Under stressed conditions, such as those found in PC, activated AMPK plays a major role in maintaining energy homeostasis by inducing ATP-producing catabolic pathways (33
). In this study, we compared the level of AMPK activity in normal and malignant prostate specimens by examining the phosphorylation status of the well characterized AMPK substrate ACC. As expected, low levels of P-ACC were detected in non-stressed normal human prostate tissues. To our knowledge, this is the first report that demonstrates prevalent AMPK activity in human prostate cancer specimens. This finding suggests that prostate cancer cells are energetically stressed due to their environment and the demands of continuous cell proliferation. The degree of AMPK activity varied among the cancerous specimens. This suggests that human prostate cancers vary in their levels of metabolic stress. Of note, we did not see a correlation between AMPK activity and tumor Gleason grade. This finding is consistent with microelectrode studies, reported by others, showing no relationship between pO2
levels and Gleason grade (10
). Alternatively, low AMPK activity in some of the cancerous specimens could be secondary to deficiencies in upstream kinases as seen in other cancer types (34
). The selective activation of AMPK in prostate cancer specimens raises the question of a possible connection between AMPK activation and prognosis which is currently being explored in our laboratory.
Surprisingly, we found that AMPK was highly activated not only in human PC specimens but also in human PC cells growing under standard tissue culture conditions. We hypothesize that AMPK activation is an important downstream effector of an unknown genomic and /or proteomic change found in transformed prostate epithelial cells (2
). Transformed cells demonstrate altered metabolism when compared to normal cells. One of the most fundamental metabolic alterations occurring with malignant transformation is the up-regulation of aerobic glycolysis, a phenomenon known as the Warburg effect (35
). Currently, the molecular mechanisms leading to constitutive upregulation of aerobic glycolysis are poorly understood (37
). Activated AMPK, a possible contributor to the Warburg effect, has been shown to promote glycolysis by enhancing glucose uptake (17
) and activating PFK-2 (17
). In addition, AMPK is known to induce numerous glycolytic genes (38
). Further studies are required to determine the relationship between transformation, aerobic glycolysis and AMPK activity in human PC.
The mechanism by which AMPK is activated in human prostate cancer cells is currently unclear. Under normal physiologic conditions, AMPK is activated under conditions that deplete cellular ATP such as glucose deprivation, heat shock, hypoxia, and ischemia (17
). However, AMPK activity may also be elevated under non-stressed conditions. For example, AMPK is activated by hormones like leptin, adiponectin and interleukin-6 (39
). These adipokines have been implicated in the development and progression of human prostate cancer (40
). LKB1 and CAMKKβ are central candidates for AMPK activation in prostate cancer since these enzymes are responsible for AMPK activation in non-cancerous tissues (17
). While LKB1 is a known tumor suppressor (41
), CAMKKβ activation has not been tied to prostate cancer progression. Alternatively, an unidentified kinase(s) could be acting as an AMP-activated kinase kinase in prostate cancer.
In normal cell physiology, AMPK activation has been proposed to protect cells from injurydue to hypoxia and other metabolic stressors (42
) by slowing cell growth and proliferation. The target-of-rapamycin (TOR) stimulates the initiation step of protein synthesis which is required for cell growth via phosphorylation of multiple targets (43
). TOR is activated by Akt phosphorylation of its binding partner Raptor and an upstream pathway involving tuberous sclerosis complex 2 (TSC2) (44
). AMPK inhibit TOR regulate protein synthesis via phosphorylation of Raptor (45
) and the TSC1-TSC2 complex (46
). To delay progression through the cell cycle, AMPK activation decreases expression of important cell cycle regulators (47
) and induces stabilization of p53 and cyclin-dependent kinase inhibitors (48
). In cancer cells, AMPK has been demonstrated to bestow tolerance to nutrient deprivation (50
) and hypoxia (33
) without restricting cell growth and proliferation. It has been proposed that AMPK promotes cancer cell survival by providing energy for essential cellular functions through processes such as fatty acid beta-oxidation and/or autophagy (50
). The opposing effect of AMPK activation in normal and transformed cells could be due to differential deletion of downstream tumor suppressors allowing AMPK activation, while mitigating the growth limiting effect of the enzyme. For example, some cancers harbor TSC2 and/or p53 mutations, allowing AMPK activation without inhibition of protein synthesis or cell cycle arrest (52
During the course of these experiments, reports suggesting that AICAR-induced AMPK activation inhibits proliferation, induces senescence and promotes apoptosis in various cancer cells were published (53
). It was proposed that this was secondary to AMPK inhibition of anabolic processes such as protein synthesis (45
) or activation of p53 (48
). While AICAR is the best characterized pharmacologic activator of AMPK and most of its effects have been prescribed to AMPK activation, AMPK-independent effects have been documented (26
). In an attempt to compare their findings with ours, we treated human prostate cancer cells with AICAR. In our hands, millimolar concentrations of AICAR induce S-phase arrest and senescence independent of AMPK activation (data not shown). These data suggest that decreased proliferation in response to AICAR is not secondary to AMPK activation but a non-specific effect of AICAR on nucleotide metabolism (26
). New selective AMPK activators (55
) may help to further define the role of AMPK in cancer therapy.
Protein kinase signaling pathways important in maintaining cell proliferation and survival under stressed conditions may provide the critical growth signals for premalignant lesions to progress to clinical prostate cancer. We show that AMPK is activated in primary prostate cancers and may promote prostate cancer proliferation and survival. The prevalent detection of activated AMPK in primary human prostate cancer specimens indicates that AMPK is a potential candidate molecular target for chemoprevention of prostate cancer.