**3.1. Angiogenic growth factors of the VEGF family**

The human VEGF family consists of five members: VEGF (VEGF-A), VEGF-B, VEGF-C, VEGF-D and PlGF. These growth factors are secreted as dimers and their biological effects are mediated by binding to three tyrosine kinase receptors, *i.e.* VEGF-R1/Flt-1, VEGF-R2/KDR (whose murine homologue is known as Flk-1) and VEGF-R3/Flt-4, and two nonenzymatic co-receptors known as neuropilin-1 and -2 (Ferrara et al., 2003; Koch et al., 2011; Roskoski, 2007).

## *3.1.1. VEGF*

VEGF is regarded as the major mediator of tumor angiogenesis. It is expressed in the majority of cancers and has a central role in tumor growth and metastasis. In fact, this growth factor is essential for the mobilization of bone-marrow-derived endothelial precursors in neovascularization (Asahara et al., 1999), and stimulates vascular endothelial cells mobility, proliferation and survival (Waltenberger et al., 1994).

High levels of VEGF are present both in malignant and non-malignant pleural effusions leading to increased vascular permeability. On the other hand, VEGF levels in serum or pleural effusions of MM patients are higher than those found in patients with nonmalignant pleuritis or lung cancer involving malignant pleural effusions. Further, in MM patients elevated serum or pleural effusion levels of VEGF correlate with a worse prognosis and may also contribute to increase resistance to chemotherapy (Hirayama et al., 2011; Yasumitsu et al., 2010; Zebrowski et al., 1999). In fact, VEGF status has proved to be of value in predicting the effectiveness of radiotherapy and chemotherapy on different cancers (Choi et al., 2008; Kumar et al., 2009; Toi et al., 2001).

In addition to its role in tumor vascularization, VEGF can directly affect the behavior of cancer cells in an autocrine or paracrine manner. Indeed, many tumor cell types express VEGF receptors. VEGF has been found to promote the growth of transformed cell lines *in vitro* (Masood et al., 2001) and to act as a survival factor for tumor cells by enhancing the expression of the antiapoptotic factors bcl-2 (Harmey & Bouchier-Hayes, 2002) and survivin (Kanwar et al., 2011). In this context, MM cells have been shown to express high amounts of VEGF, VEGF receptors and co-receptors both *in vitro* and *in vivo*, and VEGF has been demonstrated to act as an autocrine growth factor for this tumor cell type (Albonici et al., 2009; Ohta et al., 1999; Pompeo et al., 2009; Strizzi et al., 2001a).

VEGF-R1 participates in cell migration; it has an important role in monocyte chemotaxis and promotes recruitment of circulating endothelial precursor cells from bone marrow (Hattori et al., 2002). Its expression is increased in various tumors, correlates with disease progression and can predict poor prognosis, metastasis and recurrent disease in humans (Dawson et al., 2009; Fischer et al., 2008; Kerber et al., 2008). This receptor is also expressed by MM cells *in vitro* and *in vivo*, where it appears to mediate proliferative and cell survival responses (Albonici et al., 2009; Strizzi et al., 2001a).

VEGF-R2 is the main mediator of VEGF-stimulated endothelial cell migration, proliferation, survival and enhanced vascular permeability (Olsson et al., 2006; Shibuya, 2006). VEGF-R2 expression is induced in conjunction with active angiogenesis, such as during the reparative process, and in pathological conditions associated with neovascularization, such as cancer (Plate et al., 1993). VEGF-R2 is overexpressed in MM cells and specimens, and VEGF-R2 silencing by small intefering RNA has been shown to induce cell death in MM or immortalized mesotelial cells *in vitro* (Albonici et al., 2009; Catalano et al., 2009; Pompeo et al., 2009; Strizzi et al., 2001a). Interestingly, it has been reported that in MM cells this receptor can be activated also via the semaphorin-6D receptor Plexin-A1, triggering a prosurvival program that promotes anchorage-independent growth through a NF-κBdependent pathway (Catalano et al., 2009). Remarkably, the expression of plexin-A1 is induced by asbestos fibers and overexpression of plexin-A1 in non-malignant mesothelial cells inhibits cell death after asbestos exposure, thus suggesting a role for this receptor not only in MM promotion and progression but also in asbestos-induced mesothelial carcinogenesis (Catalano et al., 2009).

*In vitro* studies have shown that transfection of normal mesothelial cells with SV40 Tag potently increases VEGF protein and mRNA levels (Cacciotti et al., 2002) as well as mesothelial cell proliferation (Catalano et al., 2002). These data indicate that VEGF regulation by SV40 transforming proteins can also represent a key event in MM onset and progression.

#### *3.1.2. PlGF*

56 Malignant Mesothelioma

Roskoski, 2007).

*3.1.1. VEGF* 

al., 2007). Angiogenic growth factors include, among the others, Vascular Endothelial Growth Factor (VEGF), Placenta Growth Factor (PlGF), Platelet-Derived Growth Factor (PDGF) and acidic and basic Fibroblast Growth Factors (FGF-1 and -2, respectively). VEGF

The link between angiogenesis and tumor progression is provided by the negative prognostic value of intratumoral microvascular density (IMD) (Folkman, 2006; Kerbel, 2008). In MM the IMD has an independent prognostic value (Kumar-Singh et al., 1997). MM demonstrates a higher IMD than colon and breast tumors and, consistently, presents with minimal central

On the other hand, the involvement of angiogenic growth factors in MM goes beyond the stimulation of angiogenesis. Indeed, as discussed below, MM cells express receptors for several angiogenic factors which, accordingly, can directly modulate MM cell behavior.

The human VEGF family consists of five members: VEGF (VEGF-A), VEGF-B, VEGF-C, VEGF-D and PlGF. These growth factors are secreted as dimers and their biological effects are mediated by binding to three tyrosine kinase receptors, *i.e.* VEGF-R1/Flt-1, VEGF-R2/KDR (whose murine homologue is known as Flk-1) and VEGF-R3/Flt-4, and two nonenzymatic co-receptors known as neuropilin-1 and -2 (Ferrara et al., 2003; Koch et al., 2011;

VEGF is regarded as the major mediator of tumor angiogenesis. It is expressed in the majority of cancers and has a central role in tumor growth and metastasis. In fact, this growth factor is essential for the mobilization of bone-marrow-derived endothelial precursors in neovascularization (Asahara et al., 1999), and stimulates vascular endothelial

High levels of VEGF are present both in malignant and non-malignant pleural effusions leading to increased vascular permeability. On the other hand, VEGF levels in serum or pleural effusions of MM patients are higher than those found in patients with nonmalignant pleuritis or lung cancer involving malignant pleural effusions. Further, in MM patients elevated serum or pleural effusion levels of VEGF correlate with a worse prognosis and may also contribute to increase resistance to chemotherapy (Hirayama et al., 2011; Yasumitsu et al., 2010; Zebrowski et al., 1999). In fact, VEGF status has proved to be of value in predicting the effectiveness of radiotherapy and chemotherapy on different cancers (Choi

In addition to its role in tumor vascularization, VEGF can directly affect the behavior of cancer cells in an autocrine or paracrine manner. Indeed, many tumor cell types express VEGF receptors. VEGF has been found to promote the growth of transformed cell lines *in vitro* (Masood et al., 2001) and to act as a survival factor for tumor cells by enhancing the

is regarded as the most important player in angiogenesis (Ono, 2008).

**3.1. Angiogenic growth factors of the VEGF family** 

cells mobility, proliferation and survival (Waltenberger et al., 1994).

et al., 2008; Kumar et al., 2009; Toi et al., 2001).

necrosis despite its huge size (Gasparini & Harris, 1995; Kumar-Singh et al., 1997).

PlGF, originally identified in the placenta during the early embryonic development (Khaliq et al., 1996; Maglione et al., 1991), is expressed in several other organs including the heart, lung, thyroid, skeletal muscle and adipose tissue (Persico et al., 1999) but not normal mesothelium (Albonici et al., 2009).

Although the role exerted by PlGF in tumor growth is controversial yet, PlGF can stimulate vessel growth and maturation directly by affecting endothelial and mural cells, as well as indirectly by recruiting pro-angiogenic cell types (Barillari et al., 1998; Carmeliet, 2003). It

also promotes the recruitment and maturation of angiogenesis-competent myeloid progenitors to growing sprouts and collateral vessels (Hattori et al., 2002; Luttun et al., 2002; Rafii et al., 2003). Further, PlGF is able to protect endothelial cells from apoptosis, in a similar manner as VEGF, by inducing the expression of antiapoptotic genes such as survivin (Adini et al., 2002).

Role of Inflammation and Angiogenic Growth Factors in Malignant Mesothelioma 59

heterodimeric PDGF-AB. The biological effects of PDGF are mediated by two tyrosine kinase receptors, namely the PDGF receptor alpha (PDGFRα), which binds PDGF-A, -B, and –C chains, and the PDGF receptor beta (PDGFR), which binds PDGF-B and –D. Accordingly, upon ligand binding different receptor dimers may form depending on ligand configuration and the pattern of receptor expression. Cellular responses to PDGF signaling include stimulation of cell growth, differentiation, migration and inhibition of apoptosis (Andrae et

An increased PDGF activity has been linked with tumors, vascular and fibrotic diseases (Andrae et al., 2008). Autocrine PDGF signaling leading to enhanced proliferation of tumor cells occurs in several types of cancer (Ostman, 2004). In addition, PDGF secretion by cancer cells and activated endothelial cells promotes the formation of both fibrous and vascular tumor stroma. In particular, PDGF-BB participates in tumor angiogenesis by stimulating endothelial cell motility and pericyte recruitment to neoformed vessels, thus leading to vessel stabilization, tumor cell survival and growth. Instead, both PDGF-AA and PDGF-BB appear involved in tumor recruitment of PDGFR-positive fibroblasts which, in turn, can be activated by PDGFs to produce VEGF and other tumor-promoting growth factors (Andrae

Either high PDGF-AB serum levels or a strong expression of PDGFR signaling effectors in MM tissues have been associated with a lower survival in MM patients (Filiberti et al., 2005; Kothmaier et al., 2008). In fact, several evidence support a role for PDGF in MM promotion

While PDGFRα expression levels are lower in MM than in normal mesothelial cells, PDGFRβ, PDGF-A and PDGF-B are overexpressed in MM cells as compared to their nontransformed counterparts (Langerak et al., 1996a, 1996b; Metheny-Barlow et al., 2001). Functional studies have shown that transduction of MM cells with a hammerhead ribozyme against PDGFRβ mRNA reduced both PDGFRβ expression and MM cell proliferation, demonstrating the involvement of a PDGF-BB autocrine loop in MM cell growth (Dorai et al., 1994). Conversely, the role of PDGF-A in MM cell proliferation is controversial. Indeed, the transfection of MM cells with antisense oligonucleotides to PDGF-A has been reported to either inhibit or stimulate MM cell growth *in vitro* (Garlepp & Leong, 1995; Metheny-Barlow et al., 2001). On the other hand, PDGF-A appears to play an important role in sustaining MM cell growth *in vivo* through paracrine mechanisms. Indeed, PDGF-A overexpression in MM cells inoculated in nude mice was found to increase tumor incidence, tumor growth rate and to decrease the latency period to tumor formation (Metheny-Barlow et al., 2001). In this regard, it has been suggested that PDGF-A participates in a malignant cytokine network through which MM cells instigates tumor-associated fibroblasts to produce growth factors, such as hepatocyte

The FGF family encompasses 22 structurally related ligands in mammals. The effects of most FGF family members, including FGF-1 and -2, are mediated by binding to a family of

al., 2008).

**3.3. FGF** 

et al., 2008; Cao et al., 2008; Homsi & Daud, 2007).

and progression through both autocrine and paracrine mechanisms.

growth factor (HGF), with tumor-promoting activities (Li et al., 2011).

Under pathological conditions, PlGF abundance is elevated in various cell types and tissues, including vascular endothelial cells, and many different tumor cells (Albonici et al., 2009; Cao et al., 1996; Fischer et al., 2007; Oura et al., 2003). PlGF expression is switched on in hyperplastic/reactive mesothelium and in MM cells (Albonici et al., 2009). Moreover, in MM as well as in different types of cancer, including melanoma, gastric, colorectal and breast carcinomas, PlGF plasma levels and intratumoral expression have been found to correlate with tumor stage, vascularity, recurrence, metastasis and survival (Chen et al., 2004; Marcellini et al., 2006; Parr et al. 2005; Pompeo et al.; 2009; Wei et al., 2005).

*In vitro* studies have shown that administration of recombinant PlGF to MM cells triggers the activation of Akt but does not elicit a significant stimulation of cell growth. Conversely, the administration of PlGF-neutralizing antibodies causes a significant reduction of MM cell viability, demonstrating the PlGF acts as a survival factor for MM cells (Albonici et al., 2009).

PlGF binds VEGF-R1 and the co-receptors neuropilin-1 and -2, but, unlike VEGF, it does not bind VEGF-R2. Accordingly, it can act independently of VEGF in cells which primarily express VEGF-R1 (Fischer et al., 2007). Worthy of note, even though VEGF and PlGF both bind VEGF-R1, PlGF was reported to stimulate the phosphorylation of specific VEGF-R1 tyrosine residues and the expression of distinct downstream target genes as compared to VEGF (Autiero et al., 2003). On the other hand, PlGF can also sustain VEGF activity through different mechanisms involving both VEGF-R1 and VEGF-R2. One of these mechanisms relies on the formation of PlGF:VEGF heterodimers. Indeed, PlGF:VEGF heterodimers have been isolated from cells producing both factors and shown to bind VEGF-R1:VEGF-R2 receptor complexes, thus inducing receptor cross-talk and activation of VEGF-R2, the major mediator of VEGF activities (Autiero et al., 2003; Cao et al., 1996). In addition, the activation of VEGF-R1 by PlGF homodimers may induce the intermolecular transphosphorylation and activation of VEGF-R2 (Carmeliet et al., 2001).

It is noteworthy that *in vivo* anti-PlGF treatment was reported to inhibit tumor growth without affecting healthy vessels, thus reducing tumor infiltration by angiogenic macrophages and severe tumor hypoxia, and preventing the switch on of the angiogenic rescue program leading to the enhanced release different angiogenic factors responsible for resistance to VEGF receptors inhibitors (Fischer et al., 2007).

#### **3.2. PDGF**

PDGFs comprise a family of dimeric growth factors structurally and functionally related to VEGFs (Andrae et al., 2008). PDGF homodimers are formed by four different chains, *i.e.*  PDGF-A, PDGF-B, PDGF-C and PDGF-D. In addition, PDGF-A and –B chains can form the heterodimeric PDGF-AB. The biological effects of PDGF are mediated by two tyrosine kinase receptors, namely the PDGF receptor alpha (PDGFRα), which binds PDGF-A, -B, and –C chains, and the PDGF receptor beta (PDGFR), which binds PDGF-B and –D. Accordingly, upon ligand binding different receptor dimers may form depending on ligand configuration and the pattern of receptor expression. Cellular responses to PDGF signaling include stimulation of cell growth, differentiation, migration and inhibition of apoptosis (Andrae et al., 2008).

An increased PDGF activity has been linked with tumors, vascular and fibrotic diseases (Andrae et al., 2008). Autocrine PDGF signaling leading to enhanced proliferation of tumor cells occurs in several types of cancer (Ostman, 2004). In addition, PDGF secretion by cancer cells and activated endothelial cells promotes the formation of both fibrous and vascular tumor stroma. In particular, PDGF-BB participates in tumor angiogenesis by stimulating endothelial cell motility and pericyte recruitment to neoformed vessels, thus leading to vessel stabilization, tumor cell survival and growth. Instead, both PDGF-AA and PDGF-BB appear involved in tumor recruitment of PDGFR-positive fibroblasts which, in turn, can be activated by PDGFs to produce VEGF and other tumor-promoting growth factors (Andrae et al., 2008; Cao et al., 2008; Homsi & Daud, 2007).

Either high PDGF-AB serum levels or a strong expression of PDGFR signaling effectors in MM tissues have been associated with a lower survival in MM patients (Filiberti et al., 2005; Kothmaier et al., 2008). In fact, several evidence support a role for PDGF in MM promotion and progression through both autocrine and paracrine mechanisms.

While PDGFRα expression levels are lower in MM than in normal mesothelial cells, PDGFRβ, PDGF-A and PDGF-B are overexpressed in MM cells as compared to their nontransformed counterparts (Langerak et al., 1996a, 1996b; Metheny-Barlow et al., 2001). Functional studies have shown that transduction of MM cells with a hammerhead ribozyme against PDGFRβ mRNA reduced both PDGFRβ expression and MM cell proliferation, demonstrating the involvement of a PDGF-BB autocrine loop in MM cell growth (Dorai et al., 1994). Conversely, the role of PDGF-A in MM cell proliferation is controversial. Indeed, the transfection of MM cells with antisense oligonucleotides to PDGF-A has been reported to either inhibit or stimulate MM cell growth *in vitro* (Garlepp & Leong, 1995; Metheny-Barlow et al., 2001). On the other hand, PDGF-A appears to play an important role in sustaining MM cell growth *in vivo* through paracrine mechanisms. Indeed, PDGF-A overexpression in MM cells inoculated in nude mice was found to increase tumor incidence, tumor growth rate and to decrease the latency period to tumor formation (Metheny-Barlow et al., 2001). In this regard, it has been suggested that PDGF-A participates in a malignant cytokine network through which MM cells instigates tumor-associated fibroblasts to produce growth factors, such as hepatocyte growth factor (HGF), with tumor-promoting activities (Li et al., 2011).

#### **3.3. FGF**

58 Malignant Mesothelioma

(Adini et al., 2002).

also promotes the recruitment and maturation of angiogenesis-competent myeloid progenitors to growing sprouts and collateral vessels (Hattori et al., 2002; Luttun et al., 2002; Rafii et al., 2003). Further, PlGF is able to protect endothelial cells from apoptosis, in a similar manner as VEGF, by inducing the expression of antiapoptotic genes such as survivin

Under pathological conditions, PlGF abundance is elevated in various cell types and tissues, including vascular endothelial cells, and many different tumor cells (Albonici et al., 2009; Cao et al., 1996; Fischer et al., 2007; Oura et al., 2003). PlGF expression is switched on in hyperplastic/reactive mesothelium and in MM cells (Albonici et al., 2009). Moreover, in MM as well as in different types of cancer, including melanoma, gastric, colorectal and breast carcinomas, PlGF plasma levels and intratumoral expression have been found to correlate with tumor stage, vascularity, recurrence, metastasis and survival (Chen et al., 2004;

*In vitro* studies have shown that administration of recombinant PlGF to MM cells triggers the activation of Akt but does not elicit a significant stimulation of cell growth. Conversely, the administration of PlGF-neutralizing antibodies causes a significant reduction of MM cell viability, demonstrating the PlGF acts as a survival factor for MM cells (Albonici et al., 2009). PlGF binds VEGF-R1 and the co-receptors neuropilin-1 and -2, but, unlike VEGF, it does not bind VEGF-R2. Accordingly, it can act independently of VEGF in cells which primarily express VEGF-R1 (Fischer et al., 2007). Worthy of note, even though VEGF and PlGF both bind VEGF-R1, PlGF was reported to stimulate the phosphorylation of specific VEGF-R1 tyrosine residues and the expression of distinct downstream target genes as compared to VEGF (Autiero et al., 2003). On the other hand, PlGF can also sustain VEGF activity through different mechanisms involving both VEGF-R1 and VEGF-R2. One of these mechanisms relies on the formation of PlGF:VEGF heterodimers. Indeed, PlGF:VEGF heterodimers have been isolated from cells producing both factors and shown to bind VEGF-R1:VEGF-R2 receptor complexes, thus inducing receptor cross-talk and activation of VEGF-R2, the major mediator of VEGF activities (Autiero et al., 2003; Cao et al., 1996). In addition, the activation of VEGF-R1 by PlGF homodimers may induce the intermolecular transphosphorylation and

It is noteworthy that *in vivo* anti-PlGF treatment was reported to inhibit tumor growth without affecting healthy vessels, thus reducing tumor infiltration by angiogenic macrophages and severe tumor hypoxia, and preventing the switch on of the angiogenic rescue program leading to the enhanced release different angiogenic factors responsible for

PDGFs comprise a family of dimeric growth factors structurally and functionally related to VEGFs (Andrae et al., 2008). PDGF homodimers are formed by four different chains, *i.e.*  PDGF-A, PDGF-B, PDGF-C and PDGF-D. In addition, PDGF-A and –B chains can form the

Marcellini et al., 2006; Parr et al. 2005; Pompeo et al.; 2009; Wei et al., 2005).

activation of VEGF-R2 (Carmeliet et al., 2001).

**3.2. PDGF** 

resistance to VEGF receptors inhibitors (Fischer et al., 2007).

The FGF family encompasses 22 structurally related ligands in mammals. The effects of most FGF family members, including FGF-1 and -2, are mediated by binding to a family of

tyrosine kinase receptors designated FGF receptors (FGFR1 to FGFR5), whereas a smaller number of FGF isoforms does not bind FGFRs but interacts with voltage-gated sodium channels (Knights & Cook, 2010).

Role of Inflammation and Angiogenic Growth Factors in Malignant Mesothelioma 61

established, cancer cells promote a constant influx of myelomonocytic cells that express inflammatory mediators supporting pro-tumoral functions. In this regard, myelomonocytic cells are key orchestrators of cancer-related inflammatory processes supporting proliferation and survival of malignant cells, subversion of adaptive immune responses, stromal remodeling and angiogenesis (David Dong et al., 2009; Loges et al., 2009; Porta et al., 2009).

Tissue infiltration by macrophages is a dramatic and common feature of inflammation, angiogenesis and cancer (Pollard, 2004; Sica, 2010). High densities of tumor-infiltrating macrophages are associated with poor survival in patients with MM (Burt et al., 2011). In fact, the recruitment and infiltration of macrophages in the tumor microenvironment can activate them to support the malignant progression of cancer cells. These macrophages are called tumor-associated macrophages (TAMs) (Lawrence, 2011; Sica, 2010). Cancer cells cocultured with macrophages and incubated with inflammatory cytokines are synergistically stimulated to produce various angiogenesis-related factors (Izzi et al., 2009; Ono, 2008). This inflammatory angiogenesis is mediated, in part, by activation of NF-κB and AP-1 (Angelo & Kurzrock, 2007; Huang et al., 2000; Ono, 2008). In fact, treatment of both vascular endothelial cells and cancer cells with IL-1α/β, TNF-α and ROS *in vitro* results in a marked induction of VEGF and FGF-2, through the transcriptional activation of NF-κB, Specificity

In addition to macrophages, other tumor-infiltrating immune cells including T cells, B cells, natural killer cells and neutrophils can release cytokines, such as IL-1α/β, TNF-α and IL-6, able to sustain the synthesis of angiogenic growth factors (Angelo & Kurzrock, 2007). As for, IL-6, this pro-inflammatory cytokine has been reported to play a critical role in the stimulation of VEGF synthesis by different cell types, including MM cells (Adachi et al., 2006; Angelo & Kurzrock, 2007). Of note, MMs usually produce high levels of IL-6 but express low levels of IL-6R, so that the presence of sIL-6Rs, which may be provided by inflammatory cells recruited to the tumor region, is essential for the IL-6-dependent stimulation of VEGF expression by MM cells (Adachi et al., 2006). Inflammation can also induce the expression of receptors for angiogenic growth factors. In this regard, the expression of PDGFRs is known to be induced by inflammatory cytokines such as TNF-α and IL-1 (Andrae et al., 2008). Besides, inflammatory cells themselves can directly release angiogenic factors such as VEGF, PlGF, FGF-2 and PDGF, among many others, which exert mitogenic and migratory effects on surrounding cells (Sica 2010, Ono 2008). Inflammatory cells recruited in the tumor microenvironment can also produce matrix metalloproteinases which promote the formation of new vessels by degrading the basement membrane and by releasing angiogenic growth factors, such as VEGF, PlGF-2 and FGF-2, stored in the

extracellular matrix (Barillari et al.,1998; Cao et al., 2008; Lieu et al., 2011).

The high amount of chemokines/cytokines, growth factors, proteolytic enzymes, proteoglycans, lipid mediators and prostaglandins which is typically found in the tumor microenvironment sustains and exacerbates both inflammation and angiogenesis (Costa et al., 2007; Lin & Karin, 2007; Ono, 2008). In this context, the cross-talk between inflammation and angiogenesis is further corroborated by the evidence that, if on one hand inflammatory mediators have significant effects on angiogenesis, on the other hand angiogenic factors can

protein 1 (Sp-1), AP-1 and hypoxia response elements.

FGFs regulate cell proliferation, differentiation, survival, wound healing and angiogenesis. In cancer, FGF signaling is frequently de-regulated, resulting in mitogenic, anti-apoptotic and angiogenic responses (Knights & Cook, 2010). FGF-1 and -2, but also other less-studied FGF isoforms, exert pro-angiogenic effects by modulating proliferation and migration of endothelial cells and by stimulating the production of proteases (Lieu et al., 2011; Saylor et al., 2012). Worthy of note, it has been demonstrated that FGF-2 can synergize with both VEGF and PDGF-BB in stimulating neovascularization, this synergism relying on multiple mechanisms. For instance, FGF-2 promotes hypoxia-induced VEGF release by cancer cells and the expression of both VEGF and VEGFRs in endothelial cells, whereas VEGF, in turn, upregulates the expression of FGF-2 (Lieu et al., 2011; Saylor et al., 2012). Moreover, FGF-2 upregulates PDGFRs expression and increases the responsiveness to PDGF-BB in endothelial cells, whereas PDGF-BB enhances FGFR1 expression and FGF-2 responsiveness in vascular smooth muscle cells (Cao et al., 2008; Liu et al., 2011). Remarkably, FGFs are thought to play a critical role in the resistance to anti-VEGF therapy (Lieu et al., 2011; Saylor et al, 2005). Besides, both FGF-1 and -2 may also be involved in tumor cell growth through cell-autonomous, autocrine mechanisms (Kumar-Singh et al., 1999).

FGF-1 and -2 are expressed in the majority of MMs *in vivo* and high levels of FGF-2 in tumor tissues, serum or pleural effusions are associated with a worse prognosis in MM patients (Davidson et al., 2004; Kumar-Singh et al., 1999; Strizzi et al., 2001b). Furthermore, the combined expression levels of FGF-1, FGF-2, VEGF and Transforming Growth Factor beta (TGFβ) in MM tissues correlates with both IMD and a poorer prognosis (Kumar-Singh et al., 1999). In addition to their role in tumor angiogenesis, FGFs act as autocrine growth factors for MM cells. Indeed, MM cells express FGFs and FGF receptors and the transfection with short interfering RNAs to FGF-1 and FGF-2 reduces MM cell proliferation (Kumar-Singh et al., 1999; Liu & Klominek, 2003; Stapelberg et al., 2005). It has also been reported that treatment of MM cells with exogenous FGF-2 stimulates the secretion of matrix metalloproteinases involved in tumor invasion and angiogenesis (Liu & Klominek, 2003).
