Characterization of a new liver- and kidney-specific pfkfb3 isozyme that is downregulated by cell proliferation and dedifferentiation

Repurposing PDE5 inhibitor tadalafil and sildenafil as anticancer agent against hepatocellular carcinoma via targeting key events of glucose metabolism and multidrug resistance

Hepatocellular carcinoma (HCC) has emerged as one of the most common and lethal cancers worldwide and is caused due to contamination of diets with aflatoxin B1 and chronic viral hepatitis. Recent reports suggest that phosphodiesterase-5 inhibitor (PDE5i) exhibits anticancer properties against several forms of cancer but till now has not been evaluated against HCC. We aimed to evaluate the anticancer property of phosphodiesterase-5 inhibitors (PDE5i) tadalafil and sildenafil against aflatoxin B1 HCC. Rats of HCC group were fed with 5% alcohol via drinking water for 3 weeks, followed by administration of AFB1 (1 mg/kg/bw, i.p.) at an interval of two subsequent days. PDE5i (tadalafil and sildenafil, 10 mg/kg bw) was administered along with drinking water after 6 weeks of treatment with AFB1 for 2 weeks. In the present investigation, in HCC elevation in the level of SGOT, SGPT, ALP, and urea vis-à-vis activity of key glycolytic enzyme LDH and mRNA expression of c-myc, Akt, LDH-A, and PFKFB3 was noted. Similarly, the level of multidrug resistance protein (MDR) and breast cancer resistance protein (BCRP/ABCG2) was elevated along with increased expression of angiogenesis marker (HIF-1α, VEGF, and TGF-β1) in HCC. Post-treatment with PDE5 inhibitor (tadalafil and sildenafil) downregulated and brought back the above parameters towards normal and out of two PDE5i (tadalafil and sildenafil), sildenafil effect was more potent as compared to tadalafil. Our findings demonstrate for the first time that PDE5 inhibitors tadalafil and sildenafil are able to prohibit the development and progression of aflatoxin B1 induced HCC.

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.

Glycolysis, tumor metabolism, cancer growth and dissemination. A new pH-based etiopathogenic perspective and therapeutic approach to an old cancer question

Cancer cells acquire an unusual glycolytic behavior relative, to a large extent, to their intracellular alkaline pH (pHi). This effect is part of the metabolic alterations found in most, if not all, cancer cells to deal with unfavorable conditions, mainly hypoxia and low nutrient supply, in order to preserve its evolutionary trajectory with the production of lactate after ten steps of glycolysis. Thus, cancer cells reprogram their cellular metabolism in a way that gives them their evolutionary and thermodynamic advantage. Tumors exist within a highly heterogeneous microenvironment and cancer cells survive within any of the different habitats that lie within tumors thanks to the overexpression of different membrane-bound proton transporters. This creates a highly abnormal and selective proton reversal in cancer cells and tissues that is involved in local cancer growth and in the metastatic process. Because of this environmental heterogeneity, cancer cells within one part of the tumor may have a different genotype and phenotype than within another part. This phenomenon has frustrated the potential of single-target therapy of this type of reductionist therapeutic approach over the last decades. Here, we present a detailed biochemical framework on every step of tumor glycolysis and then proposea new paradigm and therapeutic strategy based upon the dynamics of the hydrogen ion in cancer cells and tissues in order to overcome the old paradigm of one enzyme-one target approach to cancer treatment. Finally, a new and integral explanation of the Warburg effect is advanced.

Sertoli-secreted FGF-2 induces PFKFB4 isozyme expression in mouse spermatogenic cells by activation of the MEK/ERK/CREB pathway

Sertoli cells play a central role in the control and maintenance of spermatogenesis by secreting growth factors, in response to hormonal stimulation, that participate in the paracrine regulation of this process. In this study, we investigated how the hormonal regulation of spermatogenesis modulates 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB) isozyme expression in two mouse spermatogenic cell lines, GC-1 spg and GC-2 spd (ts). For this purpose, TM4 Sertoli cells were used to obtain conditioned medium that was treated or not with dihydrotestosterone for 2 days [dihydrotestosterone conditioned medium (TCM) and basal conditioned medium (BCM), respectively]. We observed an increase in the expression of PFKFB4 along with a decrease in PFKFB3 in spermatogenic cell lines treated with TCM. These effects were inhibited by the antiandrogen drug flutamide and by heat-inactivated TCM, indicating the protein nature of the TCM mediator and its dependence on Sertoli cell stimulation by dihydrotestosterone. In addition, adult rat testes treated with the GnRH antagonist Degarelix exhibited a reduction in the expression of PFKFB4 in germ cells. Addition of exogenous FGF-2 mimicked the changes in the Pfkfb gene expression, whereas neutralizing antibodies against FGF-2 abolished them. Interestingly, similar effects on Pfkfb gene expression were observed using different MAPK inhibitors (U-0126, PD-98059, and H-89). Luciferase analysis of Pfkfb4 promoter constructs demonstrated that a putative CRE-binding sequence located at -1,463 relative to the transcription start site is required to control Pfkfb4 gene expression after TCM treatment. Pulldown assays showed the binding of the CREB transcription factor to this site. Altogether, these results show how the paracrine regulation orchestrated by Sertoli cells in response to testosterone controls glycolysis in germ cells.

Pfkfb3 is transcriptionally upregulated in diabetic mouse liver through proliferative signals

The ubiquitous isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (uPFK-2), a product of the Pfkfb3 gene, plays a crucial role in the control of glycolytic flux. In this study, we demonstrate that Pfkfb3 gene expression is increased in streptozotocin-induced diabetic mouse liver. The Pfkfb3/-3566 promoter construct linked to the luciferase reporter gene was delivered to the liver via hydrodynamic gene transfer. This promoter was upregulated in streptozotocin-induced diabetic mouse liver compared with transfected healthy cohorts. In addition, increases were observed in Pfkfb3 mRNA and uPFK-2 protein levels, and intrahepatic fructose-2,6-bisphosphate concentration. During streptozotocin-induced diabetes, phosphorylation of both p38 mitogen-activated protein kinase and Akt was detected, together with the overexpression of the proliferative markers cyclin D and E2F. These findings indicate that uPFK-2 induction is coupled to enhanced hepatocyte proliferation in streptozotocin-induced diabetic mouse liver. Expression decreased when hepatocytes were treated with either rapamycin or LY 294002. This shows that uPFK-2 regulation is phosphoinositide 3-kinase-Akt-mammalian target of rapamycin dependent. These results indicate that fructose-2,6-bisphosphate is essential to the maintenance of the glycolytic flux necessary for providing energy and biosynthetic precursors to dividing cells.

Regulation of glucose metabolism by 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases in cancer

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 has been confirmed in human studies using positron emission tomography (PET) with (18)F-2-fluoro-deoxy-glucose which have demonstrated that tumors take up 10-fold more glucose than adjacent normal tissues in vivo. The high glucose metabolism of cancer cells is caused by a combination of hypoxia-responsive transcription factors, activation of oncogenic proteins and the loss of tumor suppressor function. Over-expression of HIF-1alpha and myc, activation of ras and loss of p53 function each have been found to stimulate glycolysis in part by activating a family of regulatory bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB). The PFKFB enzymes synthesize fructose-2,6-bisphosphate (F2,6BP) which allosterically activates 6-phosphofructo-1-kinase (PFK-1), a rate-limiting enzyme and essential control point in the glycolytic pathway. PFK-1 is inhibited by ATP when energy stores are abundant and F2,6BP can override this inhibition and enhance glucose uptake and glycolytic flux. It is therefore not surprising that F2,6BP synthesis is stimulated by several oncogenic alterations which simultaneously cause both enhanced consumption of glucose and growth. Importantly, these studies suggest that selective depletion of intracellular F2,6BP in cancer cells may suppress glycolytic flux and decrease their survival, growth and invasiveness. This review will summarize the requirement of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases for the regulation of glycolysis in tumor cells and their potential utility as targets for the development of antineoplastic agents.