin D (uniprotkb: Q4R4P0 ) colocalize ( MI:0403 ) by fluorescence microscopy ( MI:0416 ) MINT‐7266176 : L‐PGDS (uniprotkb: P41222 ) and Cathepsin B (uniprotkb: Q4R5M2 ) colocalize ( MI:0403 ) by fluorescence microscopy ( MI:0416 )