Hepatic alpha 2 mu-globulin localizes to the cytosol of rat proximal tubule cells.
BACKGROUND
Alpha 2 mu-Globulin (A2), an 18.6 kD protein of hepatic origin, accumulates in the proximal tubule as an abundant, 15.5 kD cleavage product termed "A2-fragment" (A2-f). A2-f facilitates proximal tubule fatty acid oxidation, presumably by binding hydrophobic ligands. This requires some A2-f to enter the cytosol of the renal epithelial cell (REC). The localization of A2/A2-f in the proximal tubule cell was evaluated in this study.


METHODS
Immunoblot analysis of renal cortical homogenates separated by differential centrifugation and quantitative immunoelectron microscopy (IEM) was performed to localize A2/A2-f using an affinity-purified antibody that detects both proteins. To evaluate A2 as a physiologically relevant ligand, the accumulation of A2-f in the female rat kidney (normally devoid of A2-f) was examined after the induction of hepatic A2 synthesis. Ligand binding, uptake, and degradation assays were used to assess A2 processing by RECs in vitro.


RESULTS
Although A2 and A2-f were detected in the "lysosomal" fraction, only A2-f was found in the soluble protein fraction. IEM confirmed the presence of significant signal in the vesicular and lysosomal as well as the cytosolic compartments. In contrast, both beta 2 mu globulin (B2) and cathepsin B were restricted to endosomes. In the female rat, induction of hepatic A2 production resulted in A2-f accumulation in the renal cortex. In RECs in culture, uptake of A2 and B2 demonstrated nonsaturable, nondisplacable surface binding and similar uptake rates. Compared with B2, A2 was markedly resistant to degradation.


CONCLUSIONS
A fraction of A2 escapes lysosomal degradation, permitting A2-f to accumulate in the cytosol of the proximal tubule epithelial cell. A2 may represent an unusual example of a physiologic protein capable of accumulating in a distant cell type.
Wang
Y
Y
Renal Section, Boston Medical Center, Massachusetts, USA.
Shia
M A
MA
Christensen
T G
TG
Borkan
S C
SC
D000510:Alpha-Globulins / Q000378:metabolism*; D000818:Animals; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism*; D004847:Epithelial Cells / Q000378:metabolism; D005260:Female; D007150:Immunohistochemistry; D007425:Intracellular Membranes / Q000378:metabolism; D007668:Kidney / Q000378:metabolism / Q000648:ultrastructure; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D016253:Microscopy, Immunoelectron; D009893:Opossums; D051381:Rats; D014018:Tissue Distribution; D001613:beta 2-Microglobulin / Q000378:metabolism
D000510:Alpha-Globulins; C013803:alpha 2u globulin; D001613:beta 2-Microglobulin