Hepatic alpha(2 mu)-globulin localizes to the cytosol of rat proximal tubule cells

Background. alpha(2 mu)-Globulin (A(2)) an 18.6 kD protein of hepatic origi n, accumulates in the proximal tubule as an abundant, 15.5 kD cleavage prod uct termed "A(2)-fragment" (A(2)-f). A(2)-f facilitates proximal tubule fat ty acid oxidation, presumably by binding hydrophobic ligands. This requires some A(2)-f to enter the cytosol of the renal epithelial cell (REC). The l ocalization of A(2)/A(2)-f in the proximal tubule cell was evaluated in thi s study. Methods. Immunoblot analysis of renal cortical homogenates separated by dif ferential centrifugation and quantitative immunoelectron microscopy (IEM) w as performed to localize A(2)/A(2)-f using an affinity-purified antibody th at detects both proteins To evaluate A(2) as a physiologically relevant lig and, the accumulation of A(2)-f in the female rat kidney (normally devoid o f A(2)-f) was examined after the induction of hepatic A(2) synthesis. Ligan d binding, uptake, and degradation assays were used to assess A(2) processi ng by RECs in vitro. Results. Although A(2) and A(2)-f were detected in the "lysosomal" fraction , only A(2)-f was found in the soluble protein fraction. TEM confirmed the presence of significant signal in the vesicular and lysosomal as well as th e cytosolic compartments. In contrast, both beta(2 mu) globulin (B-2) and c athepsin B were restricted to endosomes. In the female rat, induction of he patic A(2) production resulted in A(2)-f accumulation in the renal cortex. In RECs in culture, uptake of A(2) and B-2 demonstrated nonsaturable, nondi splacable surface binding and similar uptake rates. Compared with B-2, A(2) was markedly resistant to degradation. Conclusions. A fraction of A(2) escapes lysosomal degradation, permitting A (2)-f to accumulate in the cytosol of the proximal tubule epithelial cell. A(2) may represent an unusual example of a physiologic protein capable of a ccumulating in a distant cell type.