br Materials and methods br Results br Discussion

Materials and methods
Results
Discussion Prostate cancer is the most common cancer in men in the United States and the second leading cause of cancer death. Therapeutic interventions include mainly: radical prostatectomy or radiation therapy (RT), with or without androgen deprivation therapy (ADT). However, nearly 30% of patients treated with potentially curative radiation doses relapse at the sites of irradiation and there is a need for new strategies to radiosensitize prostate cancer cells [21], [22]. Eicosanoids, the metabolites of arachidonic GSK2269557 mg (AA), are potent lipid mediators involved in a number of homeostatic biological functions and inflammation [23]. They are also important players in cancer [24]. Arachidonic acid is metabolized by the cyclooxygenase pathway to prostaglandins and thromboxane [25], by the lipoxygenase (LOX) pathway to leukotrienes [26], lipoxins [27], [28], hepoxilins [29], and hydroxyeicosatetraenoic acids (HETE) [2], [30], and by the epoxygenase pathway to epoxy eicosatetraenoic acids (EpETrE) [31], [32], [33]. Oxidative metabolism of AA by LOXs, especially 12-LOX, plays an important role in cancer pathophysiology [34], [35]. Many studies have demonstrated that 12-LOX and its metabolite 12(S) HETE regulate a plethora of biological functions such as cell survival, matrix adhesion [36], [37], cell motility [7] and secretion of matrix degrading enzymes [34]. Subsequently, in several cancer types where 12-LOX is active, it is involved in tumor progression and metastasis [34], [38], [39]. The work from our lab and others’ has identified 12-LOX as a regulator of the cell survival and cell cycle machineries in prostate cancer cells. We have elucidated the underlying mechanisms that affect cell cycle control by 12-LOX inhibitors. In particular, we have shown that 12-LOX inhibition results in cell cycle arrest in G0–G1, which subsequently induces caspase- and Bcl-2- dependent apoptosis [19]. Cell cycle and apoptosis are critical determinants of the response to various therapies, including radiation therapy [40], [41], [42], [43], [44]. As radiotherapy plays a paramount role in the treatment of prostate cancer, there is a great interest in enhancing the effect of radiation. Radiosensitivity of prostate cancer depends on the apoptotic propensity and hypoxia of cancer cells. It was shown that expression of the antiapoptotic proteins BCL2, BCL-XL (the BCL2/BAX ratio), clusterin and caspase-1 were associated with radiosensitivity [45], [46], [47]. Furthermore, BCL2 inhibitors have been shown to have radiosensitizing effects [48]. Building on the above-mentioned findings, we hypothesized that inhibiting the activity of 12-LOX in prostate cancer cells could be effective at promoting radiation-induced cytotoxicity. In this preclinical study we have shown that inhibition of 12-LOX activity by various selective inhibitors promoted the effect of radiation on human prostate cancer cells. All three human prostate cancer cell lines have been shown earlier to express 12-LOX and 15LO1 at various levels [38] and to produce 12-HETE [49]. The difference between these cell lines is that DU-145 showed only additive effect upon 12-LOX inhibition while PC-3 and LNCaP, showed synergistic effects. It is interesting that both hormone refractory and sensitive cell lines are radiosensitized by 12-LOX inhibition. Hormone sensitivity plays an important role in the treatment of prostate cancer and hormone-refractory cancer is known to have a worse prognosis [50]. Moreover, to apply curative radiotherapy for prostate cancer currently, it is advised to employ a 2–3months hormonal treatment prior to radiotherapy, in order to sensitize cancer cells to radiation [51]. The add-back experiments where 12(S)-HETE could suspend the effect of 12-LOX inhibition when administered alone or in conjunction with radiation, further confirm that this radiosensitization effect is mediated through the 12-LOX pathway. Using a cellular model of PC3 cells transfected with the platelet-type 12-LOX, we found that the latter increased the radioresistance significantly. These data demonstrate the translational validity of the 12-LOX inhibitors as radiosensitizers in prostate cancer. This is further confirmed by using a xenograft model, whereby 12-LOX inhibitors could efficiently potentiate the radioresponse of prostate cancer cells in vivo.