www.impactjournals.com/oncotarget/ Oncotarget, 2017, Vol. 8, (No. 56), pp: 95504-95515 Research Paper UCP2 overexpression enhanced glycolysis via activation of PFKFB2 during skin cell transformation Annapoorna Sreedhar1, Petra Petruska1, Sumitra Miriyala2, Manikandan Panchatcharam2 and Yunfeng Zhao1 1Department of Pharmacology, Toxicology & Neuroscience, LSU Health Sciences Center in Shreveport, Shreveport, LA 71130, USA 2Department of Anatomy and Cell Biology, LSU Health Sciences Center in Shreveport, Shreveport, LA 71130, USA Correspondence to: Yunfeng Zhao, email: [email protected] Keywords: mitochondrial uncoupling 2; PFKFB2; mitochondria; glycolysis; skin cells Received: June 24, 2017 Accepted: August 03, 2017 Published: September 08, 2017 Copyright: Sreedhar et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License 3.0 (CC BY 3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. ABSTRACT Uncoupling protein 2 (UCP2) is an inner mitochondrial membrane transporter which is often upregulated in human cancers. However, how this anion transporter affects tumorigenesis is not well understood. Using the skin cell transformation JB6 model, we demonstrated that UCP2 overexpression activated phosphofructokinase 2/ fructose-2,6-bisphosphatase 2 (PFKFB2), a key regulator of glycolysis. In conjunction, upregulation of PFKFB2 expression correlated with elevated fructose 2,6-bisphosphate (Fru-2,6-P2) levels, 6-phosphofructo-1-kinase (PFK-1) activity, glucose uptake, and lactate production. Inhibiting PFKFB2 expression suppressed UCP2-mediated skin cell transformation, decreased cell proliferation, and enhanced mitochondrial respiration, while dampening aerobic glycolysis. The AKT signaling pathway was activated in the UCP2 overexpressed cells; furthermore, the activated AKT signaling contributed to the activation of PFKFB2. Whereas AKT inactivation blocked PFKFB2 activation, suggesting that AKT activation is an important step in PFKFB2 activation. Collectively, our data suggest that UCP2 is a critical regulator of cellular metabolism during cell transformation. Our data also demonstrate a potentially novel mechanism to understand UCP2’s tumor-promoting role, which is through the AKT-dependent activation of PFKFB2 and thereby, the metabolic shift to glycolysis (the Warburg effect). INTRODUCTION According to a recent review by Hanahan and Weinberg [1], much attention has been given to a Unlike normal cells, cancer cells are characterized dysregulated metabolism as an emerging hallmark by uncontrolled growth, enhanced proliferation, and of cancer cells with a primary focus directed at the replicative immortality [1]. Cancer cells often reprogram dependence of tumor cells on enhanced glycolysis [10– their metabolism to fuel their accelerated growth [2]. This 13]. This realization has brought renewed attention to the observation was first made by Otto Warburg, one of the Warburg effect, and this metabolic alteration of tumor pioneers in the field of cancer metabolism, who claimed cells is being extensively studied. that the root cause of cancer is too much acidity in the Cancer cells undergo various metabolic and body [3–5]. In other words, the Warburg effect is the autonomous changes that drive tumorigenesis [11, 14]. In observation that cancer cells heavily depend on glycolysis addition to our work, other studies have also suggested leading to lactic acid production even in the presence of that one such driver of tumorigenesis is the mitochondrial oxygen [6–7]. However, the definitive reason for this uncoupling protein 2 (UCP2) [15–16]. Uncoupling observation remained debatable [8–9]. proteins are a family of anion transporters present www.impactjournals.com/oncotarget 95504 Oncotarget in the inner mitochondrial membrane whose protein RESULTS expressions are closely related to changes during cell proliferation and tumorigenesis [16–19]. We and others Glycolysis versus mitochondrial respiration in have demonstrated that UCP2 is often highly expressed UCP2 overexpressed cells in human cancers [20–22]. Although the exact role of UCP2 during carcinogenesis remains elusive, studies To understand how upregulated UCP2 affects from our lab (manuscript under review) and others’ cellular bioenergetics, oxygen consumption rate (OCR) research demonstrate that UCP2 is a crucial player in and extracellular acidification rate (ECAR), as indicators of the cascade of mitochondrial molecular events, redox mitochondrial respiration and glycolytic rate, respectively, regulation, cell cycle, cell proliferation, cell survival, were measured. The measurements were taken using a and apoptosis [23–26]. Since UCP2 overexpression is Seahorse Bioscience XF Analyzer in control pCMV cells closely associated with enhanced cell proliferation and and UCP2 overexpressed cells [35–36]. Modulators of tumorigenesis, targeting UCP2 expression may be a oxidative phosphorylation such as oligomycin, FCCP, winning therapeutic strategy to treat UCP2 overexpressed and antimycin/rotenone were added sequentially to the cancers. Moreover, it was recently reported that UCP2 cells and OCR was determined. The basal OCR rates is not just an uncoupling protein but also a metabolite were 255 pmol/min in pCMV cells and 170 pmol/min transporter [27]. This observation would put UCP2 in in UCP2 overexpressed cells; and TPA treatment further a position to influence cellular metabolism in favor of reduced the basal OCR rates (Figure 1A). To determine tumorigenesis. Hence, it is important to understand the the maximal capacity of these cells, p-trifluoromethoxy molecular basis of UCP2 overexpression in tumor cell carbonyl cyanide phenyl hydrazone (FCCP), an uncoupler metabolism, differentiation, and survival. Therefore, in an was added to the respiring cells. The maximal respiratory attempt to understand how UCP2 regulates cellular energy rate as determined by FCCP was significantly lower in metabolism during skin carcinogenesis, we performed a the UCP2 overexpressed cells compared to the control protein microarray analysis using the skin tumor tissues pCMV cells (Figure 1A). Mitochondrial reserve capacity, obtained from wild-type and UCP2 knockout mice at the amount of oxygen consumption that is available for the end of a skin carcinogenesis study [28]. In this study, cells to use in times of stress or increased ATP demand, we identified that PFKFB2 was upregulated in skin was calculated by subtracting basal respiration from tumor tissues of the wild-type mice but not in the UCP2 maximal respiration (FCCP). As shown in Figure 1A, knockout mice, suggesting that PFKFB2 expression may reserve capacity was significantly decreased in the UCP2 be positively regulated by UCP2 [28]. PFKFBs are a overexpressed cells compared to the control pCMV cells family of bifunctional enzymes that control the levels of and TPA treatment further reduced the reserve capacity. fructose 2,6-bisphosphate (Fru-2,6-P2). They are termed These results show that OCR is significantly dampened ‘bifunctional’ because they catalyze the synthesis and in UCP2 overexpressed cells. To determine whether the degradation of Fru-2,6-P2. Fru-2,6-P2 is a powerful the decreased OCR was compensated with an elevated allosteric activator of 6-phosphofructo-1-kinase (PFK- glycolytic rate, ECAR was also measured in these cells. 1), a key enzyme in controlling the glycolytic flux. As shown in Figure 1B, in contrast to the OCR results, PFK-1 catalyzes the conversion of Fru-6-P (fructose 6- glycolysis was at maximum in the presence of glucose in phosphate) into Fru-1,6-P2. [29–30]. Therefore, increased the UCP2 overexpressed cells and TPA treatment further levels of Fru-2,6-P2 would allow cells to maintain a high elevated the glycolytic rates. Basal glycolysis, glycolytic glycolytic flux. In mammals, there are four isoforms of capacity, and glycolytic reserve in UCP2 overexpressed PFKFB genes (PFKFB1, PFKB2, PFKFB3, and PFKB4) cells were significantly higher than that of control pCMV which are characterized by tissue expression [31]. In fact, cells. Therefore, both the OCR and ECAR data together PFKFB2, PFKFB3, and PFKFB4 are overexpressed in the suggest that mitochondrial respiration is dampened, majority cancers, leading to a higher concentration of Fru- whereas glycolysis is enhanced in UCP2 overexpressed 2,6-P2 and increased glycolytic flux to lactate [32–34]. cells; therefore, UCP2 overexpression may cause a switch Given that PFKFB2 was activated in wild-type from mitochondrial respiration to glycolysis. Furthermore, mice but not in UCP2 knockout mice during the skin intracellular ATP levels were elevated in the UCP2 carcinogenesis study [28], we speculated that UCP2 overexpressed cells (Figure 1C), suggesting that UCP2 overexpression may bring a metabolic shift towards overexpression may serve as an important bioenergetic glycolysis by activating PFKFB2 as a potential novel regulator which enhances glycolysis to produce more mechanism of tumor promotion. Utilizing the well- ATP to meet the need for proliferation. The increase in the established JB6 skin cell transformation model, we have levels of lactate (Figure 1D) in the UCP2 overexpressed performed mechanistic studies to determine whether cells is further confirmation of the Warburg effect PFKFB2 activation is important for skin tumorigenesis observed in UCP2 overexpressed cells. and how UCP2 overexpression regulated PFKFB2 activation. www.impactjournals.com/oncotarget