SYNTHESIS AND FUNCTION OF A TRANSFORMATION-DEPENDENT SECRETED LYSOSOMAL PROTEASE

点击翻译按钮获取中文摘要

Cultured mouse fibroblasts which are malignantly transformed or treated with TPA or growth factors such as PDGF synthesize and secrete the 39,000 Mr precursor to cathepsin L in large amounts. This purified procathepsin L, originally called MEP for major excreted protein, contains mannose 6-phosphate, the lysosomal recognition marker. It is processed intracellularly in both transformed and nontransformed cells to give two specific lower molecular weight forms with a lysosomal localization. Overproduction of cloned cathepsin L in a nontransformed cell results in secretion of this lysosomal enzyme. Secreted procathepsin L can bind to the mannose 6-phosphate receptor of many cells and be endocytosed and processed intracellularly. In antigen presenting cells, procathepsin L uptake reduces the efficiency of antigen presentation, thereby interfering with immune response. Transformation, TPA and PDGF stimulate procathepsin L synthesis by increasing levels of procathepsin L specific mRNA transcription. We have cloned a functional procathepsin L gene from the mouse and have identified and sequenced the 5' flanking region which acts as a promoter in CAT constructions. A human procathepsin L cDNA has also been cloned and sequenced. Human procathepsin L mRNA has been detected in all normal human tissues analyzed with highest levels in liver, spleen, kidney, and many malignant tumors. Recombinant human procathepsin L produced in bacteria is enzymatically active.