ProMMP-13 is constitutively produced and endocytosed by chondrocytes. LRP1 is a key modulator of extracellular levels of proMMP-13 and MMP-13. ProMMP-13 and MMP-13 directly bind to LRP1 via the hemopexin domain. Unique sites on LRP1 for MMP-13 binding have been mapped. Co-endocytosis of proMMP-13 with ADAMTS-4, -5 and TIMP-3.