Reduction of adenylyl cyclase activity by cholera toxin in myeloid cells. Long-term down-regulation of Gs alpha subunits by cholera toxin treatment

Disarming the enemy: targeting bacterial toxins with small molecules

The rapid emergence of antibiotic-resistant bacterial strains has prompted efforts to find new and more efficacious treatment strategies. Targeting virulence factors produced by pathogenic bacteria has gained particular attention in the last few years. One of the inherent advantages of this approach is that it provides less selective pressure for the development of resistance mechanisms. In addition, antivirulence drugs could potentially be the answer for diseases in which the use of conventional antibiotics is counterproductive. That is the case for bacterial toxin-mediated diseases, in which the severity of the symptoms is a consequence of the exotoxins produced by the pathogen. Examples of these are haemolytic-uraemic syndrome produced by Shiga toxins, the profuse and dangerous dehydration caused by Cholera toxin or the life-threatening colitis occasioned by clostridial toxins. This review focuses on the recent advances on the development of small molecules with antitoxin activity against Enterohaemorrhagic Escherichia coli, Vibrio cholerae and Clostridium difficile given their epidemiological importance. The present work includes studies of small molecules with antitoxin properties that act directly on the toxin (direct inhibitors) or that act by preventing expression of the toxin (indirect inhibitors).

Galpha(s) sensitizes human SH-SY5Y cells to apoptosis independently of the protein kinase A pathway

Disturbances in Galpha(s-L) levels and function have been implicated in the pathophysiology of bipolar disorder, but the role of these changes in the development of the illness is not clear. In view of the critical role of Galpha(s)-mediated cAMP signaling in regulating cell survival, we investigated the potential role of Galpha(s-L) in modulating susceptibility to cellular stressors in human SH-SY5Y neuroblastoma cells. Overexpression of Galpha(s-L) to a level twice that of the vector-transfected cells did not directly affect cell viability but significantly increased the sensitivity to induction of cell death by serum deprivation and other apoptotic stimuli, including staurosporine, H(2)O(2), and tunicamycin. This enhanced sensitivity was associated with increased caspase-3 activation and appearance of fragmented nuclei (Hoechst 33342 staining). The broad-spectrum caspase inhibitor z-VAD-fmk completely suppressed cell death evoked by these apoptotic insults in both vector-transfected and Galpha(s-L)-overexpressing cells. The increased vulnerability conferred by increased Galpha(s-L) expression was neither mimicked by cAMP analogs 8-Br-cAMP, 8-CPT-cAMP, and 8-CPT-2Me-cAMP nor attenuated by PKA inhibitors Rp-cAMPS and KT5720. These data indicate that Galpha(s-L) may modulate apoptotic processes in a caspase-dependent manner through a signaling cascade that is independent of the cAMP/PKA or cAMP/Epac pathway. These results suggest that enhanced Galpha(s-L) expression, as was observed in post-mortem brain of bipolar patients, may impair cellular resilience in response to intracellular stress signals resulting from mitochondrial and/or endoplasmic reticulum dysfunction implicated in this disorder.

The human MT1 melatonin receptor stimulates cAMP production in the human neuroblastoma cell line SH-SY5Y cells via a calcium-calmodulin signal transduction pathway

Melatonin regulates circadian and seasonal physiology via melatonin receptors expressed in the brain. However, little is known about the signal transduction mechanisms that mediate the action of melatonin in neuronal cells. To begin to address this issue, we expressed the human MT(1) receptor in the human neuroblastoma SH-SY5Y cell line. In this cell line, melatonin acutely stimulated cAMP synthesis through a calcium-calmodulin dependent pathway. This stimulatory effect was independent of an interaction with G(i) or G(s) G proteins and dependent upon internal calcium stores. Melatonin also potentiated forskolin-activated cAMP synthesis. Differentiation of the neuroblastoma cells with retinoic acid to the neuronal phenotype did not alter the ability of melatonin to acutely stimulate cAMP. These data may be relevant to the neuronal action of melatonin and highlight the importance of the cellular context of expression of melatonin and other G protein-coupled receptors.

Cholera toxin-induced Gs alpha down-regulation in neural tissue: studies on the pineal gland

Cholera toxin (CT) treatment (50 micrograms/ml) was used to down regulate the alpha subunit of the stimulatory guanine nucleotide binding protein (Gs alpha) in pineal glands in organ culture, as has been seen in non-neural tissue. A 15 h treatment reduces Gs alpha by approximately 75% as measured using semi-quantitative Western blot technology. In contrast, this treatment does not alter the abundance of G beta, Gi alpha or Go alpha. This effect on Gs alpha was still apparent following a 36-h washout period. The 48-h CT treatment increased cyclic AMP accumulation 10- to 17-fold but blocked the norepinephrine (NE)-induced increase in cyclic AMP accumulation, presumably reflecting the loss of Gs alpha. This treatment did not, however, inhibit protein synthesis or stimulation of arylalkylamine N-acetyltransferase (NAT) activity produced by treatment with either DB-cyclic AMP (N6,2'-O-dibutyryl adenosine 3',5' monophosphate) or 8 Br-cyclic AMP, stable cyclic AMP derivatives. This indicates that a 48-h CT treatment was not generally toxic. In contrast, this treatment blocked subsequent CT stimulation of NAT. The effects of CT treatment on the adrenergic stimulation of NAT was examined using treatments which selectively produced alpha- or beta-adrenergic stimulation. alpha 1-Adrenergic activation of the pineal gland elevates [Ca2+]i, which potentiates effects of cyclic AMP; in these studies the response to alpha-adrenergic activation was markedly increased in 48 h CT-treated glands, reflecting Ca2+ potentiation of the effects of elevated levels of cyclic AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Gs alpha availability to cholera toxin-catalysed ADP-ribosylation is decreased in membranes of retinoic acid-treated leukemic cell lines HL-60 and THP-1. A posttranslational effect

Retinoic acid (RA) induces HL-60 and THP-1 leukemic cell lines to differentiate into granulocyte-like and monocyte-like cells. Limited data are available concerning the effects of RA on components of the cyclic AMP pathway in human myeloid leukemic cells. We showed previously a decrease in adenylate cyclase activity in the presence of histamine, prostaglandin E1 and forskolin in RA-treated HL-60 cells as compared to untreated cells. We examined the elements of the signal transduction pathway utilized by RA in the human myeloid cell line HL-60 and the human monocytic cell line THP-1. We therefore studied the effect of RA on the activity of the stimulatory G-protein (Gs). We demonstrate that addition of RA to two human myeloid leukemia cell lines, HL-60 and THP-1, does not induce a reduction of the 2 subunit of Gs (Gs alpha) RNA or Gs alpha protein in the plasma membrane but leads to a rapid decrease in the cholera toxin (CTX)-catalysed ADP-ribosylation of Gs alpha. In addition, this effect seems to be specific to RA, since there was no modification in Gs alpha ADP-ribosylation in the membranes of cells treated with dimethyl sulfoxide (DMSO), another inducer of differentiation in HL-60 cells.