**1. Introduction**

48 Malignant Mesothelioma

New York

ISSN 0022-1007

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Malignant mesothelioma (MM) is a highly aggressive tumor which arises from the mesothelial cell lining of the serosal surfaces, most cases (>90%) being of pleural origin (Attanoos & Gibbs, 1997; Robinson & Lake, 2005). The pathogenesis of MM has been mainly associated with previous asbestos exposure (Berman & Crump, 2008), with a latency period of up to 40 years, although other agents such as Simian virus 40 (SV40) or genetic susceptibility factors have been linked to the development of this tumor (Carbone et al., 2002; Pisick & Salgia, 2005). Indeed, human mesothelial cells are highly susceptible to SV40 mediated transformation *in vitro* and SV40 DNA sequences and large T antigen (Tag) have been detected in human MM cells (Bocchetta et al., 2000; Carbone et al., 2012; Gazdar et al., 2003).

MM is largely unresponsive to conventional chemotherapy or radiotherapy and, despite its low metastatic efficiency, it is highly invasive to surrounding tissues so that its extensive growth leads to the failure of the organs underlying the serosal membranes (Astoul, 1999). In fact, the primary cause of fatality in MM is related to the propensity of the tumor cells to invade locally, even though MM metastasis are more common after surgery and, at the autopsy, metastatic diffusion is observed in 50% of patients (Astoul, 1999). At present, the median survival from diagnosis of MM is less than two years (Palumbo et al., 2008).

The mesothelium is not just a passive protective surface, but a highly dynamic membrane (Mutsaers, 2004). It consists of a single layer of elongated, flattened, squamous-like cells of mesodermal origin, characterized by dual epithelial/mesenchymal features. Cuboidal mesothelial cells can also be found at various locations in physiological conditions. Further, mesothelial cells can adopt a cuboidal morphology, which reflects a metabolically activated state, after injury or stimulation of the serosal surface (Mutsaers, 2004). Indeed, mesothelial cells are sentinel cells that can sense and respond to a variety of signals within their

microenvironment. They participate in serosal inflammation by secreting both pro- and antiinflammatory as well as immunomodulatory mediators. Besides, these cells can act as antigen presenting cells for T lymphocytes (Hausmann et al., 2000), regulate tissue repair, control fibrin deposition and breakdown, and modulate adhesion, growth and dissemination of tumor cells metastasizing to the serosal membranes (Mutsaers, 2002). In particular, in response to different types of stimuli, including cytokines and asbestos fibers, mesothelial cells have been reported to release prostaglandins, chemokines, reactive oxygen and nitrogen species and growth factors which represent key effectors in the modulation of inflammatory reactions that occur in response to pleural injury (Fleury-Feith et al., 2003; Mutsaers, 2002).

Role of Inflammation and Angiogenic Growth Factors in Malignant Mesothelioma 51

leads to the persistent activation of the nuclear factor kappa B (NF-κB) transcription factor, which in turn mediates the activation of prosurvival genes and prevents apoptosis of the damaged mesothelial cells (Mantovani et al., 2008; Micheau & Tschopp, 2003; Philip et al., 2004). This allows mesothelial cells with asbestos-induced DNA damage to survive and divide rather than die and, if sufficient genetic damage accumulates, to eventually develop into a MM (Miura et al., 2006; Nymark, 2007). In fact, apoptosis is an important mechanism by which cells with DNA damage are eliminated without eliciting an inflammatory response (Ullrich et al., 2008; Yoshida et al., 2010). However, failure of apoptosis in cells with unrepaired DNA and chromosomal damage after chronic exposure to asbestos may lead to permanent genetic alterations and trigger the development of a clone of cancerous cells (Roos & Kaina, 2006; Wu, 2006). Consistently, MM cells are found to be apoptosis-resistant as compared to

primary cultured mesothelial cells (Fennel & Rudd, 2004; Villanova et al., 2008).

**2.1. Tumor Necrosis Factor-α and other pro-inflammatory cytokines** 

transformation (Haegens et al., 2007; Janssen-Heininger et al., 1999; Yang et al., 2006).

sIL-6R are typical of several chronic inflammatory conditions (Rose-John et al., 2007).

Thus, inflammatory cytokines such as TNF-α and IL-6 appear to play a dual role in MM pathogenesis: they induce and sustain pleural inflammation and at the same time can act as survival or mitogenic factors for normal and transformed mesothelial cells, respectively.

It has been reported that rats receiving a single intratracheal instillation of fibrogenic chrysotile asbestos developed lung chronic inflammatory reactions characterized by the accumulation of alveolar macrophages producing elevated levels of both Interleukin (IL)-1 and IL-6 (Lemaire & Ouellet, 1996). An increased production and/or release of these cytokines triggers inflammatory cell recruitment, thus amplifying and sustaining local inflammation. It has also been demonstrated that crocidolite asbestos and TNF-α can stimulate a dose-dependent increase in IL-6 expression and secretion from cultured, transformed and normal, human alveolar type II epithelial cells that is dependent upon intracellular redox potential (Simeonova et al., 1997). Interestingly, although MM cells appear to express low levels of IL-6 receptor (IL-6R), IL-6 can act as a growth factor for these cells through a trans-signaling mechanism involving the interaction of macromolecular complexes of IL-6 and soluble IL-6R (sIL-6R) with the transmembrane glycoprotein gp130 expressed on the surface of MM cells (Adachi et al., 2006; Rose-John et al., 2007). High levels of both IL-6 and

Tumor Necrosis Factor-α (TNF-α) is probably the most studied candidate for initiating inflammatory and fibrotic events linked to lung diseases such as asbestosis. Asbestos fibers cause the accumulation of macrophages in the pleura and lung. When these macrophages encounter asbestos, they release TNF-α. At the same time, asbestos induces the secretion of TNF-α and the expression of TNF-α receptor I (TNF-RI) in mesothelial cells (Yang et al., 2006). Remarkably, treatment of mesothelial cells with TNF-α significantly reduced asbestos cytotoxicity. Indeed, TNF-α activates NF-κB, which in turn promotes mesothelial cell survival and resistance to the cytotoxic effects of asbestos. Thus, TNF-α signaling through NF-κBdependent mechanisms increases the percentage of mesothelial cells that survive asbestos exposure, thereby increasing the pool of asbestos-damaged cells susceptible to malignant
