A high rate of glycolytic flux, even in the presence of oxygen, is a central metabolic hallmark of neoplastic tumors. Cancer cells preferentially utilize glycolysis in order to satisfy their increased energetic and biosynthetic requirements. This metabolic phenotype is confirmed by positron emission tomography (PET) with 2-[18F]-fluoro-2-deoxy-glucose which demonstrates that tumors take up 10-fold more glucose than adjacent normal tissues in vivo. Over-expression of HIF-1a and myc, ras activation and loss of p53 function stimulate glycolysis by activating a family of four bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB) which synthesize fructose-2,6-bisphosphate (F2,6BP), an allosteric activator of 6-phosphofructo-1- kinase (PFK-1) which is an essential control point in the glycolytic pathway. The PFKFB enzymes are encoded by four genes (PFKFB1-4) reportedly expressed in distinct tissues: liver/skeletal muscle (PFKFB1), heart (PFKFB2), placenta (PFKFB3) and testis (PFKFB4). Previous studies have focused on the PFKFB3 isozyme as the dominant source of F2,6BP in neoplastic cells due to its high kinase:phosphatase ratio (~740:1) and overexpression in multiple tumors. The identification of the specific PFKFB isozyme responsible for the high rate of glycolysis in cancer cells may allow for the development of novel agents that suppress tumor growth through inhibition of glycolysis. In preliminary studies, we measured mRNA expression of the four PFKFB isozymes in 20 tumor tissues by multiplex RT-PCR and found that PFKFB4 mRNA (and not PFKFB3) was markedly increased in 17/20 tumors relative to adjacent, normal tissues. We speculated that the high PFKFB4 expression may be caused by disparate oncogenic pathways converging to enhance glycolytic flux. We found that hypoxic exposure of K-rasG12S+ A549 lung adenocarcinoma cells and introduction of H-rasG12V into immortalized human bronchial epithelial cells increased PFKFB4 mRNA and protein expression. Further, cell fractionation revealed that the PFKFB4 protein, and not PFKFB3, localized to the cytoplasm, the cellular compartment of both PFK-1 and glycolysis. Last, transient siRNA silencing of PFKFB4 mRNA expression in K- rasG12S+ A549 cells decreased steady-state concentration of F2,6BP, glycolytic flux and anchorage- independent tumor growth in mice. Taken together, these preliminary studies provide rationale for further investigation of the PFKFB4 isozyme as a possible novel target for the development of anti-neoplastic agents. We hypothesize that the PFKFB4 isozyme is required for the high glycolytic flux, survival, growth and spread of transformed cells. We plan to test this hypothesis by pursuing the following specific aims: 1. To examine the requirement for F2,6BP synthesized by PFKFB4 for glycolytic and mitochondrial metabolism. 2. To determine the requirement for F2,6BP synthesized by PFKFB4 for growth, invasiveness and survival of normal, immortalized and ras-transformed epithelial cells. 3. To examine the effect of PFKFB4 genomic deletion on growth and metabolism of ras-dependent tumors in vivo using a Cre-lox inducible mouse knockout of PFKFB4.
We anticipate that shRNA silencing or genomic deletion of the PFKFB4 isozyme will attenuate the neoplastic potential of transformed epithelial cells and conversely, that increased F2,6BP (from overexpression of PFKFB4) will enhance glycolytic flux and augment tumor growth. These findings may validate PFKFB4 as a new target for the inhibition of glycolysis. Accordingly, suppression of glycolytic flux in cancer via the development of small molecule inhibitors of PFKFB4 through computational screening for competitive inhibitors of the PFKFB4 substrate-binding domain may reduce the high mortality that is associated with cancer.
