Enhanced activity of lysosomal proteases in activated rat hepatic stellate cells is associated with a concomitant increase in the number of the mannose-6-phosphate/insulin-like growth factor II receptor

Activated hepatic stellate cells (HSCs) play a central role during hepatic tissue repair through their influence on extracellular matrix remodelling. We have determined whether the activity levels of cathepsin B and D are affected by in vitro activation of rat HSCs, and whether the enzymes were released from the cells. Furthermore, given the important role of the mannose-6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR) in the intracellular transport of lysosomal enzymes, we have examined whether changes in the activity of these proteases were associated with parallel changes in the level of the M6P/IGF-IIR. The activity of cathepsin B and D increased ∼4 times between 2 and 8 days of HSC culture. This result was supported by analysing mRNA expression by RT-PCR. The cells released the enzymes into the culture medium, amounting to ∼10% of the cell-associated activity over 24 h. The release of enzymes was not affected by reducing medium pH from 7.4 to 6.2, indicating that the enzymes were transported to the medium independently of the M6P/IGF-II-R. The released cathepsin B was mostly in the inactive proenzyme form. HSC activation led to a particularly large increase in M6P/IGF-IIR expression. A large proportion of the receptors was located on the cell surface and was found to be very suitable for measuring endocytosis of 125I-IGF-II. The results show that the endocytic activity increased in parallel with the increase in surface receptors and activity of lysosomal enzymes. Degradation of the ligand was reduced by inhibitors of lysosomal proteases and therefore took place in lysosomes.