Regulation of the collagenase-3 receptor and its role in intracellular ligand processing in rat osteoblastic cells

We have previously described a specific, saturable receptor for rat collage nase-3 in the rat osteosarcoma cell line, UMR 106-01. Binding of rat collag enase-3 to this receptor is coupled to the internalization and eventual deg radation of the enzyme and correlates with observed extracellular levels of the enzyme. In this study we have shown that decreased binding, internaliz ation, and degradation of I-125-rat collagenase-3 were observed in cells af ter 24 h of parathyroid hormone treatment; these activities returned to con trol values after 48 h and were increased substantially (twice control leve ls) after 96 h of treatment with the hormone. Subcellular fractionation stu dies to identify the route of uptake and degradation of collagenase-3 local ized intracellular accumulation of I-125-rat collagenase-3 initially in Gol gi-associated lysosomes and later in secondary lysosomes. Maximal lysosomal accumulation of the radiolabel and stimulation of general lysosomal activi ty occurred after 72 h of parathyroid hormone treatment. Preventing fusion of endosomes with lysosomes (by temperature shift, colchicine, or monensin) resulted in no internalized I-125-collagenase-3 in either lysosomal fracti on. Treatment of UMR cells with the above,agents or ammonium chloride decre ased excretion of I-125-labeled degradation products of collagenase-3. Thes e experiments demonstrated that degradation of collagenase-3 required recep tor-mediated endocytosis and sequential processing by endosomes and lysosom es. Thus, parathyroid hormone regulates the expression and synthesis of col lagenase-3 as well as the abundance and functioning of the collagenase-3 re ceptor and the intracellular degradation of its ligand. The coordinate chan ges in the secretion of collagenase-3 and expression of the receptor determ ine the net abundance of the enzyme in the extracellular space. J Cell Phys iol 177:563-574, 1998. (C) 1998 Wiley-Liss, Inc.