TGF‐β Initiates β‐Catenin‐Mediated CTGF Secretory Pathway in Old Bovine Nucleus Pulposus Cells: A Potential Mechanism for Intervertebral Disc Degeneration

We have recently demonstrated that overexpression of Smurf2 under the control of type II collagen alpha 1 (Col2a1) promoter induces an intervertebral disc degeneration phenotype in Col2a1‐Smurf2 transgenic mice. The chondrocyte‐like cells that express type II collagen and Smurf2 in the transgenic mouse discs are prone to degenerate. However, how the chondrocyte‐like cells contribute to disc degeneration is not known. Here, we utilized primary old bovine nucleus pulposus (NP) cells as substitutes for the chondrocyte‐like cells in Col2a1‐Smurf2 transgenic mouse discs to identify mechanism. We found that 35% of the cells were senescent; TGF‐β treatment of the cells induced a rapid moderate accumulation of β‐catenin, which interacted with connective tissue growth factor (CTGF/CCN2) in the cytoplasm and recruited it to the membrane for secretion. The TGF‐β‐initiated β‐catenin‐mediated CTGF secretory cascade did not occur in primary young bovine NP cells; however, when Smurf2 was overexpressed in young bovine NP cells, the cells became senescent and allowed this cascade to occur. These results suggest that Smurf2‐induced disc degeneration in Col2a1‐Smurf2 transgenic mice occurs through activation of CTGF secretory pathway in senescent disc cells. © 2018 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.