LOH on 10p14-p15 targets the PFKFB3 gene locus in human glioblastomas

Canonical and Non-Canonical Roles of PFKFB3 in Brain Tumors

PFKFB3 is a bifunctional enzyme that modulates and maintains the intracellular concentrations of fructose-2,6-bisphosphate (F2,6-P2), essentially controlling the rate of glycolysis. PFKFB3 is a known activator of glycolytic rewiring in neoplastic cells, including central nervous system (CNS) neoplastic cells. The pathologic regulation of PFKFB3 is invoked via various microenvironmental stimuli and oncogenic signals. Hypoxia is a primary inducer of PFKFB3 transcription via HIF-1alpha. In addition, translational modifications of PFKFB3 are driven by various intracellular signaling pathways that allow PFKFB3 to respond to varying stimuli. PFKFB3 synthesizes F2,6P2 through the phosphorylation of F6P with a donated PO4 group from ATP and has the highest kinase activity of all PFKFB isoenzymes. The intracellular concentration of F2,6P2 in cancers is maintained primarily by PFKFB3 allowing cancer cells to evade glycolytic suppression. PFKFB3 is a primary enzyme responsible for glycolytic tumor metabolic reprogramming. PFKFB3 protein levels are significantly higher in high-grade glioma than in non-pathologic brain tissue or lower grade gliomas, but without relative upregulation of transcript levels. High PFKFB3 expression is linked to poor survival in brain tumors. Solitary or concomitant PFKFB3 inhibition has additionally shown great potential in restoring chemosensitivity and radiosensitivity in treatment-resistant brain tumors. An improved understanding of canonical and non-canonical functions of PFKFB3 could allow for the development of effective combinatorial targeted therapies for brain tumors.

The Role of HIF1α-PFKFB3 Pathway in Diabetic Retinopathy

Diabetic retinopathy (DR) is the leading cause of blindness for adults in developed countries. Both microvasculopathy and neurodegeneration are implicated in mechanisms of DR development, with neuronal impairment preceding microvascular abnormalities, which is often underappreciated in the clinic. Most current therapeutic strategies, including anti-vascular endothelial growth factor (anti-VEGF)-antibodies, aim at treating the advanced stages (diabetic macular edema and proliferative diabetic retinopathy) and fail to target the neuronal deterioration. Hence, new therapeutic approach(es) intended to address both vascular and neuronal impairment are urgently needed. The hypoxia-inducible factor 1α (HIF1α)-6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) pathway is critically implicated in the islet pathology of diabetes. Recent evidence highlighted the pathway relevance for pathologic angiogenesis and neurodegeneration, two key aspects in DR. PFKFB3 is key to the sprouting angiogenesis, along with VEGF, by determining the endothelial tip-cell competition. Also, PFKFB3-driven glycolysis compromises the antioxidative capacity of neurons leading to neuronal loss and reactive gliosis. Therefore, the HIF1α-PFKFB3 signaling pathway is unique as being a pervasive pathological component across multiple cell types in the retina in the early as well as late stages of DR. A metabolic point-of-intervention based on HIF1α-PFKFB3 targeting thus deserves further consideration in DR.

Functional diversity of PFKFB3 splice variants in glioblastomas

Tumor cells tend to metabolize glucose through aerobic glycolysis instead of oxidative phosphorylation in mitochondria. One of the rate limiting enzymes of glycolysis is 6-phosphofructo-1-kinase, which is allosterically activated by fructose 2,6-bisphosphate which in turn is produced by 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2 or PFKFB). Mounting evidence suggests that cancerous tissues overexpress the PFKFB isoenzyme, PFKFB3, being causing enhanced proliferation of cancer cells. Initially, six PFKFB3 splice variants with different C-termini have been documented in humans. More recently, additional splice variants with varying N-termini were discovered the functions of which are to be uncovered. Glioblastoma is one of the deadliest forms of brain tumors. Up to now, the role of PFKFB3 splice variants in the progression and prognosis of glioblastomas is only partially understood. In this study, we first re-categorized the PFKFB3 splice variant repertoire to simplify the denomination. We investigated the impact of increased and decreased levels of PFKFB3-4 (former UBI2K4) and PFKFB3-5 (former variant 5) on the viability and proliferation rate of glioblastoma U87 and HEK-293 cells. The simultaneous knock-down of PFKFB3-4 and PFKFB3-5 led to a decrease in viability and proliferation of U87 and HEK-293 cells as well as a reduction in HEK-293 cell colony formation. Overexpression of PFKFB3-4 but not PFKFB3-5 resulted in increased cell viability and proliferation. This finding contrasts with the common notion that overexpression of PFKFB3 enhances tumor growth, but instead suggests splice variant-specific effects of PFKFB3, apparently with opposing effects on cell behaviour. Strikingly, in line with this result, we found that in human IDH-wildtype glioblastomas, the PFKFB3-4 to PFKFB3-5 ratio was significantly shifted towards PFKFB3-4 when compared to control brain samples. Our findings indicate that the expression level of distinct PFKFB3 splice variants impinges on tumorigenic properties of glioblastomas and that splice pattern may be of important diagnostic value for glioblastoma.

PFKFB3: A Potential Key to Ocular Angiogenesis

The current treatment for ocular pathological angiogenesis mainly focuses on anti-VEGF signals. This treatment has been confirmed as effective despite the unfavorable side effects and unsatisfactory efficiency. Recently, endothelial cell metabolism, especially glycolysis, has been attracting attention as a potential treatment by an increasing number of researchers. Emerging evidence has shown that regulation of endothelial glycolysis can influence vessel sprouting. This new evidence has raised the potential for novel treatment targets that have been overlooked for a long time. In this review, we discuss the process of endothelial glycolysis as a promising target and consider regulation of the enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as treatment for ocular pathological angiogenesis.

Role of PFKFB3 and PFKFB4 in Cancer: Genetic Basis, Impact on Disease Development/Progression, and Potential as Therapeutic Targets

Glycolysis is a crucial metabolic process in rapidly proliferating cells such as cancer cells. Phosphofructokinase-1 (PFK-1) is a key rate-limiting enzyme of glycolysis. Its efficiency is allosterically regulated by numerous substances occurring in the cytoplasm. However, the most potent regulator of PFK-1 is fructose-2,6-bisphosphate (F-2,6-BP), the level of which is strongly associated with 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase activity (PFK-2/FBPase-2, PFKFB). PFK-2/FBPase-2 is a bifunctional enzyme responsible for F-2,6-BP synthesis and degradation. Four isozymes of PFKFB (PFKFB1, PFKFB2, PFKFB3, and PFKFB4) have been identified. Alterations in the levels of all PFK-2/FBPase-2 isozymes have been reported in different diseases. However, most recent studies have focused on an increased expression of PFKFB3 and PFKFB4 in cancer tissues and their role in carcinogenesis. In this review, we summarize our current knowledge on all PFKFB genes and protein structures, and emphasize important differences between the isoenzymes, which likely affect their kinase/phosphatase activities. The main focus is on the latest reports in this field of cancer research, and in particular the impact of PFKFB3 and PFKFB4 on tumor progression, metastasis, angiogenesis, and autophagy. We also present the most recent achievements in the development of new drugs targeting these isozymes. Finally, we discuss potential combination therapies using PFKFB3 inhibitors, which may represent important future cancer treatment options.

Prognostic Value of PFKFB3 to PFKFB4 mRNA Ratio in Patients With Primary Glioblastoma (IDH-Wildtype)

A hallmark of glioblastoma is the high level of aerobic glycolysis. PFKFB3 and PFKFB4 are regulatory glycolytic enzymes, which are overexpressed in glioblastomas. Selective inhibition of these enzymes has emerged as a new approach in tumor therapy. We investigated the ratios of PFKFB3 to PFKFB4 mRNA expression in 66 astrocytic tumors of different malignancy grades. PFKFB3 mRNA levels were considerably higher than those of PFKFB4 in all analyzed tumors. IDH-wildtype glioblastomas showed lower PFKFB3 to PFKFB4 mRNA ratios (7.7:1) than IDH-mutant low-grade astrocytomas (36.5:1), indicating a dependency of the ratio on malignancy grade. In IDH-wildtype glioblastomas exhibiting loss of heterozygosity (LOH) of the PFKFB3 gene locus, the decrease of PFKFB3 mRNA levels was accompanied by lower PFKFB4 mRNA levels, but the PFKFB3 to PFKFB4 mRNA ratio did not differ between tumors with or without PFKFB3 LOH. IDH-wildtype primary glioblastoma patients with high PFKFB3 to PFKFB4 mRNA ratios above the average of 7.7:1 had a significantly longer overall survival time (14 months) than patients with lower ratios (9 months). Our results indicate that low PFKFB3 to PFKFB4 expression ratio is a poor prognostic factor in patients with IDH-wildtype primary glioblastoma and that PFKFB3 and PFKFB4 might represent promising targets for astrocytoma and glioblastoma treatment.

Comparable Genomic Copy Number Aberrations Differ across Astrocytoma Malignancy Grades

A collection of intracranial astrocytomas of different malignancy grades was analyzed for copy number aberrations (CNA) in order to identify regions that are driving cancer pathogenesis. Astrocytomas were analyzed by Array Comparative Genomic Hybridization (aCGH) and bioinformatics utilizing a Bioconductor package, Genomic Identification of Significant Targets in Cancer (GISTIC) 2.0.23 and DAVID software. Altogether, 1438 CNA were found of which losses prevailed. On our total sample, significant deletions affected 14 chromosomal regions, out of which deletions at 17p13.2, 9p21.3, 13q12.11, 22q12.3 remained significant even at 0.05 q-value. When divided into malignancy groups, the regions identified as significantly deleted in high grades were: 9p21.3; 17p13.2; 10q24.2; 14q21.3; 1p36.11 and 13q12.11, while amplified were: 3q28; 12q13.3 and 21q22.3. Low grades comprised significant deletions at 3p14.3; 11p15.4; 15q15.1; 16q22.1; 20q11.22 and 22q12.3 indicating their involvement in early stages of tumorigenesis. Significantly enriched pathways were: PI3K-Akt, Cytokine-cytokine receptor, the nucleotide-binding oligomerization domain (NOD)–like receptor, Jak-STAT, retinoic acid-inducible gene (RIG)-I-like receptor and Toll-like receptor pathways. HPV and herpex simplex infection and inflammation pathways were also represented. The present study brings new data to astrocytoma research amplifying the wide spectrum of changes that could help us identify the regions critical for tumorigenesis.

Fructose 2,6-Bisphosphate in Cancer Cell Metabolism

For a long time, pioneers in the field of cancer cell metabolism, such as Otto Warburg, have focused on the idea that tumor cells maintain high glycolytic rates even with adequate oxygen supply, in what is known as aerobic glycolysis or the Warburg effect. Recent studies have reported a more complex situation, where the tumor ecosystem plays a more critical role in cancer progression. Cancer cells display extraordinary plasticity in adapting to changes in their tumor microenvironment, developing strategies to survive and proliferate. The proliferation of cancer cells needs a high rate of energy and metabolic substrates for biosynthesis of biomolecules. These requirements are met by the metabolic reprogramming of cancer cells and others present in the tumor microenvironment, which is essential for tumor survival and spread. Metabolic reprogramming involves a complex interplay between oncogenes, tumor suppressors, growth factors and local factors in the tumor microenvironment. These factors can induce overexpression and increased activity of glycolytic isoenzymes and proteins in stromal and cancer cells which are different from those expressed in normal cells. The fructose-6-phosphate/fructose-1,6-bisphosphate cycle, catalyzed by 6-phosphofructo-1-kinase/fructose 1,6-bisphosphatase (PFK1/FBPase1) isoenzymes, plays a key role in controlling glycolytic rates. PFK1/FBpase1 activities are allosterically regulated by fructose-2,6-bisphosphate, the product of the enzymatic activity of the dual kinase/phosphatase family of enzymes: 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase (PFKFB1-4) and TP53-induced glycolysis and apoptosis regulator (TIGAR), which show increased expression in a significant number of tumor types. In this review, the function of these isoenzymes in the regulation of metabolism, as well as the regulatory factors modulating their expression and activity in the tumor ecosystem are discussed. Targeting these isoenzymes, either directly or by inhibiting their activating factors, could be a promising approach for treating cancers.

The potential utility of PFKFB3 as a therapeutic target

INTRODUCTION

It has been known for over half a century that tumors exhibit an increased demand for nutrients to fuel their rapid proliferation. Interest in targeting cancer metabolism to treat the disease has been renewed in recent years with the discovery that many cancer-related pathways have a profound effect on metabolism. Considering the recent increase in our understanding of cancer metabolism and the enzymes and pathways involved, the question arises as to whether metabolism is cancer's Achilles heel. Areas covered: This review summarizes the role of 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) in glycolysis, cell proliferation, and tumor growth, discussing PFKFB3 gene and isoenzyme regulation and the changes that occur in cancer and inflammatory diseases. Pharmacological options currently available for selective PFKFB3 inhibition are also reviewed. Expert opinion: PFKFB3 plays an important role in sustaining the development and progression of cancer and might represent an attractive target for therapeutic strategies. Nevertheless, clinical trials are needed to follow up on the promising results from preclinical studies with PFKFB3 inhibitors. Combination therapies with PFKFB3 inhibitors, chemotherapeutic drugs, or radiotherapy might improve the efficacy of cancer treatments targeting PFKFB3.

Roles of PFKFB3 in cancer

The understanding of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFK-2/FBPase 3, PFKFB3) has advanced considerably since its initial identification in human macrophages in the mid-1990s. As a vital regulator of glycolysis, accumulating studies have suggested that PFKFB3 is associated with many aspects of cancer, including carcinogenesis, cancer cell proliferation, vessel aggressiveness, drug resistance and tumor microenvironment. In this review, we summarize current knowledge of PFKFB3 regulation by several signal pathways and its function in cancer development in different cell types in cancer tissues. Ubiquitous PFKFB3 has emerged as a potential target for anti-neoplastic therapy.

Observations of the genomic landscape beyond 1p19q deletions and EGFR amplification in glioma

BACKGROUND

With recent advancements in molecular techniques, the opportunities to gather whole genome information have increased, even in degraded samples such as FFPE tissues. As a result, a broader view of the genomic landscape of solid tumors may be explored. Whole genome copy number and loss of heterozygosity patterns can advance our understanding of mechanisms and complexity of various tumors.

RESULTS

Genome-wide alterations involving copy number changes and loss of heterozygosity were identified in 17 glioma samples with positive FISH results for 1p19q co-deletions (n = 9) or EGFR amplification (n = 8). Gliomas positive for 1p19q co-deletions did not have other frequently recurrent genomic alterations. Additional copy-number alterations were observed in individual cases, and consisted primarily of large-scale changes, including gains or losses of entire chromosomes. The genomic architecture of EGFR amplified gliomas was much more complex, with a high number of gains and losses across the genome. Recurrent alterations in EGFR amplified gliomas were both focal, such as CDKN2A homozygous deletions, and large, such as chromosome 10 loss.

CONCLUSIONS

Microarray enabled a broader picture of the genomic alterations occurring in glioma cases. Gliomas with 1p19q co-deletion had a relatively quiet genome, apart from the selected co-deletion. Additional alterations in isolated cases, involved primarily larger aberrations. On the other hand, EGFR amplified cases tended to be more complex and have specific abnormalities associated with the EGFR amplification. Furthermore, 1p19q co-deletions and EGFR amplification associated copy number changes appeared to often be mutually exclusive.

SORBS2 and TLR3 induce premature senescence in primary human fibroblasts and keratinocytes

Background

Genetic aberrations are required for the progression of HPV-induced cervical precancers. A prerequisite for clonal expansion of cancer cells is unlimited proliferative capacity. In a cell culture model for cervical carcinogenesis loss of genes located on chromosome 4q35→qter and chromosome 10p14-p15 were found to be associated with escape from senescence. Moreover, by LOH and I-FISH analyses a higher frequency of allele loss of these regions was also observed in cervical carcinomas as compared to CIN3. The aim of this study was to identify candidate senescence-related genes located on chromosome 4q35→qter and chromosome 10p14-p15 which may contribute to clonal expansion at the transition of CIN3 to cancer.

Methods

Microarray expression analyses were used to identify candidate genes down-regulated in cervical carcinomas as compared to CIN3. In order to relate these genes with the process of senescence their respective cDNAs were overexpressed in HPV16-immortalized keratinocytes as well as in primary human fibroblasts and keratinocytes using lentivirus mediated gene transduction.

Results

Overall fifteen genes located on chromosome 4q35→qter and chromosome 10p14-p15 were identified. Ten of these genes could be validated in biopsies by RT-PCR. Of interest is the novel finding that SORBS2 and TLR3 can induce senescence in primary human fibroblasts and keratinocytes but not in HPV-immortalized cell lines. Intriguingly, the endogenous expression of both genes increases during finite passaging of primary keratinocytes in vitro.

Conclusions

The relevance of the genes SORBS2 and TLR3 in the process of cellular senescence warrants further investigation. In ongoing experiments we are investigating whether this increase in gene expression is also characteristic of replicative senescence.

Balancing glycolytic flux: the role of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatases in cancer metabolism

The increased glucose metabolism in cancer cells is required to fulfill their high energetic and biosynthetic demands. Changes in the metabolic activity of cancer cells are caused by the activation of oncogenes or loss of tumor suppressors. They can also be part of the metabolic adaptations to the conditions imposed by the tumor microenvironment, such as the hypoxia response. Among the metabolic enzymes that are modulated by these factors are the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatases (PFKFBs), a family of bifunctional enzymes that control the levels of fructose 2,6-bisphosphate (Fru-2,6-P2). This metabolite is important for the dynamic regulation of glycolytic flux by allosterically activating the rate-limiting enzyme of glycolysis phosphofructokinase-1 (PFK-1). Therapeutic strategies designed to alter the levels of this metabolite are likely to interfere with the metabolic balance of cancer cells, and could lead to a reduction in cancer cell proliferation, invasiveness and survival. This article will review our current understanding of the role of PFKFB proteins in the control of cancer metabolism and discuss the emerging interest in these enzymes as potential targets for the development of antineoplastic agents.