Tissue factor pathway inhibitor TFPI is also known as

Tissue factor pathway inhibitor-2 (TFPI-2) is also known as matrix-associated serine protease inhibitor (MSPI) and placental protein 5 (PP5) (Rao et al., 1995; Kisiel et al., 1994). TFPI-2 is a kunitz-type serine proteinase inhibitor, which is abundantly expressed in a variety of human tissues such as liver, pancreas, skeletal and directionally secreted into the extracellular matrix (ECM) (Miyagi et al., 1994; Sugiyama et al., 2002; Herman et al., 2001). TFPI-2 is thought to negatively regulate the enzymatic activity including matrix metalloproteinase (MMP), plasmin, cathepsin G, trypsin, and plasma kallikrein (Stamenkovic, 2003; Kempaiah et al., 2007). Previous studies have suggested that the expression of TFPI-2 is down-regulated in many malignant tumors, including breast cancer, gastric stromal tumor, cervical cancer, gliomas and non-small-cell lung cancer, and low expression of TFPI-2 was associated with poor prognosis in cancer patients (Wang et al., 2014; Zhang et al., 2013; Rao et al., 2001; Rollin et al., 2005; Xu et al., 2013).
The therapy of induced differentiation of tumors is proposed in recent years, which indicates a new direction for the treatment of hepatocellular carcinoma. Commonly used differentiation-inducing agents are mostly substances that might work on other malignancies. But generally speaking, the therapy of induced differentiation of hepatocellular carcinoma has not yet yielded satisfactory results. It has been reported that transmembrane protease, serine 4 (TMPRSS4) is upregulated by the silencing of TFPI-2 through aberrant DNA methylation in non-small-cell lung cancer (Hamamoto et al., 2015). TMPRSS4 has been shown to be an important regulator during the epithelial-mesenchymal transition (EMT) in human epithelial cancer atipamezole (Li et al., 2011). EMT is a physiological mechanism which is present during development, including mesoderm formation and neural tube formation (Kalluri and Weinberg, 2009). Previous studies showed that the EMT process may facilitate the generation of cancer cells with the mesenchymal traits needed for dissemination as well as the self-renewal properties needed for initiating secondary tumors (Hollier et al., 2009). Our previous studies indicated that TFPI-2 could not only inhibit the proliferation, invasion and metastasis of Hep3B and HepG2, but also significantly reduce the expression and secretion of alpha-fetal protein (AFP), a maker of HCC (Xu et al., 2011). Therefore, we hypothesize that TFPI-2 may show an effect on inducing the differentiation of hepatocellular carcinoma cells (HCC) into mature hepatocytes and serve as a novel way for the treatment of hepatocellular carcinoma.

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Discussion
TFPI-2, also known as placental protein (PP5), is identified as a tumor suppressor gene (Bretz et al., 2012). As a member of the Kunitz structure superfamily, TFPI-2 is a broad-spectrum inhibitor of serine protease. Since the promoter of TFPI-2 is rich in CPG islands, its expression is silenced in many malignant tumors through epigenetic modifications, including promoter methylation and histone deacetylation (Dong et al., 2015; Glockner et al., 2009). In addition, the aberrant splicing form of TFPI-2 was detected during cancer progression, which represented an untranslated form providing another mechanism (Bretz et al., 2012). Moreover, TFPI-2 could mediate dephosphorylation of residues outside the T-loop of ERK, which may directly impact kinase function (Mazalouskas et al., 2014). The ERK1/2 pathway integrates various cytosolic signals to regulate cellular proliferation, differentiation, and apoptosis, which contributes to the formation and development of a variety of tumors (Deng et al., 2013; George et al., 2007).
Tumors are organized in a hierarchy of heterogeneous cell populations with different biologic properties comprising proliferating transit-amplifying cells, terminally differentiated cells, and dying cells and that the populations consist of cancer stem cells (CSCs). Some of the proliferating cells do not differentiate into mature cells, which could continue to proliferate. The CSCs are thought to maintain tumor cells self-renewal capacity, high proliferation rate and are more resistant to chemotherapy than differentiated cancer cells (Ciurea et al., 2014; Puglisi et al., 2013). Differentiation therapy could force hepatocellular carcinoma cells to differentiate and lose self-renewal capacity. Cell differentiation is assumed to be regulated by an informational network, including transacting factors, soluble transmitters and cell-matrix adhesion molecules. But, to our knowledge, little is known on the role of TFPI-2 inducing differentiation in hepatocellular carcinoma.