Differential regulation of the signaling modules downstream of gp130 receptor in regeneration and disease

Degenerative joint diseases, such as osteoarthritis (OA), are a consequence of extracellular matrix (ECM) degradation attributed to chronic inflammation comprising a broad range of tissues including cartilage, endothelium, perivascular tissue and immune cells. Progression of OA leads to cartilage degradation, subchondral bone remodeling, hypertrophy of the joint capsule and osteophyte formation which are all a consequence of low-grade, chronic inflammation mediated by pro-inflammatory cytokines. No disease-modifying drugs are currently available for OA reinforcing the need for novel therapeutic targets and strategies that can regenerate the ECM and improve patient quality of life.

Interleukin 6 (IL-6) family of cytokines, which signal through gp130 (IL6ST) transmembrane receptor, have previously been shown to play a role in regulation of chronic inflammation involving the stroma and immune cell interaction, either promoting regeneration and homeostasis or aggravating the progression of the disease. Previous studies have identified downstream signaling pathways of gp130 that play a role in normal physiology and that also play a role in chronic inflammation, matrix destruction and oncogenesis via upregulation of kinases and pro-inflammatory transcription factors. Gp130 regulates these signaling cascades via recruitment of proteins on its various tyrosine and serine residues; however, most of these signaling residues have not been well-characterized.

The purpose of this study was to elucidate the role of gp130 signaling in the context of the joint and provide novel targets to show how pharmacological modulation, not inhibition, of this pathway results in amelioration of ECM breakdown in the joint during OA. First (Chapter 2), we identified a tyrosine 814 (Y814) residue within gp130 protein that represents an initiating factor in upregulation of pro-inflammatory SRC-NF-κB pathway. We have shown that genetic and pharmacological manipulation of Y814-gp130 molecular mechanism prevents ECM degeneration and progression of OA. Based on our data, Y814 serves as a pivotal novel therapeutic target providing a new way pro-inflammatory signaling can be regulated in a highly selective manner. Second (Chapter 3 and 4), we identified a novel small molecule regulator, RCGD 423, that is capable of modulating gp130 signaling towards anti-inflammatory and anti-degenerative outputs. RCGD 423 prevents degenerative changes in chondrocytes from human patients with OA, promotes formation of gp130 dimers via STAT3/MYC signaling, induces proliferation and migration of chondrocytes, outcompetes pro-inflammatory IL-6 cytokines for binding inhibiting the hypertrophic and catabolic effects of this pathway mediated by MAPK and NF-κB and prevents matrix breakdown in animal models of OA. This small molecule selectively shifts the output of this pathway to prevent cartilage degeneration. Subsequently (Chapter 5), an optimized analog of RCGD 423 was synthesized termed R805. This novel compound efficiently prevents activation of Y814 residue by IL-6 cytokines in cartilage and synovial cells; R805 also inhibits pro-degenerative effects mediated via NF-κB through SRC and MAPK signaling mechanisms downstream of Y814 while permitting pro-regenerative effects mediated via STAT3 signaling. Finally, we showed that R805 has a major therapeutic effect in a rat model of degenerative joint disease. All of these findings provide insight on potential targets and tools for development of novel pharmacological therapies and interventions for treatment of OA and other ECM degenerative disorders.