Transglutaminase 2 Stimulates Cell Proliferation and Modulates Transforming Growth Factor-Beta Signaling Pathway Independently of Epithelial–Mesenchymal Transition in Hepatocellular Carcinoma Cells

Transglutaminase 2 (TG2) is a multifunctional protein and plays a role in cancer progression. We previously identified TG2 as an early-recurrence biomarker in hepatocellular carcinoma (HCC). TG2-knockdown (shTG2) and control (shCtl) HCC cell lines were used for comparative analyses to clarify the molecular mechanisms underlying the contribution of this protein to HCC malignancy. The proliferation of shTG2 cells was slightly but significantly decreased compared with that of shCtl cells. Differential gene expression profiling based on GeneChip arrays revealed the enrichment of the PI3K-Akt signaling pathway and showed that the expression of Dickkopf-1 and -3 (DKK1 and DKK3, respectively), inhibitors and modulators of the Wnt/β-catenin signaling pathway, was increased in shTG2 cells. The expression of epithelial-mesenchymal transition (EMT)-related genes was similar in both shCtl and shTG2 cells before and after TGF-β1 treatment, even though TGF-β1 markedly upregulated TG2. Thus, TG2 may contribute to cancer malignancy via the stimulation of cell proliferation signaling, such as PI3K-Akt and Wnt/β-catenin signaling, but not EMT. This effect might be further enhanced by humoral factors such as TGF-β1 from the tumor microenvironment.