**6. HCC tumor microenvironment**

42 Hepatocellular Carcinoma – Basic Research

to reduce cellular retinoblastoma stability (Li et al., 2005). In addition, gankyrin increases both the association and activity of MDM2 for p53. This inactivates p53 by increasing the ubiquitylation level of p53 and in turn driving proteasomal degradation of p53 (Higashitsuji et al., 2005). Gankyrin also promotes HCC growth through the activation of oncoprotein D cyclin-dependent kinase 4 (CDK4). The activity of the kinase is negatively regulated by p16 during stress condition, but this inhibitory effect is removed when gankyrin competes with p16 for the binding of CDK4 and thus allows the activation of CDK4 (Dawson et al., 2002). Other than tumor promoting effect, gankyrin contributes to cancer drug resistance. It desensitizes cancer cells to the effect of DNA-damaging chemical agents by preventing p53-

The use of RNAi to knock down gankyrin in HCC resulted in a decrease of cell growth, as well as reduction in observed levels of hyperphosphorylated retinoblastoma (Li et al., 2005) and restoration of caspase 8/9-dependent apoptosis (Higashitsuji et al., 2005). Silencing of gankyrin expression also reportedly attenuated epithelial to mesenchymal transition together with cell migration and invasion. The inhibition of gankyrin also reduced the level of nuclear β-catenin (Dong et al., 2011), c-myc, cyclin D1 (Fu et al., 2011) and insulin-like growth factor binding protein 5 (Umemura et al., 2008). The close relationship with several

X-linked inhibitor of apoptosis (XIAP) belongs to the inhibitor of apoptosis (IAP) domaincontaining family, and is famous of its anti-apoptotic ability. They are induced by NF-κB signaling to circumvent the pro-apoptotic effect induced by JNK pathway (Kaur et al., 2005), by inhibiting caspase-mediated apoptosis. Apart from this, XIAP participates in the regulation of transforming growth factor β (TGF-β) -induced apoptosis through an ubiqutinproteasomal regulating machine. XIAP is able to complex with TGF-β activated kinase 1 (TAK1), as such this negatively regulates the TGF-β signaling (Chen, 2005). The RING domain of XIAP is responsible for the poly-ubiquitylation of TAK1, resulting the proteasome-mediated degradation of TAK1. Subsequently, it disrupts the activation of JNK signaling and halts apoptosis. It is speculated that the involvement of XIAP in HCC is common given that most HCCs acquire resistance to TGF-β-mediated cell killing (Chen, 2005). Moreover, TAK1 is important for the phosphorylation and activation of the IKK complex. Increased activity of IKK leads to the degradation of the IκB-α inhibitor of NF-κB and subsequent activation of classical NF-κB signaling. Activation of IKK also causes the degradation of MKK7, the upstream kinase essential for activation of JNK signaling. Reduction of MKK7 level could

Inhibition of XIAP sensitizes HCC cells to apoptotic signal owing to retaining of TAK1. In doing so, persistent activation of JNK signaling is resulted whenever the TGF-β-mediated apoptotic signal is induced. Stabilized TAK1 also potentially attenuate the influence of NFκB signals (Chen, 2005). Besides, it is reported XIAP inhibition in HCC enhanced TRAILmediated cell killing. The combination of XIAP silencing shRNA and tumor-necrosis factorrelated apoptosis TRAIL is reported to generate potent antitumor effect in HCC cells and tumors in animal models (Pan et al., 2008). Targeting XIAP further renders HCC cells vulnerable to other therapeutic effect by releasing the break for caspase-mediated apoptosis

pro-tumorgenic events makes gankyrin a rationale target during HCC treatment.

ablate the JNK signaling and inhibit apoptosis (Kaur et al., 2005).

dependent apoptosis (Higashitsuji et al., 2005).

**5.2.2 X-linked inhibitor of apoptosis** 

It is gradually accepted that the progression and aggressiveness of cancer cells are defined by the tumor-stromal interaction. In HCC, the tumor microenvironment plays a pivotal role in affecting cancer development. Through paracrine and autocrine mechanisms, the stromal components communicate with the tumor, promoting the HCC cell proliferation, survival, and allowing them to invade and metastasize. In the past, majority of targeting therapies are derived from the research focusing on intracellular events of cancer cells. However, limited studies are able to be translated into effective therapies, because they ignored the influence from the surrounding components. Although the field is still in its embryonic stage waiting to be explored, targeting the interaction between tumor-stromal may be a more logical approach against HCC.
