Regulation of apoptosis by cyclic nucleotides in human erythroleukemia (HEL) cells and human myelogenous leukemia (K-562) cells

The Role of Pseudomonas aeruginosa ExoY in an Acute Mouse Lung Infection Model

The effector protein Exotoxin Y (ExoY) produced by Pseudomonas aeruginosa is injected via the type III secretion system (T3SS) into host cells. ExoY acts as nucleotidyl cyclase promoting the intracellular accumulation of cyclic nucleotides. To what extent nucleotidyl cyclase activity contributes to the pathogenicity of ExoY and which mechanisms participate in the manifestation of lung infection is still unclear. Here, we used an acute airway infection model in mice to address the role of ExoY in lung infection. In infected lungs, a dose-dependent phenotype of infection with bacteria-expressing ExoY was mirrored by haemorrhage, formation of interstitial oedema in alveolar septa, and infiltration of the perivascular space with erythrocytes and neutrophilic granulocytes. Analyses of the infection process on the cellular and organismal level comparing infections with Pseudomonas aeruginosa mutants expressing either nucleotidyl cyclase-active or -inactive ExoY revealed differential cytokine secretion, increased prevalence of apoptosis, and a break of lung barrier integrity in mice infected with cyclase-active ExoY. Notably, of all measured cyclic nucleotides, only the increase of cyclic UMP in infected mouse lungs coincides temporally with the observed early pathologic changes. In summary, our results suggest that the nucleotidyl cyclase activity of ExoY can contribute to P. aeruginosa acute pathogenicity.

cCMP and cUMP in Apoptosis: Concepts and Methods

The cyclic nucleotides cAMP and cGMP are well-characterized second messenger molecules regulating many important intracellular processes, such as differentiation, proliferation, and apoptosis. The latter is a highly regulated process of programmed cell death wherein several regulatory proteins, like those belonging to the Bcl-2 family, are involved. The initiation of apoptosis is regulated by three different pathways: the intrinsic or mitochondrial, the extrinsic, and the ER stress pathway. Recently, it has been published that the pyrimidine cyclic nucleotides cCMP and cUMP also function as second messenger molecules, and additionally have an effect on apoptosis signaling pathways. cCMP induced PKA-independent apoptosis via the intrinsic and ER-stress pathway in S49 mouse lymphoma cells, and cCMP as well as cUMP induced apoptosis in human HEL cells via the intrinsic pathway. However, in human K-562 cells, which are known to be multidrug-resistant, cCMP and cUMP had no effect. Summarized in this chapter are the initiation of apoptosis by cCMP and cUMP regarding the various apoptotic pathways, the enzymes involved in apoptosis, as well as the most relevant methods for the detection and examination of apoptosis and the corresponding signaling pathways.

cCMP and cUMP Across the Tree of Life: From cCMP and cUMP Generators to cCMP- and cUMP-Regulated Cell Functions

The cyclic purine nucleotides cAMP and cGMP are well-established second messenger molecules that are generated by distinct nucleotidyl cyclases (NCs) and regulate numerous cell functions via specific effector molecules. In contrast, the existence of the cyclic pyrimidine nucleotides cCMP and cUMP has been controversial for many years. The development of highly specific and sensitive mass spectrometry methods has enabled the unequivocal detection and quantitation of cCMP and cUMP in biological systems. These cNMPs occur broadly in numerous mammalian cell lines and primary cells. cCMP has also been detected in mouse organs, and both cCMP and cUMP occur in various developmental stages of the zebrafish Danio rerio. So far, the soluble guanylyl cyclase (sGC) and soluble adenylyl cyclase (sAC) have been identified as cCMP and cUMP generators. Dissociations in the expression patterns of sAC and sGC relative to cCMP and cUMP abundance may point to the existence of hitherto unidentified cCMP- and cUMP-generating NCs. The broad occurrence of cCMP and cUMP in vertebrates and the distinct cNMP patterns suggest specific roles of these cNMPs in the regulation of numerous cell functions.

cUMP hydrolysis by PDE3A

As previously reported, the cardiac phosphodiesterase PDE3A hydrolyzes cUMP. Moreover, cUMP-degrading activity was detected in cow and dog hearts several decades ago. Our aim was to characterize the enzyme kinetic parameters of PDE3A-mediated cUMP hydrolysis and to investigate whether cUMP and cUMP-hydrolyzing PDEs are present in cardiomyocytes. PDE3A-mediated cUMP hydrolysis was characterized in time course, inhibitor, and Michaelis-Menten kinetics experiments. Intracellular cyclic nucleotide (cNMP) concentrations and the mRNAs of cUMP-degrading PDEs were quantitated in neonatal rat cardiomyocytes (NRCMs) and murine HL-1 cardiomyogenic cells. Moreover, we investigated cUMP degradation in HL-1 cell homogenates and intact cells. Educts (cNMPs) and products (NMPs) of the PDE reactions were detected by HPLC-coupled tandem mass spectrometry. PDE3A degraded cUMP (measurement of UMP formation) with a K _M value of ~143 μM and a V _max value of ~42 μmol/min/mg. PDE3A hydrolyzed cAMP with a K _M value of ~0.7 μM and a V _max of ~1.2 μmol/min/mg (determination of AMP formation). The PDE3 inhibitor milrinone inhibited cUMP hydrolysis (determination of UMP formation) by PDE3A (K _i = 57 nM). Significant amounts of cUMP as well as of PDE3A mRNA (in addition to PDE3B and PDE9A transcripts) were detected in HL-1 cells and NRCMs. Although HL-1 cell homogenates contain a milrinone-sensitive cUMP-hydrolyzing activity, intact HL-1 cells may use additional PDE3-independent mechanisms for cUMP disposal. PDE3A is a low-affinity and high-velocity PDE for cUMP. Future studies should investigate biological effects of cUMP in cardiomyocytes and the role of PDE3A in detoxifying high intracellular cUMP concentrations under pathophysiological conditions.