Malignant pleural mesothelioma (MPM) is a rare, aggressive tumour. Despite recent advances in chemotherapy, it is associated with a universally poor prognosis. In 80% of cases, MPM can be attributed to asbestos fibre exposure with a median latency of at least 32 years [
1,
2]. Histologically MPM can be classified into epithelial, biphasic and sarcomatoid subtypes. The epithelial subtype is associated with longer survival when compared with sarcomatoid [
3,
4]. With the administration of cisplatin/pemetrexed the median survival of MPM is still only 12 months and currently there is no widely approved second line regimen after failure of first line treatment [
5‐
8].
Arachidonic acid is metabolised by lipoxygenase (LOX) enzymes to form leukotrienes (LTs) and by cyclooxygenase (COX) enzymes to form prostanoids, including prostaglandin E
2 (PGE
2) which has been implicated in inflammation and carcinogenesis [
9‐
13]. COX exists in two forms, COX-1 and COX-2. COX-2 is overexpressed in a wide variety of tumours and this feature has been correlated with the malignant properties of cancers. Inhibition of COX-2 has been reported to reverse malignant behaviour such as antiapoptosis, angiogenesis and invasion [
14,
15] and epidemiological evidence suggests that regular use of COX-2 inhibitors may reduce the risk of several cancers [
16]. We have previously shown using immunohistochemistry (IHC) that COX-2 is overexpressed in MPM and that the specific COX-2 inhibitor DuP-697 can potentiate the in vitro cytotoxic effects of pemetrexed in MPM cell lines [
4,
17]. Functional interactions between COX-2 and LOX enzymes have been identified [
18] and, of the three known LOX isoenzymes (5-LOX, 12-LOX and 15-LOX), 5-LOX and 12-LOX have also been implicated in carcinogenesis [
13,
19]. The 5-LOX enzyme interacts with 5-LOX activating protein (FLAP) and converts arachidonic acid into LTA4. LTA4 can be converted into 5-hydroxyeicosatetraenoic acid (HETE) or can be hydrolysed into LTB4 or LTC4 [
12,
20]. LTB4 has been shown to cause cell proliferation and survival via its action on the ERK pathway in colon cancer cell lines and via the PI3K-AKT and MAPK pathways in pancreatic cancer cell lines [
21,
22]. Arachidonic acid is converted into 12-HETE by its interaction with 12-LOX [
20]. The 12-HETE enzyme interacts with the NFkB pathway, resulting in an antiapoptotic effect, in prostate cancer cells [
23] and may interact with various growth factors resulting in angiogenesis, invasion and metastasis [
24]. The expression of 5-LOX and 12-LOX has been associated with carcinogenesis in various solid tumours. Hong and colleagues reported the first evidence of the potential role for 5-LOX in cancer through the demonstration of expression of 5-LOX and FLAP transcripts in different epithelial cancer cell lines [
25]. Increased expression of 5-LOX and LTB4 transcripts was later demonstrated in human pancreatic cancer cells, when compared with cultivated normal pancreatic ductal cells [
26] and a 6-fold upregulation of 5-LOX transcripts was demonstrated in prostate cancer tissue, when compared with a matched benign tissue sample [
27] The overexpression of 5-LOX protein was demonstrated in 22 matched samples of benign and malignant tissue obtained from patients with prostate cancer, when assessed by immunoblotting and IHC [
27]. This immunoblotting analysis demonstrated overexpression of 5-LOX protein in 73% of samples with a 2.6 fold greater expression in the malignant tissue and the immunohistochemical analysis confirmed the up-regulation of 5-LOX protein in malignant tissue [
27]. Further studies on the overexpression of LOX enzymes have since been documented in other epithelial cancers, including bladder [
28], breast [
29], colon [
30], melanoma [
31], oesophagus [
32,
33], and oral [
34]. The role of LOX enzymes in MPM has not been widely studied. 5-LOX transcripts were demonstrated to be upregulated in MPM cell lines, when compared with normal mesothelial cells, whilst the expression of 12-LOX transcripts was detected in both normal and malignant cells [
35]. We therefore aimed to investigate the importance of 5-LOX and 12-LOX protein expression in MPM and benign pleura tissue samples and to study the effects of in vitro inhibition of the arachidonic acid pathway in MPM cell lines.