Vascular endothelial growth factor (VEGF) is a member of the VEGF/platelet-derived factor gene family and consists of at least six isoforms (VEGF-121, -145, -165, -183, -189 and -206) which are regulated by splicing at the mRNA level. VEGF can bind to two tyrosine kinase receptors, which are VEGFR-1 (flt-1) and VEGFR-2 (FLK-1/KDR) seated on the surface of endothelial cells. Once VEGF binds to these receptors, it induces dimerization, autophosphorylation and signal transduction. Notably, VEGF has potent angiogenic, mitogenic and vascular permeability-enhancing properties (13–18). Studies have shown that VEGF is overexpressed in human tumors (19) and plays an important role in the development of certain types of effusion (17,20–23). In agreement with previous studies (24–27), our group recently showed that VEGF levels are significantly higher in exudates than in transudates. Moreover, we found higher levels of VEGF in malignant than in parapneumonic effusions, and VEGF levels were correlated with markers of increased vascular permeability and pleural inflammation, such as LDH and protein ratios (28) in accord with recent studies (27,29).

Since it was shown that VEGF contributes to the formation of malignant pleural effusions, there have been some attempts to implicate, therapeutically, this finding. ZD6474 is an orally active inhibitor of VEGFR-2 with some additional activity against epidermal growth factor receptor tyrosine kinase (30–35). Matsumori et al investigated whether this drug controls experimental metastasis and pleural effusions produced by human non-small-cell lung carcinoma (NSCLC). They found that treatment with ZD6474 inhibits activation of VEGFR-2 and reduces tumor vascularization and tumor cell proliferation (36). Shibuya et al combined ZD6474 with radiation therapy using an orthotopic nude mouse model of NSCLC that closely mimics the patterns of tumor growth (37). This combination significantly enhanced the antiangiogenic, antivascular and anti-tumor effects of radiotherapy and was more effective than combined radiotherapy and chemotherapy (37).

PTK 787 is a VEGF receptor tyrosine kinase phosphorylation inhibitor. The efficiency of this agent has been examined in malignant pleural effusion developed in mice (38). It has been noted that oral administration of PTK 787 suppressed pleural effusion formation by inhibiting vascular permeability (38). It has been shown that, given orally, this agent exhibits excellent activity and tolerability, thus it can be used for long-term therapy of malignancies and for other diseases where VEGF mediates angiogenesis and plays an important role in their pathogenesis (39).

Bevacizumab is a humanized anti-VEGF monoclonal neutralizing antibody which blocks the binding of VEGF to its receptor and neutralizes all the isoforms of human VEGF. Li and co-workers used a patient-like severe combined immunodeficient mouse model by the orthotopic implantation of malignant pleural mesothelioma cells in order to examine the therapeutic efficiency of this agent (40). Administration of bevacizumab managed to suppress pleural effusion formation. Moreover, they showed that the combination of bevacizumab with pemetrexed, a new anticancer drug recently approved for the therapy of malignant pleural mesothelioma, augmented this effect (40).

Malignant mesothelioma cells express a number of receptor tyrosine kinase including VEGF receptor, epidermal growth factor receptor, platelet-derived growth factor receptor and Eph receptors (41–43). The Eph transmembrane tyrosine kinases constitute the largest family of receptor tyrosine kinases. EphA2 receptor is overexpressed during various processes such as tumor growth, angiogenesis and metastasis, and is also overexpressed in aggressive malignances (44–47). A recent in vitro study showed that silencing the EphA2 receptor using small interfering RNA inhibits the growth and migration of malignant mesothelioma cells. Silencing the EphA2 gene can induce apoptosis in malignant mesothelioma cells through caspase-9 activation (41–43). Conversely, when the receptor is overexpressed, this increases the proliferation and migration of malignant mesothelioma cells. Thus, knocking down the oncogenic protein EphA2 may have therapeutic implications in patients with malignant mesothelioma (48). It has also been noted that activation of the EphA2 receptor by its ligand ephrinA1 downregulates total EphA2 expression via phosphorylation and inhibits mesothelioma cell formation (49).

Endostatin, a 20-kDa C-terminal fragment of collagen XVIII, is released by normal cells and tissues. Endostatin specifically inhibits endothelial proliferation and potently inhibits angiogenesis and tumor growth (50,51). It has been recently observed that thoracoscopic talc insufflation induces pleural mesothelial cells to release endostatin (52). It seems that talc insufflation may alter the angiogenic balance in the pleural space upregulating angiostasis (52). However, further studies are needed to clarify these findings in humans.

Hyaluronan (HA) is a non-sulfated glycosaminoglycan that is secreted in significant quantities by pleural mesothelial cells and malignant mesothelioma cells (53). Its receptor, CD44, is expressed by malignant cells with a predilection for the pleura. High molecular weight HA is inactive, while low molecular weight HA produced by hydrolysis of the high molecular weight HA is chemotactic for malignant cells and also has angiogenic properties increasing permeability of the mesothelial monolayer. Low molecular weight HA induces malignant mesothelial cell proliferation and haptotaxis via interaction of the CD44 receptor (54).

Angiopoietin (Ang)-1 and -2 are important regulators of angiogenesis and exert their actions through binding a common tyrosine kinase receptor (Tie-2) that is mainly expressed on the surface of endothelial cells (55). Ang-1 and -2 act in conjunction with VEGF in promoting angiogenesis and occur under both normal and disease conditions (55).

In vivo (56,57) and in vitro studies (58–60) have demonstrated that Ang-1 has anti-inflammatory and antipermeability properties. It blocks the expression of adhesion molecules on the endothelial cell surface, leukocyte adherence on endothelial cells and transmigration into tissues and IL-8 production by endothelial cells. In addition, Ang-1 inhibits vascular permeability induced by VEGF and other inflammatory agents. On the other hand, Ang-2 antagonizes the effect of Ang-1 in endothelial cells, suggesting that Ang-2 promotes vascular permeability (61). In agreement with this, Ang-2 destabilizes the endothelial cell monolayer integrity leading to the detachment of endothelial cells in vitro (62). More evidence supporting a hyperpermeability and pro-inflammatory function of Ang-2 comes from an in vivo study in which Ang-2 was found to induce edema formation and to exert a weak stimulatory effect on leukocyte migration when injected into mouse paw (63).

In malignant diseases, Ang-2 induces the instability of blood vessels and facilitates angiogenesis in the presence of VEGF, and it is one of the poor prognostic factors of NSCLC (64). Moreover, another study revealed that Ang-2 levels but not Ang-1 levels are elevated in exudative pleural effusions, suggesting that Ang-2 along with VEGF participate in pleural inflammation and in the pathogenesis of exudative pleural effusions (27). Tomimoto et al confirmed that a high level of VEGF and Ang-2 coexisted in exudative effusion, especially in bloody effusion, and there was a significant correlation between the levels of VEGF and Ang-2 (65). Our group recently demonstrated that, while VEGF is one of the main mediators in exudative malignant pleural effusions, this effect is not mediated through the angiogenetic pathway of angiopoietin receptor Tie-2 (28). In addition, it has been shown that Ang-2 along with VEGF, but not Ang-1, is increased in pleural inflammation (28). However, the role of the above biological axis of VEGF and angiopoietins needs further investigation.