In human CCC tissues and cell lines, El Khatib and colleagues[132] demonstrated that inhibition of Hedgehog signaling attenuates carcinogenesis in vitro and increases necrosis in CCC. Chen et al[133] showed that enhanced Hedgehog

signaling activity may be responsible for the invasion and Kinesin chemoresistance of hepatoma subpopulations. In a fibrosis-associated hepatocarcinogenesis model, Philips et al[134] further established that Hedgehog signaling pathway activation promotes hepatocarcinogenesis while inhibiting Hedgehog signaling safely reverses this process even in advanced HCC. TGF-β signaling pathway The TGF-β signaling pathway is involved in various cellular functions in both the developing embryo and the adult organism including cell growth, cell differentiation, apoptosis, and cellular

homeostasis. The pathway is activated upon binding of TGF-β to its receptors, TGF-β receptor I (TGFBR1) and TGFBR2, resulting in the translocation of Smad proteins to the nucleus where they act as transcription factors and participate in the regulation of target gene expression[135,136]. The role of TGF-β in tumors is rather complicated. In healthy tissue, it acts as a tumor suppressor controlling the cell cycle and inducing apoptosis. During carcinogenesis, TGF-β acts as a potent inducer of cell motility, invasion and metastasis. In liver cancer, TGF-β has been shown to have both tumor-promoting and tumor-suppressing effects, and its expression is decreased in early but increased in later stages of carcinogenesis. Although the underlying molecular mechanisms remain largely undefined, it had been speculated that the dual role of TGF-β signaling in liver cancer results from its effect on the tumor microenvironment[135,136]. It has long been known that TGF-β signaling

is vitally involved in stem cell renewal and lineage specification, including in LSCs[137]. Recently, TGF-β signaling has also been linked to the malignant transformation of LSCs in hepatocarcinogenesis. Nishimura et al[138] reported that TGF-β treatment increases the percentage of SP cells in a hepatoma cell line. Yuan et al[139] reported that HCC cells with aberrantly high expression of TGF-β signaling that are positive for Oct4 (octamer-binding transcription factor 4) are likely cancer progenitor cells with the potential to give rise to HCC. Entinostat Using several experimental approaches, Wu et al[140] confirmed that long-term treatment of oval cells with TGF-β impaired their LSC potential but granted them tumor-initiating cell (TIC) properties including the expression of TIC markers, increased self-renewal capacity, stronger chemoresistance, and tumorigenicity in nude mice. In opposition to these findings, however, Tang et al[141,142] showed that activation of the interleukin-6 (IL-6) signaling pathway induces neoplastic transformation of LSCs along with inactivation of the TGF-β signaling pathway.