IκB is a substrate for a selective pathway of lysosomal proteolysis

In lysosomes isolated from rat liver and spleen, a percentage of the intracellular inhibitor of the nuclear factor kappa B (I kappa B) can be detected in the lysosomal matrix where it is rapidly degraded. Levels of I kappa B are significantly higher in a lysosomal subpopulation that is active in the direct uptake of specific cytosolic proteins. I kappa B is directly transported into isolated lysosomes in a process that requires binding of I kappa B to the heat shock protein of 73 kDa (hsc73), the cytosolic molecular chaperone involved in this pathway, and to the lysosomal glycoprotein of 96 kDa (lgp96), the receptor protein in the lysosomal membrane. Other substrates for this degradation pathway competitively inhibit I kappa B uptake by lysosomes. Ubiquitination and phosphorylation of I kappa B are not required for its targeting to lysosomes. The lysosomal degradation of I kappa B is activated under conditions of nutrient deprivation. Thus, the half-life of a long-lived pool of I kappa B is 4.4 d in serum-supplemented Chinese hamster ovary cells but only 0.9 d in serum-deprived Chinese hamster ovary cells. This increase in I kappa B degradation can be completely blocked by lysosomal inhibitors. In Chinese hamster ovary cells exhibiting an increased activity of the hsc73-mediated lysosomal degradation pathway due to overexpression of lamp2, the human form of lgp96, the degradation of I kappa B is increased. There are both short- and long-lived pools of I kappa B, and it is the long-lived pool that is subjected to the selective lysosomal degradation pathway. In the presence of antioxidants, the half-life of the long-lived pool of I kappa B is significantly increased. Thus, the production of intracellular reactive oxygen species during serum starvation may be one of the mechanisms mediating I kappa B degradation in lysosomes. This selective pathway of lysosomal degradation of I kappa B is physiologically important since prolonged serum deprivation results in an increase in the nuclear activity of nuclear factor kappa B. In addition, the response of nuclear factor kappa B to several stimuli increases when this lysosomal pathway of proteolysis is activated.