Radiotherapy is an effective therapy for localized prostate cancer (PrCa) but this disease is highly resistant to ionizing radiation (IR). Conventional radiotherapy doses up to 70 Gy show biochemical failure rates of 30% or more in localized disease , leading to a need for RT dose escalation, which is associated with rectal and bladder toxicity. Therefore, there is a need for rational development of effective radiosensitizers for PrCa.
The phosphatidylinositol 3-kinase (PI3k)-Protein kinase B/Akt (henceforth: Akt) pathway is known to promote proliferation, cell cycle progression and resistance to cytotoxic therapies in PrCa . PI3k is an effector of the epidermal growth factor receptor (EGFR) , that leads to recruitment of Akt and its activators to plasma membrane. Akt is activated by phosphorylation on residues T308 and serine S473, both of which are required for activation . T308 phosphorylation is mediated by the phosphoinoisitide-dependent kinase 1 (PDK1)  but the kinase mediating S473 phosphorylation (PDK2) is not clearly defined. Candidate kinases include the DNA damage sensor Ataxia Telangiectasia Mutated (ATM) . Activated Akt mediates transcription of genes involved in survival and inhibition of those involved in apoptosis (see [2, 7] for review). It promotes cell cycle progression through inhibition of the cell cycle regulators p53  and the cyclin-dependent kinase inhibitors (CDKI) p21cip1 and p27kip1 [9, 10]. Furthermore, it regulates metabolic and nuclear processes through activation of the mammalian target-of-rapamycin (mTOR). Importantly, IR elicits cytoprotective responses mediated in part through activation of the PI3k-Akt pathway . Akt is a mediator of radioresistance and PI3k-Akt pathway inhibitors are shown to enhance radiosensitivity of cancer cells [7, 12].
AMPK is a heterotrimeric enzyme that consists of an α-catalytic and β-and γ-regulatory subunits . It is a key regulator of carbohydrate and lipid metabolism and of proliferation in normal and cancer cells. AMPK detects an elevated AMP/ATP ratio in conditions of metabolic stress such as starvation and exercise  and promotes energy conservation by inhibiting protein synthesis, through mTOR inhibition while it also functions as a metabolic checkpoint to induce cell cycle arrest via p53 . Recently, we showed that IR activates AMPK in human lung, breast and PrCa cells and suggested that AMPK participates in a signaling pathway involving ATM-AMPK-p53-p21cip1 leading to regulation of the cell cycle and survival .
RSV (3,4',5-trihydroxystilbene) is a polyphenolic phytoalexin with widely reported anti-aging and anti-cancer properties [16, 17]. It inhibits cancer cell proliferation and is suggested to enhance radiation responses [18, 19]. RSV has also been reported to increase metabolic rate and reduce fat mass in wild-type mice but not in AMPK α subunit knockout mice . Further, it was shown to suppress tumor growth and metastasis in the mouse Lewis lung carcinoma model . RSV is known to regulate both Akt and AMPK [22, 23] but the effects of this compound on the two signaling pathways have not been studied in radiated cells.
Here, we investigated the polyphenol RSV due to the reported ability of this natural compound to modulate both the radioresistance-mediating Akt and the tumour suppressor AMPK pathways [24, 25].