Direct lysosomal uptake of alpha(2)-microglobulin contributes to chemically induced nephropathy

Background. An abnormal accumulation of alpha(2)-microglobulin (alpha 2 mu) in kidney lysosomes of male rats has been described in the nephropathy res ulting from exposure to a variety of chemicals. The increment in lysosomal levels of alpha 2 mu, cannot be explained by a decrease in its proteolytic susceptibility. Because a portion of alpha 2 mu resides in the cytosol of k idney cells, we decided to analyze whether this cytosolic form also contrib utes to the abnormal lysosomal accumulation of alpha 2 mu, after exposure t o chemicals. Methods. Intact kidney lysosomes were isolated from untreated or 2,2,4-trim ethylpentane (TMP) treated rats, and their ability to take up alpha 2 mu wa s compared, Results. alpha 2 mu can be directly transported into isolated lysosomes in the presence of the heat shock cognate protein of 73 kDa (hsc73). alpha 2 m u specifically binds to a lysosomal membrane glycoprotein of 96 kDa, previo usly identified as the receptor for the hsc73-mediated lysosomal pathway of protein degradation. In rats exposed to TMP, the specific lysosomal transp ort of alpha 2 mu increases, as well as the ability of lysosomes to directl y transport other substrates for this pathway. The increased lysosomal tran sport is mainly due to an increase in the levels of the receptor protein in the lysosomal membrane. Conclusions. The hsc73-mediated lysosomal pathway contributes to the normal degradation of alpha 2 mu, in rat kidney and liver, and the activity of th is pathway is increased after exposure to TMP. Our results suggest that the chemically induced accumulation of cytosolic alpha 2 mu in lysosomes is me diated by an increased rate of direct uptake into lysosomes.