Misoprostol br Introduction Fructose bisphosphatase FBPase E

Introduction Fructose 1,6-bisphosphatase (FBPase; EC 3.1.3.11) catalyses the irreversible reaction of hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate and inorganic phosphate [1]. Genetic and kinetic studies so far have demonstrated that at least two distinct isoenzymes of FBPase exist in vertebrates [2], [3], [4]: the liver isozyme – which is the regulatory enzyme of gluconeogenesis [2], and the muscle isozyme – which participates in glycogen synthesis from carbohydrate precursors [5], but its physiological role is not yet fully understood. In contrast to its liver isozyme, muscle FBPase is highly sensitive to inhibition by AMP and calcium ions [2], [6], [7], [8], [9], and it is presumed that the isozyme may be catalytically active and participate in glycogen synthesis from non-glucidic precursors only upon association with the muscle isoform of aldolase [6], [8], [10]. Unlike the liver FBPase, the muscle isoform is widely expressed, both in Misoprostol which are not known to synthesize glycogen from carbohydrate precursors (e.g. neurons, [11]) and in cells which predominantly express the liver isozyme (e.g. liver, [3]). The role of muscle FBPase in these tissues remains unexplained. Quite recently we have presented evidence that FBPase is present inside the cells' nuclei in cardiomyocytes, in smooth muscle cells, and in dividing muscle satellite cells [7], [12], [13], [14]. Such unexpected nuclear localization of muscle FBPase seems to accompany the cells' potential to divide, while cytoplasm-restricted localization of the enzyme appears to characterize terminally differentiated cells [14]. This might suggest that, in addition to its ‘classic’ glyconeogenic role in cytoplasm, muscle FBPase participates in some nuclear processes during division and regeneration of muscle cells [14] and should be considered another member of the growing family of multifaceted proteins [15]. On the other hand, parallel studies of Slebe's group demonstrated that the liver gluconeogenic isozyme may also localize within cells' nuclei, however, such localization depends mainly on the metabolic state of the cells and is modulated by glucose, dihydroxyacetone and insulin [16]. Our preliminary experiments revealed that changes in glucose concentration (within the physiological range) have no effect on the nuclear localization of muscle FBPase. Consequently, to address the question concerning the nuclear role of the isozyme in non-gluconeogenic tissues we have sought to identify the signaling pathways engaged in the reversible FBPase targeting to the nucleus. In our experiments we have used the murine cardiomyocyte cell line HL-1, first established by Dr. W.C. Claycomb [17]. The immortalized HL-1 cardiomyocytes continuously divide and spontaneously contract, while maintaining adult cell phenotype, and as such they can be used to study normal cardiomyocyte function with regard to signaling, metabolic and transcriptional regulation [17], [18]. Results of our studies indicate that, in HL-1 cardiomyocytes, nuclear transport of FBPase is mediated by norepinephrine, acting through β1 receptors and the Gs protein signaling cascade, involving cAMP-dependent protein kinase (PKA). Nuclear transport is also influenced by the activity of phosphoinositide 3-kinase (PI3K). The physiological meaning of these findings is discussed.
Materials and methods
Results
Discussion We have presented evidence that, in HL-1 cardiomyocytes, nuclear targeting of FBPase is controlled by the β-1 adrenergic receptor-activated Gs protein signaling cascade. The physiological role of muscle FBPase, present in a wide range of non-gluconeogenic cells, is not yet fully understood. Our previous research has shown that muscle FBPase localizes in the nuclei of several cell types: cardiac, smooth muscle and dividing muscle satellite cells, and the current study has managed to extend this list to include HL-1 cardiomyocytes. These immortalized cells not only divide continuously, but they also display morphological and biochemical properties of differentiated cardiac cells, therefore being a convenient model to study normal cardiomyocyte functions, with regard to signaling and metabolic regulation [17].