Pleural fluid osteopontin, vascular endothelial growth factor, and urokinase-type plasminogen activator levels as predictors of pleurodesis outcome and prognosticators in patients with malignant pleural effusion: a prospective cohort study

Recent research has elucidated the pivotal role of OPN in regulating cell signaling that controls tumor progression and metastasis. Enhanced OPN expression has been noted in the plasma of advanced lung cancer patients and associated with MPE [26‐29]. Moreover, OPN has been speculated to be involved in the formation of MPE [30]. On the other hand, VEGF is a mitogen for endothelial cells involved in angiogenesis and is a potent inducer of vascular permeability. It has also been reported as a key mediator in pleural fluid formation [9]. Together, OPN has been reported to up-regulate VEGF through the Akt activation and the induction of HIF-1α expression [31]. In the present study, the positively correlated pleural fluid OPN and VEGF concentrations across different sub-groups support the assumption that OPN can induce the release of VEGF and promote MPE formation [30, 32‐34].

The plasminogen activator system consists of plasminogen activators (urokinase and tissue-type plasminogen activators), their receptors, and their inhibitors. Plasminogen activators split plasminogen to plasmin, which can then break down extracellular matrix components. Thus, OPN-induced cancer cell migration and invasion requires uPA [35‐37], while VEGF can induce pro-uPA activation on the surface of endothelial cells. The angiogenic response of endothelial cells initiated by VEGF is accompanied by the assembly of cell surface-bound proteolytic machinery as a prerequisite for focal invasion [38, 39]. Stepwise analysis also identifies uPA as an independent factor correlated with micro-vessel density [40].

The positively correlated pleural fluid OPN and uPA, and VEGF and uPA concentrations in patients with MPE provide clinical evidences of prior laboratory findings. Additional experiments are needed to clarify the sequence of action of OPN, VEGF, and uPA in the formation of MPE, which may include quantification of pleural fluid mRNA by quantitative real-time PCR, immuno-histochemical staining of freshly isolated pleural cells, measurement of their production by cultured cells from pleural fluids, in situ mRNA hybridization, and zymography to evaluate the spatial and temporal expression [41‐44].

Tumor tissue expression and plasma concentration of OPN and VEGF have been correlated with advanced stage and poor survival in most common malignancies, although a small number of studies have also shown no correlation [45]. In patients with lung cancer and MPE, Zhang et al. reported that OPN in pleural effusion was an independent prognostic factor [46]. Kotyza et al. and Hooper et al. demonstrated a significant association between increased pleural fluid VEGF and reduced survival in malignant pleural effusion [47, 48]. Except for OPN in the lung cancer sub-group, the present study did not find any survival difference between sub-groups with higher and those with lower pleural fluid OPN, VEGF, or uPA concentrations in the others. The conflicting results may be attributed to the heterogeneity of the underlying malignancies, with different expected survival rates, small number of each cancer sub-group, and different specimens tested.

Survival analysis was also different across studies. While some employed the log-rank test applied to Kaplan Meier curves above and below the cut-off value of each parameter, others used Spearman’s rank correlation. Further validation of the pleural fluid processing, measurement, and unification of the statistical methods are necessary. The introduction of chemotherapy or targeted therapy before the presence of MPE also affected the survival analysis compared to chemotherapy- or targeted therapy-naïve patients with MPE as one of the first manifestations at diagnosis.

However, compared to OPN or uPA, VEGF is significantly higher in patients with MPE than in the controls, especially in patients with distant metastases in addition to MPE. The potential relevance of pleural fluid VEGF concentration in selecting patients undergoing adjuvant treatment targeting the VEGF pathway is a particularly interesting area for future research.

Although the uPA concentration is elevated in the MPE, the more substantially elevated concentrations of its inhibitors, namely PAI-1 and PAI-2, likely result in depressed fibrinolytic activity and favor the deposition and maintenance of intra-pleural fibrin. Pedersen et al. proposed that PAI-1 served to protect cancer tissue against the proteolytic degradation that the tumor imposed upon the surrounding normal tissue [49, 50]. In contrast, breast cancer patients with high uPA/PAI-1 ratio in tumor tissue extracts are reported to have increased relapse risk [23, 24]. The failure to counterbalance the exuberant production of uPA characteristic of tumor cells by the timely production of sufficient PAI-1 by host cells may contribute to neoplastic proliferation and formation of metastasis. The latter is more consistent with the hypothesis of the present study. However, there is no association of increased pleural fluid uPA concentrations or uPA/PAI-1 ratio with lower success rate of pleurodesis or shorter survival. Such contradictory results may be attributed to the case number of this study, which may be too small to discern differences and different specimens tested.

Pleurodesis is traditionally regarded as symptomatic treatment, with rare mention in literature of its benefit on survival. Hirata et al. reported that patients with breast cancer and MPE receiving systemic therapy following the initial talc pleurodesis had longer survival than those receiving systemic therapy alone [51]. Recently, Korsic et al. reported that patients who underwent successful talc pleurodesis had significantly longer survival than patients in the control group treated with serial thoracentesis, especially in the groups with breast cancer and better performance status [52]. Antony et al. demonstrated a strong apoptotic effect of talc on cells of malignant mesothelioma in vitro [53]. They also found that pleural mesothelial cells released great amounts of endostatin after pleurodesis [54].

Talc alters angiogenic balance in the pleural space from a biologically active and angiogenic environment to an angiostatic milieu. However, the above-mentioned survival benefit does not seem to be limited to talc. Patients with successful minocycline pleurodesis in this study also have significantly longer survival than those with failed pleurodesis (Fig. 1). The fibrin-generation potential of patients following installation of sclerosing agent is decisive for pleurodesis outcome and survival. Successfully induced pleural fibrosis is supposed to prohibit tumor invasion and metastasis. Recruiting more patients with MPE in the future may help to confirm these findings.

To date, this is the first study to prospectively evaluate the presentations and relationship among pleural fluid OPN, VEGF, and uPA concentrations in patients with MPE and examine their association with pleurodesis outcome and survival. There are several limitations that need to be overcome, including the small sample size and heterogeneity of underlying malignancies.