Although pharmacological studies from the end

Although pharmacological studies from the end of the 20th century suggested that β1-AR induces a cAMP-dependent apoptosis [30], more recently it has been demonstrated that cAMP, acting through PKA, may be a crucial anti-apoptotic factor in cardiac myocytes [31]. cAMP can also stimulate PI3K, and the signaling cascades activated by PKA and PI3K jointly contribute to the cAMP-afforded cell survival [26], [31]. Here we demonstrate that inhibition of PI3K results in FBPase withdrawal from the nucleus of cardiomyocytes. These results point to the conclusion that both of the kinases promoting anti-apoptotic pathways also activate nuclear translocation of FBPase in HL-1 cells. Available data on the nongluconegenic role of FBPase is inconsistent: it has been suggested that FBPase might be a proapoptotic [32] or an anti-apoptotic protein [33]. cAMP can either inhibit or stimulate cell growth and survival, depending on the cell type (for example see [26]), and it is not unlikely that the same might be true for FBPase. Whatever its additional role, the enzyme appears to be more than just a component of the glyconeogenic pathway. Taken together, the obtained results offer strong evidence that, in HL-1 cardiomyocytes, nuclear transport of FBPase is a controlled process, mediated by norepinephrine acting through β1 receptors and the Gs protein signaling cascade. These findings motivate future studies to clarify the link between nuclear localization of FBPase and the regulation of the cardiomyocyte cell survival, and to unravel all components of the signaling pathway regulating nuclear transport of the enzyme.
Acknowledgements
We are very grateful to Malgorzata Gizak from Oxford Conversis Ltd (www.conversisglobal.com) for the linguistic assistance.
Introduction In oxygenic photosynthetic organisms, the conversion of fructose-1,6-biphosphate to fructose-6-biphosphate is connected with Calvin cycle, oxidative pentosephosphate polymyxin sulfate and gluconeogenesis, representing an important regulatory step in each of these pathways. The hydrolytic dephosphorylation of fructose-1,6-bisphosphate is catalyzed by fructose-1,6-bisphosphatase (FBPase). In plants, two different FBPase forms are found in chloroplasts and cytoplasm [1], [2]. In certain cyanobacterial species, such as Synechococcus leopoliensis [3], [4] and Synechococcus sp. PCC 7942 [5], there are also two forms of FBPase. In other cyanobacterial species, such as Synechocystis sp. PCC 6803, Anabaena sp. PCC 7120 and Plectonema boryanum, genes encoding the two FBPase forms are also found in genomes, but only one form of FBPase, F-I, is detectable with antiserum, while the second form F-II is probably not expressed under normal conditions [6]. Sedoheptulose-1,7-bisphosphatase (SBPase), which converts sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, is another key enzyme in all photosynthetic organisms [7]. From sedoheptulose-7-phosphate, the CO2 acceptor ribulose-1,5-bisphosphate (RuBP) is regenerated. The balance between export, regeneration of RuBP and starch storage is controlled by conversion of fructose and sedoheptulose bisphosphates into their monophosphate forms [8]. In plants, FBPase and SBPase are separate enzymes [8]. In cyanobacteria, F-I form FBPase also hydrolyzes sedoheptulose-1,7-bisphosphate at the carbon 1-ester [5], [6]. Synechocystis sp. PCC 6803 (hereafter called Synechocystis 6803), a unicellular cyanobacterium, is naturally transformable and incorporates exogenous DNA into its genome via double homologous recombinations. Its entire genomic sequence has been available for more than a decade [9]. In addition to photoautotrophic growth, this cyanobacterium can perform heterotrophic growth. Synechocystis 6803 mutants impaired in photosystems can grow on glucose under photoheterotrophic or light-activated heterotrophic growth conditions. Therefore, Synechocystis 6803 has been widely used as a research model for photosynthesis studies. By random insertion of a kanamycin resistance cassette into the genome of Synechocystis PCC6803, we constructed a mutant library of this cyanobacterium. Some mutants of this library were impaired in photoautotrophic growth. A mutant disrupted in slr2094 lost the activity of fructose-1,6-/sedoheptulose-1,7-bisphosphatase bifunctional enzyme and photoautotrophic growth capability. Under mixotrophic conditions, this mutant showed sensitiveness to light.