TGF-β signaling alters the pattern of liver tumorigenesis induced by Pten inactivation

Hepatocarcinogenesis results from the accumulation of genetic and epigenetic changes in liver cells. A common mechanism through which these alterations induce liver cancer is by deregulating signaling pathways. A number of signaling pathways, including the PI3K/PTEN/AKT and transforming growth factor β (TGF-β) pathways have been implicated in normal liver development as well as in cancer formation. In this study, we assessed the effect of the TGF-β signaling pathway on liver tumors induced by phosphatase and tensin homolog (Pten) loss. Inactivation of only the TGF-β receptor type II, Tgfbr2, in the mouse liver ( Tgfbr2 LKO ) had no overt phenotype, while inactivation of Pten alone ( Pten LKO ), resulted in the formation of both hepatocellular carcinomas and cholangiocarcinomas (CC). Interestingly, deletion of both Pten and Tgfbr2 ( Pten LKO ;Tgfbr2 LKO ) in the mouse liver resulted in a dramatic shift in tumor type to predominantly CC. Assessment of the PI3K/PTEN/AKT pathway revealed increased phosphorylation of AKT and glycogen synthase kinase 3 beta (GSK-3β) in both the Pten LKO and Pten LKO ;Tgfbr2 LKO mice, suggesting that this pathway is constitutively active regardless of the status of the TGF-β signaling pathway. However, phosphorylation of p70 S6 kinase was observed in the liver of all three phenotypes ( Tgfbr2 LKO , Pten LKO , Pten LKO ;Tgfbr2 LKO ) indicating that the loss of Tgfbr2 and/or Pten leads to an increase in this signaling pathway. Analysis of markers of liver progenitor/stem cells revealed that the loss of TGF-β signaling resulted in increased expression of c-Kit and CD133 . Furthermore, in addition to increased c-Kit and CD133 , Scf and EpCam expression were also increased in the double knock-out mice. These results suggest that the alteration in tumor types between the Pten LKO mice and Pten LKO ;Tgfbr2 LKO mice is secondary to the altered regulation of stem-cell features induced by the loss of TGF-β signaling.