Direct lysosomal uptake of α₂-microglobulin contributes to chemically induced nephropathy

Background. An abnormal accumulation of α₂-microglobulin (α2μ) in kidney lysosomes of male rats has been described in the nephropathy resulting from exposure to a variety of chemicals. The increment in lysosomal levels of α2μ cannot be explained by a decrease in its proteolytic susceptibility. Because a portion of α2μ resides in the cytosol of kidney cells, we decided to analyze whether this cytosolic form also contributes to the abnormal lysosomal accumulation of α2μ after exposure to chemicals. Methods. Intact kidney lysosomes were isolated from untreated or 2,2,4-trimethylpentane (TMP) treated rats, and their ability to take up α2μ was compared. Results. α2μ can be directly transported into isolated lysosomes in the presence of the heat shock cognate protein of 73 kDa (hsc73). α2μ specifically binds to a lysosomal membrane glycoprotein of 96 kDa, previously identified as the receptor for the hsc73-mediated lysosomal pathway of protein degradation. In rats exposed to TMP, the specific lysosomal transport of α2μ increases, as well as the ability of lysosomes to directly transport other substrates for this pathway The increased lysosomal transport 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 α2μ in rat kidney and liver, and the activity of this pathway is increased after exposure to TMP. Our results suggest that the chemically induced accumulation of cytosolic α2μ in lysosomes is mediated by an increased rate of direct uptake into lysosomes.