Mechanisms for Relaxin’s Modulation of Fibrocartilage MMPs and Matrix Loss

Objectives: We have previously shown that the fibrocartilaginous TMJ disc responds to relaxin by upregulation of MMPs and loss of key matrix molecules. Here we determined the in vitro and in vivo cellular and molecular mechanisms of MMP regulation and associated matrix loss by relaxin and estrogen in mouse TMJ disc fibrocartilage.
Methods: Responses of immortalized fibroblastic and chondrocytic disc cell clones to estrogen and relaxin, and the receptors involved in the regulation of MMPs by these hormones were characterized. The effects of systemic hormone administration on MMP expression and matrix loss were determined in wild-type and MMP-9 null ovariectomized female mice.
Results: Fibroblastic clones expressed higher baseline levels of relaxin receptors, while chondrocytic clones had higher levels of estrogen receptors. Consequently, fibroblastic clones showed greater upregulation of MMP-9 and -13 and relaxin receptors by relaxin than chondrocytic cells, while the reverse was true for cellular responses to estrogen. This finding was validated in disc fibrocartilage in vivo. Relaxin’s induction of MMP-9 and -13 was mediated via its receptor RXFP1 but not RXFP2, while estrogen modulated the MMP expression via ESR-1 but not ESR-2. In vivo administration of relaxin and/or estrogen contributed to the upregulation of MMP-9 and -13 concomitant with the loss of matrices. Treatment of MMP-9 null mice with estrogen or relaxin resulted in loss of disc collagen and glycosaminoglycan to a similar extent as in WT mice. The hormone-mediated loss of matrices in the absence of MMP-9 may be explained by our observation that relaxin and estrogen cause significantly greater induction of MMP-13 in MMP-9 null than in WT mice.
Conclusions: Relaxin and estrogen cause matrix loss in TMJ disc by signaling via RXFP1 and ESR-1, respectively, and concomitant with the induction of their respective receptors as well as MMP-9 and -13. Relaxin-mediated tissue turnover occurs either by MMP-9 and -13 acting in concert or via mediators other than these MMPs.