Transforming Growth Factor β Activation Primes Canonical Wnt Signaling Through Down-Regulation of Axin-2

Objective. Aberrant activation of Wnt signaling has been observed in tissues from patients with systemic sclerosis (SSc). This study aimed to determine the role of transforming growth factor beta (TGF beta) in driving the increased Wnt signaling, through modulation of axis inhibition protein 2 (Axin-2), a critical regulator of the Wnt canonical pathway. Methods. Canonical Wnt signaling activation was analyzed by TOPflash T cell factor/lymphoid enhancer factor promoter assays. Axin-2 was evaluated invitro by analysis of Axin-2 primary/mature transcript expression and decay, TGF beta receptor type I (TGF beta RI) blockade, small interfering RNA-mediated depletion of tristetraprolin 1, and XAV-939-mediated Axin-2 stabilization. In vivo, Axin-2 messenger RNA (mRNA) and protein expression was determined in skin and lung biopsy samples from mice that express a kinase-deficient TGF beta RII specifically on fibroblasts (T beta RII Delta k-fib-transgenic mice) and from littermate controls. Results. SSc fibroblasts displayed an increased response to canonical Wnt ligands despite basal levels of Wnt signaling that were comparable to those in healthy control fibroblasts invitro. Notably, we showed that SSc fibroblasts had reduced basal expression of Axin-2, which was caused by an endogenous TGF beta-dependent increase in Axin-2 mRNA decay. Accordingly, we observed that TGF beta decreased Axin-2 expression both invitro in healthy control fibroblasts and invivo in T beta RII Delta k-fib-transgenic mice. Additionally, using Axin-2 gain- and loss-of-function experiments, we demonstrated that the TGF beta-induced increased response to Wnt activation characteristic of SSc fibroblasts depended on reduced bioavailability of Axin-2. Conclusion. This study highlights the importance of reduced bioavailability of Axin-2 in mediating the increased canonical Wnt response observed in SSc fibroblasts. This novel mechanism extends our understanding of the processes involved in Wnt/beta-catenin-driven pathology and supports the rationale for targeting the TGF beta pathway to regulate the aberrant Wnt signaling observed during fibrosis.