Nuclear targeting of FBPase in HL-1 cells is controlled by beta-1 adrenergic receptor-activated Gs protein signaling cascade

Fructose-1,6-bisphosphatase 1 in cancer: Dual roles, mechanistic insights, and therapeutic potential - A comprehensive review

Fructose-1,6-bisphosphatase 1 (FBP1) is a key gluconeogenic enzyme that plays complex and context-dependent roles in cancer biology. This review comprehensively examines FBP1's dual functions as both a tumor suppressor and an oncogene across various cancer types. In many cancers, such as hepatocellular carcinoma, clear cell renal cell carcinoma, and lung cancer, downregulation of FBP1 contributes to tumor progression through metabolic reprogramming, promoting glycolysis, and altering the tumor microenvironment. Conversely, in certain contexts like breast and prostate cancers, FBP1 overexpression is associated with tumor promotion, indicating its oncogenic potential. The review explores FBP1's interactions with immune cells within the tumor microenvironment, influencing immune surveillance and tumor immune escape mechanisms. Additionally, FBP1 emerges as a promising diagnostic and prognostic biomarker, with expression levels correlating with patient outcomes in multiple cancers. Future therapeutic strategies targeting FBP1 are discussed, including inhibitors, activators, epigenetic modulation, and combination therapies, while addressing the challenges posed by its dual nature. Understanding the multifaceted roles of FBP1 offers valuable insights into cancer metabolism and opens avenues for personalized therapeutic interventions.

FBP2–A New Player in Regulation of Motility of Mitochondria and Stability of Microtubules in Cardiomyocytes

Recently, we have shown that the physiological roles of a multifunctional protein fructose 1,6-bisphosphatase 2 (FBP2, also called muscle FBP) depend on the oligomeric state of the protein. Here, we present several lines of evidence that in HL-1 cardiomyocytes, a forced, chemically induced reduction in the FBP2 dimer-tetramer ratio that imitates AMP and NAD⁺ action and restricts FBP2-mitochondria interaction, results in an increase in Tau phosphorylation, augmentation of FBP2-Tau and FBP2-MAP1B interactions, disturbance of tubulin network, marked reduction in the speed of mitochondrial trafficking and increase in mitophagy. These results not only highlight the significance of oligomerization for the regulation of FBP2 physiological role in the cell, but they also demonstrate a novel, important cellular function of this multitasking protein—a function that might be crucial for processes that take place during physiological and pathological cardiac remodeling, and during the onset of diseases which are rooted in the destabilization of MT and/or mitochondrial network dynamics.

Biased activation of β2-AR/Gi/GRK2 signal pathway attenuated β1-AR sustained activation induced by β1-adrenergic receptor autoantibody

Heart failure is the terminal stage of many cardiac diseases, in which β₁-adrenoceptor (β₁-AR) autoantibody (β₁-AA) has a causative role. By continuously activating β₁-AR, β₁-AA can induce cytotoxicity, leading to cardiomyocyte apoptosis and heart dysfunction. However, the mechanism underlying the persistent activation of β₁-AR by β₁-AA is not fully understood. Receptor endocytosis has a critical role in terminating signals over time. β₂-adrenoceptor (β₂-AR) is involved in the regulation of β₁-AR signaling. This research aimed to clarify the mechanism of the β₁-AA-induced sustained activation of β₁-AR and explore the role of the β₂-AR/Gi-signaling pathway in this process. The beating frequency of neonatal rat cardiomyocytes, cyclic adenosine monophosphate content, and intracellular Ca²⁺ levels were examined to detect the activation of β₁-AA. Total internal reflection fluorescence microscopy was used to detect the endocytosis of β₁-AR. ICI118551 was used to assess β₂-AR/Gi function in β₁-AR sustained activation induced by β₁-AA in vitro and in vivo. Monoclonal β₁-AA derived from a mouse hybridoma could continuously activate β₁-AR. β₁-AA-restricted β₁-AR endocytosis, which was reversed by overexpressing the endocytosis scaffold protein β-arrestin1/2, resulting in the cessation of β₁-AR signaling. β₂-AR could promote β₁-AR endocytosis, as demonstrated by overexpressing/interfering with β₂-AR in HL-1 cells, whereas β₁-AA inhibited the binding of β₂-AR to β₁-AR, as determined by surface plasmon resonance. ICI118551 biasedly activated the β₂-AR/Gi/G protein-coupled receptor kinase 2 (GRK2) pathway, leading to the arrest of limited endocytosis and continuous activation of β₁-AR by β₁-AA in vitro. In vivo, ICI118551 treatment attenuated myocardial fiber rupture and left ventricular dysfunction in β₁-AA-positive mice. This study showed that β₁-AA continuously activated β₁-AR by inhibiting receptor endocytosis. Biased activation of the β₂-AR/Gi/GRK2 signaling pathway could promote β₁-AR endocytosis restricted by β₁-AA, terminate signal transduction, and alleviate heart damage.

Cobalt Regulates Activation of Camk2α in Neurons by Influencing Fructose 1,6-bisphosphatase 2 Quaternary Structure and Subcellular Localization

Fructose 1,6-bisphosphatase 2 (Fbp2) is a gluconeogenic enzyme and multifunctional protein modulating mitochondrial function and synaptic plasticity via protein-protein interactions. The ability of Fbp2 to bind to its cellular partners depends on a quaternary arrangement of the protein. NAD+ and AMP stabilize an inactive T-state of Fbp2 and thus, affect these interactions. However, more subtle structural changes evoked by the binding of catalytic cations may also change the affinity of Fbp2 to its cellular partners. In this report, we demonstrate that Fbp2 interacts with Co2+, a cation which in excessive concentrations, causes pathologies of the central nervous system and which has been shown to provoke the octal-like events in hippocampal slices. We describe for the first time the kinetics of Fbp2 in the presence of Co2+, and we provide a line of evidence that Co2+ blocks the AMP-induced transition of Fbp2 to the canonical T-state triggering instead of a new, non-canonical T-state. In such a state, Fbp2 is still partially active and may interact with its binding partners e.g., Ca2+/calmodulin-dependent protein kinase 2α (Camk2α). The Fbp2-Camk2α complex seems to be restricted to mitochondria membrane and it facilitates the Camk2α autoactivation and thus, synaptic plasticity.

Fructose 1,6-Bisphosphatase 2 Plays a Crucial Role in the Induction and Maintenance of Long-Term Potentiation

Long-term potentiation (LTP) is a molecular basis of memory formation. Here, we demonstrate that LTP critically depends on fructose 1,6-bisphosphatase 2 (Fbp2)—a glyconeogenic enzyme and moonlighting protein protecting mitochondria against stress. We show that LTP induction regulates Fbp2 association with neuronal mitochondria and Camk2 and that the Fbp2–Camk2 interaction correlates with Camk2 autophosphorylation. Silencing of Fbp2 expression or simultaneous inhibition and tetramerization of the enzyme with a synthetic effector mimicking the action of physiological inhibitors (NAD⁺ and AMP) abolishes Camk2 autoactivation and blocks formation of the early phase of LTP and expression of the late phase LTP markers. Astrocyte-derived lactate reduces NAD⁺/NADH ratio in neurons and thus diminishes the pool of tetrameric and increases the fraction of dimeric Fbp2. We therefore hypothesize that this NAD⁺-level-dependent increase of the Fbp2 dimer/tetramer ratio might be a crucial mechanism in which astrocyte–neuron lactate shuttle stimulates LTP formation.

Cell-to-cell lactate shuttle operates in heart and is important in age-related heart failure

Recent studies have revealed a resemblance of a HIF-regulated heart and brain glycolytic profiles prompting the hypothesis that the classical cell-to-cell lactate shuttle observed between astrocytes and neurons operates also in heart - between cardiac fibroblasts and cardiomyocytes. Here, we demonstrate that co-culturing of cardiomyocytes with cardiac fibroblasts leads to orchestrated changes in expression and/or localization pattern of glucose metabolism enzymes and lactate transport proteins in both cell types. These changes are regulated by paracrine signaling using microvesicle-packed and soluble factors released to the culture medium and, taken together, they concur with the cardiac lactate shuttle hypothesis. The results presented here show that similarity of heart and brain proteomes demonstrated earlier extend to physiological level and provide a theoretical rationale for designing novel therapeutic strategies for treatment of cardiomyopathies resulting from disruption of the maturation of cardiac metabolic pathways, and of heart failure associated with metabolic complications and age-related heart failure linked with extracellular matrix deposition and hypoxia.

Fructose-1,6-Bisphosphatase 2 Inhibits Sarcoma Progression by Restraining Mitochondrial Biogenesis

The remarkable cellular and genetic heterogeneity of soft tissue sarcomas (STSs) limits the clinical benefit of targeted therapies. Here, we show that expression of the gluconeogenic isozyme fructose-1,6-bisphosphatase 2 (FBP2) is silenced in a broad spectrum of sarcoma subtypes, revealing an apparent common metabolic feature shared by diverse STSs. Enforced FBP2 expression inhibits sarcoma cell and tumor growth through two distinct mechanisms. First, cytosolic FBP2 antagonizes elevated glycolysis associated with the "Warburg effect," thereby inhibiting sarcoma cell proliferation. Second, nuclear-localized FBP2 restrains mitochondrial biogenesis and respiration in a catalytic-activity-independent manner by inhibiting the expression of nuclear respiratory factor and mitochondrial transcription factor A (TFAM). Specifically, nuclear FBP2 colocalizes with the c-Myc transcription factor at the TFAM locus and represses c-Myc-dependent TFAM expression. This unique dual function of FBP2 provides a rationale for its selective suppression in STSs, identifying a potential metabolic vulnerability of this malignancy and possible therapeutic target.

Dimeric and tetrameric forms of muscle fructose-1,6-bisphosphatase play different roles in the cell

Muscle fructose 1,6-bisphosphatase (FBP2), besides being a regulatory enzyme of glyconeogenesis also protects mitochondria against calcium stress and plays a key role in regulation of the cell cycle, promoting cardiomyocytes survival. However, in cancer cells, FBP2 acts as an anti-oncogenic/anti-proliferative protein. Here, we show that the physiological function of FBP2 depends both on its level of expression in a cell as well as its oligomerization state. Animal fructose-1,6-bisphosphatases are thought to function as tetramers. We present evidence that FBP2 exists in an equilibrium between tetramers and dimers. The dimeric form is fully active and insensitive to AMP, the main allosteric inhibitor of FBP2. Tetramerization induces the sensitivity of the protein to AMP, but it requires the presence of a hydrophobic central region in which leucine 190 plays a crucial role. Only the tetrameric form of FBP2 is retained in cardiomyocyte cell nucleus whereas only the dimeric form associates with mitochondria and protects them against stress stimuli, such as elevated calcium and H₂O₂ level. Remarkably, in hypoxic conditions, which are typical for many cancers, FBP2 ceases to interact with mitochondria and loses its pro-survival potential. Our results throw new light on the basis of the diverse role of FBP2 in cells.

Insulin/IGF1-PI3K-dependent nucleolar localization of a glycolytic enzyme--phosphoglycerate mutase 2, is necessary for proper structure of nucleolus and RNA synthesis

Phosphoglycerate mutase (PGAM), a conserved, glycolytic enzyme has been found in nucleoli of cancer cells. Here, we present evidence that accumulation of PGAM in the nucleolus is a universal phenomenon concerning not only neoplastically transformed but also non-malignant cells. Nucleolar localization of the enzyme is dependent on the presence of the PGAM2 (muscle) subunit and is regulated by insulin/IGF-1–PI3K signaling pathway as well as drugs influencing ribosomal biogenesis. We document that PGAM interacts with several 40S and 60S ribosomal proteins and that silencing of PGAM2 expression results in disturbance of nucleolar structure, inhibition of RNA synthesis and decrease of the mitotic index of squamous cell carcinoma cells. We conclude that presence of PGAM in the nucleolus is a prerequisite for synthesis and initial assembly of new pre-ribosome subunits.

T-to-R switch of muscle fructose-1,6-bisphosphatase involves fundamental changes of secondary and quaternary structure

Fructose-1,6-bisphosphatase (FBPase) catalyzes the hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate and is a key enzyme of gluconeogenesis and glyconeogenesis and, more generally, of the control of energy metabolism and glucose homeostasis. Vertebrates, and notably Homo sapiens, express two FBPase isoforms. The liver isozyme is expressed mainly in gluconeogenic organs, where it functions as a regulator of glucose synthesis. The muscle isoform is expressed in all cells, and recent studies have demonstrated that its role goes far beyond the enzymatic function, as it can interact with various nuclear and mitochondrial proteins. Even in its enzymatic function, the muscle enzyme is different from the liver isoform, as it is 100-fold more susceptible to allosteric inhibition by AMP and this effect can be abrogated by complex formation with aldolase. All FBPases are homotetramers composed of two intimate dimers: the upper dimer and the lower dimer. They oscillate between two conformational states: the inactive T form when in complex with AMP, and the active R form. Parenthetically, it is noted that bacterial FBPases behave somewhat differently, and in the absence of allosteric activators exist in a tetramer-dimer equilibrium even at relatively high concentrations. [Hines et al. (2007), J. Biol. Chem. 282, 11696-11704]. The T-to-R transition is correlated with the conformation of the key loop L2, which in the T form becomes `disengaged' and unable to participate in the catalytic mechanism. The T states of both isoforms are very similar, with a small twist of the upper dimer relative to the lower dimer. It is shown that at variance with the well studied R form of the liver enzyme, which is flat, the R form of the muscle enzyme is diametrically different, with a perpendicular orientation of the upper and lower dimers. The crystal structure of the muscle-isozyme R form shows that in this arrangement of the tetramer completely new protein surfaces are exposed that are most likely targets for the interactions with various cellular and enzymatic partners. The cruciform R structure is stabilized by a novel `leucine lock', which prevents the key residue, Asp187, from locking loop L2 in the disengaged conformation. In addition, the crystal structures of muscle FBPase in the T conformation with and without AMP strongly suggest that the T-to-R transition is a discrete jump rather than a shift of an equilibrium smooth transition through multiple intermediate states. Finally, using snapshots from three crystal structures of human muscle FBPase, it is conclusively demonstrated that the AMP-binding event is correlated with a β→α transition at the N-terminus of the protein and with the formation of a new helical structure.

A new level of regulation in gluconeogenesis: metabolic state modulates the intracellular localization of aldolase B and its interaction with liver fructose-1,6-bisphosphatase

Understanding how glucose metabolism is finely regulated at molecular and cellular levels in the liver is critical for knowing its relationship to related pathologies, such as diabetes. In order to gain insight into the regulation of glucose metabolism, we studied the liver-expressed isoforms aldolase B and fructose-1,6-bisphosphatase-1 (FBPase-1), key enzymes in gluconeogenesis, analysing their cellular localization in hepatocytes under different metabolic conditions and their protein-protein interaction in vitro and in vivo. We observed that glucose, insulin, glucagon and adrenaline differentially modulate the intracellular distribution of aldolase B and FBPase-1. Interestingly, the in vitro protein-protein interaction analysis between aldolase B and FBPase-1 showed a specific and regulable interaction between them, whereas aldolase A (muscle isozyme) and FBPase-1 showed no interaction. The affinity of the aldolase B and FBPase-1 complex was modulated by intermediate metabolites, but only in the presence of K(+). We observed a decreased association constant in the presence of adenosine monophosphate, fructose-2,6-bisphosphate, fructose-6-phosphate and inhibitory concentrations of fructose-1,6-bisphosphate. Conversely, the association constant of the complex increased in the presence of dihydroxyacetone phosphate (DHAP) and non-inhibitory concentrations of fructose-1,6-bisphosphate. Notably, in vivo FRET studies confirmed the interaction between aldolase B and FBPase-1. Also, the co-expression of aldolase B and FBPase-1 in cultured cells suggested that FBPase-1 guides the cellular localization of aldolase B. Our results provide further evidence that metabolic conditions modulate aldolase B and FBPase-1 activity at the cellular level through the regulation of their interaction, suggesting that their association confers a catalytic advantage for both enzymes.

Changes in quaternary structure of muscle fructose-1,6-bisphosphatase regulate affinity of the enzyme to mitochondria

Muscle fructose-1,6-bisphosphatase (FBP2), a regulatory enzyme of glyconeogenesis, binds to mitochondria where it interacts with proteins involved in regulation of energy homeostasis. Here, we show that the quaternary structure of FBP2 plays a crucial role in this interaction, and that the AMP-driven transition of the FBP2 subunit arrangement from active to inactive precludes its association with the mitochondria. Moreover, we demonstrate that truncation of the evolutionarily conserved N-terminal residues of FBP2 results in a loss of its mitochondria-protective functions. This strengthens the recently raised hypothesis that FBP2 evolved as a regulator not only for glycogen storage but also for mitochondrial function in contracting muscle.

Biochemical characterization and functional analysis of fructose-1,6-bisphosphatase from Clonorchis sinensis

Fructose-1,6-bisphosphatase (FBPase), a key regulatory enzyme of gluconeogenesis, plays an essential role in metabolism and development of most organisms. To the wealth of available knowledge about FBPase from Clonorchis sinensis (CsFBPase), in this study, the characteristics of CsFBPase and its potential role in pathogenesis of clonorchiasis were investigated. The Km value of CsFBPase was calculated to be 41.9 uM. The optimal temperature and pH of CsFBPase were 37 °C and pH 7.5-8.0, respectively. In addition, Mg(2+) or K(+) played a regulatory role in enzyme activity of CsFBPase. Both transcriptional and translational level of CsFBPase were higher in metacercariae (one of larva stages) than those in adult worm (P < 0.05). CsFBPase were observed to extensively express in the intestine, vitellaria and tegument of adult worms and ubiquitously in metacercariae. Moreover, CsFBPase was confirmed as a component of excretory/secretory products. Consequently, the translocation of CsFBPase could be detected on epithelial cells of bile duct in liver of C. sinensis infected rat. Recombinant CsFBPase can specifically bind to the membrane of human hepatic stellate cell line LX-2 by immunofluorescence analysis and stimulated proliferation and activation of LX-2 which demonstrated by Cell Counting Kit-8 and upregulation of key fibrosis-related factors, such as α-smooth muscle actin, collagen I and collagen III using qRT-PCR. Thus, we predicated that CsFBPase might be a multifunctional enzyme which played as both regulatory enzyme and virulence factor in pathogenesis of C. sinensis infection.

Destabilization of fructose 1,6-bisphosphatase-Z-line interactions is a mechanism of glyconeogenesis down-regulation in vivo

Although it is well known that insulin controls the synthesis of glycogen from non-carbohydrates by down-regulating expression of several glyconeogenic enzymes, a mechanism of short-term inhibition of glyconeogenesis remains unknown. In recent years, we have shown that in skeletal muscle, fructose 1,6-bisphosphatase (FBPase) is a part of the hypothetical glyconeogenic complex located on sarcomeric Z-line. Here, we show that inhibition of glycogen synthase kinase-3 causes disruption of the FBPase-Z-line interactions and reduction of muscle glycogen content in vivo. The normal, striated pattern of muscle FBPase localization is also disturbed by insulin treatment but preserved when insulin is applied together with Akt inhibitor. We suggest that destabilization of FBPase-Z-line interaction is a universal cellular mechanism of glyconeogenesis down-regulation, allowing for preferential utilization of glucose for insulin-stimulated muscle glycogen synthesis.

Nuclear accumulation of fructose 1,6-bisphosphatase is impaired in diabetic rat liver

Using a streptozotocin-induced type 1 diabetic rat model, we analyzed and separated the effects of hyperglycemia and hyperinsulinemia over the in vivo expression and subcellular localization of hepatic fructose 1,6-bisphosphatase (FBPase) in the multicellular context of the liver. Our data showed that FBPase subcellular localization was modulated by the nutritional state in normal but not in diabetic rats. By contrast, the liver zonation was not affected in any condition. In healthy starved rats, FBPase was localized in the cytoplasm of hepatocytes, whereas in healthy re-fed rats it was concentrated in the nucleus and the cell periphery. Interestingly, despite the hyperglycemia, FBPase was unable to accumulate in the nucleus in hepatocytes from streptozotocin-induced diabetic rats, suggesting that insulin is a critical in vivo modulator. This idea was confirmed by exogenous insulin supplementation to diabetic rats, where insulin was able to induce the rapid accumulation of FBPase within the hepatocyte nucleus. Besides, hepatic FBPase was found phosphorylated only in the cytoplasm, suggesting that the phosphorylation state is involved in the nuclear translocation. In conclusion, insulin and not hyperglycemia plays a crucial role in the nuclear accumulation of FBPase in vivo and may be an important regulatory mechanism that could account for the increased endogenous glucose production of liver of diabetic rodents.

A comparative study on the sensitivity of Cyprinus carpio muscle and liver FBPase toward AMP and calcium

The activity of fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) isozymes is influenced by AMP, Ca2+ and by reversible interactions with subcellular structures. In contrast to mammalian and avian isozymes, the kinetic properties of FBPases from ectothermal vertebrates are not fully described. To get some insight into mechanism of glycogen resynthesis in ectothermal vertebrates we examined the features of FBPases isolated from Cyprinus carpio skeletal muscle and liver. To investigate the evolutionary origin of the sensitivity of FBPase to effectors, we performed a phylogenetic analysis of known animal amino acids sequences of the enzyme. Based on our findings, we hypothesize that the high, mammalian-like, sensitivity of FBPase to Ca2+ is not essential for controlling the stability of glyconeogenic complex in striated muscles, instead it ensures the precise regulation of mitochondrial metabolism during prolonged Ca2+ elevation in contracting muscle fibers. Comparison of the kinetic properties of vertebrate and insect FBPases suggests that the high sensitivity of muscle isozyme to inhibitors has arisen as an adaptation enabling coordination of energy metabolism in warm-blooded animals.

Muscle FBPase binds to cardiomyocyte mitochondria under glycogen synthase kinase-3 inhibition or elevation of cellular Ca2+ level

A growing body of research suggests that fructose 1,6-bisphosphatase (FBPase) might be involved in regulation of cell mortality/survival. However, the precise role of FBPase in the process remains unknown. Here, we show for the first time that in HL-1 cardiomyocytes, inhibition of glycogen synthase kinase-3 results in translocation of FBPase to mitochondria. In vitro experiments demonstrate that FBPase reduces the rate of calcium-induced mitochondrial swelling, affects ATP synthesis and interacts with mitochondrial proteins involved in regulation of volume and energy homeostasis. We suggest that FBPase might be engaged in a regulation of cell survival by influencing mitochondrial function.

Cell cycle-dependent expression and subcellular localization of fructose 1,6-bisphosphatase

Recently a gluconeogenic enzyme was discovered—fructose 1,6-bisphosphatase (FBPase)—that localizes in the nucleus of a proliferating cell, but its physiological role in this compartment remains unclear. Here, we demonstrate the link between nuclear localization of FBPase and the cell cycle progression. Results of our studies indicate that in human and mouse squamous cell lung cancer, as well as in the HL-1 cardiomyocytes, FBPase nuclear localization correlates with nuclear localization of S and G2 phase cyclins. Additionally, activity and expression of the enzyme depends on cell cycle stages. Identification of FBPase interacting partners with mass spectrometry reveals a set of nuclear proteins involved in cell cycle regulation, mRNA processing and in stabilization of genomic DNA structure. To our knowledge, this is the first experimental evidence that muscle FBPase is involved in cell cycle events.

Ubiquitous presence of gluconeogenic regulatory enzyme, fructose-1,6-bisphosphatase, within layers of rat retina

To shed some light on gluconeogenesis in mammalian retina, we have focused on fructose-1,6-bisphosphatase (FBPase), a regulatory enzyme of the process. The abundance of the enzyme within the layers of the rat retina suggests that, in mammals in contrast to amphibia, gluconeogenesis is not restricted to one specific cell of the retina. We propose that FBPase, in addition to its gluconeogenic role, participates in the protection of the retina against reactive oxygen species. Additionally, the nuclear localization of FBPase and of its binding partner, aldolase, in the retinal cells expressing the proliferation marker Ki-67 indicates that these two gluconeogenic enzymes are involved in non-enzymatic nuclear processes.