The role of Wnt catenin signaling

The role of Wnt/β-catenin signaling in reprogramming has also been investigated. Exogenously introduced WNT3A enhances fibroblast reprogramming in the absence of c-Myc (Marson et al., 2008). Knockdown or knockout of T cell factors or treatments with several drugs that control the Wnt pathway can change the reprogramming efficiency (Aulicino et al., 2014; Ho et al., 2013; Lluis et al., 2011; Ross et al., 2014; Zhang et al., 2014). However, it remains controversial whether endogenous Wnt/β-catenin signaling has a stimulatory or inhibitory effect on reprogramming. Furthermore, the dynamics and role of endogenous Wnt ligands or β-catenin in reprogramming remain largely unanswered.
Results
Discussion β-Catenin nuclear accumulation decreased after day 6 while Wnt2 expression still remained high until day 10. There may be two possibilities to explain this time difference between Wnt2 downregulation and nuclear β-catenin diminution. (1) β-Catenin can function as a scaffolding protein for E-cadherin and the binding of E-cadherin to β-catenin prevents β-catenin nuclear localization (Orsulic et al., 1999). It can be speculated that while WNT2 promotes the stabilization of β-catenin in the early to middle stage of reprogramming, E-cadherin binds to β-catenin and prevents β-catenin nuclear localization in the middle stage of reprogramming (days 8–10). (2) Wnt/β-catenin signaling induces the transcription of Axin2, which promotes phosphorylation and degradation of β-catenin (Behrens et al., 1998; Ikeda et al., 1998; Jho et al., 2002). During reprogramming this negative feedback could occur, and thus β-catenin may be degraded in the middle phase of reprogramming, although Wnt2 is still expressed. Our results showed that Wnt2 was upregulated in the early stage of reprogramming and expressed mainly in THY1-positive cells. A transition from a THY1-positive state to a THY1-negative state also occurred in the early stage. WNT2 may act on THY1-positive prolyl hydroxylase inhibitor to change them to THY1-negative cells. Alternatively, it is possible that WNT2 secreted from THY1-positive cells supports the reprogramming of THY1-negative cells. Whether WNT2 acts on THY1-positive or -negative cells in an autocrine or paracrine manner should be examined in future studies. WNT2 promotes metastasis in several cancers (Fu et al., 2011; Jiang et al., 2014; Pu et al., 2009). Epithelial-to-mesenchymal transition takes place in the initiation of metastasis. On the other hand, mesenchymal-to-epithelial transition (MET) occurs in the early stage of fibroblast reprogramming (Li et al., 2010; Samavarchi-Tehrani et al., 2010). Thus, prolonged activation of WNT2/β-catenin signaling might result in the inhibition of MET during reprogramming. In fact, our results showed that the expression of Axin2 and CyclinD1 remained high in THY1-positive cells during the middle to late phase. It would be important that WNT2/β-catenin signaling is “on” in the early stage and switches to “off” in later stages. Several studies have investigated the involvement of Wnt signaling in different stages of reprogramming. However, their results have been controversial (Aulicino et al., 2014; Ho et al., 2013; Ross et al., 2014; Zhang et al., 2014). The reprogramming-inducing systems (retroviral infection or doxycycline-inducible system; OKSM or OKS) or donor cell types (fibroblasts or neural stem cells) differ among these studies. In ESCs, the Wnt/β-catenin signaling pathway contributes to the maintenance of pluripotency, and on the contrary plays a critical role in early development (Miki et al., 2011), implying the context-dependent role of Wnt/β-catenin signaling. In cell-fusion-mediated reprogramming, the level of Wnt/β-catenin signaling activity is critical for successful reprogramming; very high or very low activities have an inhibitory effect on reprogramming (Lluis et al., 2008). Therefore, one hypothesis for the opposite effects of endogenous Wnt/β-catenin signaling on reprogramming is that different reprogramming systems may cause different Wnt/β-catenin activity levels. The changes in Wnt/β-catenin activity levels in different reprogramming systems should be investigated in future studies.