Isolation and properties of an ATP transporter from a strain of Aspergillus niger

Released ATP is an extracellular cytotoxic mediator in salivary histatin 5-induced killing of Candida albicans

Salivary histatins (Hsts) are antifungal peptides with promise as therapeutic agents against candidiasis. Hst 5 kills the fungal pathogen Candida albicans via a mechanism that involves release of cellular ATP in the absence of cytolysis. Here we demonstrate that released ATP has a further role in Hst 5 killing. Incubation of the cells with ATP analogues induced cell death, and addition of the ATP scavenger apyrase to remove extracellular ATP released during Hst 5 treatment resulted in a reduction in cell killing. Experiments using anaerobically grown C. albicans with decreased susceptibility to Hst 5 confirmed that depletion of cellular ATP as a result of ATP efflux was not sufficient to cause cell death. In contrast to Hst-susceptible aerobic cultures, anaerobically grown cells were not killed by exogenously applied ATP. These findings established that Hst binding, subsequent entry into the cells, and ATP release precede the signal for cytotoxicity, which is mediated by extracellular ATP. In a higher-eukaryote paradigm, released ATP acts as a cytotoxic mediator by binding to membrane nucleotide P2X receptors. Based on a pharmacological profile and detection of a C. albicans 60-kDa membrane protein immunoreactive with antibody to P2X(7) receptor, we propose that released ATP in response to Hst 5 activates candidal P2X(7)-like receptors to cause cell death.

Characterization of the ATP transporter in the reconstituted rough endoplasmic reticulum proteoliposomes

Adenosine triphosphate (ATP) transporter from rat liver rough endoplasmic reticulum (RER) was solubilized and reconstituted into phosphatidylcholine liposomes. The RER proteoliposomes, resulting from optimizing some reconstitution parameters, had an apparent K(m) value of 1.5 microM and a V(max) of 286 pmol min(-1) (mg protein)(-1) and showed higher affinity for ATP and a lower V(max) value than intact RER (K(m) of 6.5 microM and V(max) of 1 nmol). ATP transport was time- and temperature-dependent, inhibited by 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid, which is known as an inhibitor of anion transporters including ATP transporter, but was not affected by atractyloside, a specific inhibitor of mitochondrial ADP/ATP carrier. The internal and external effects of various nucleotides on the ATP transport were examined. ATP transport was cis-inhibited strongly by ADP and weakly by AMP. ADP-preloaded RER proteoliposomes showed a specific increase of ATP transport activity while AMP-preloaded RER proteoliposomes did not show the enhanced overshoot peak in the ATP uptake plot. These results demonstrate the ADP/ATP antiport mechanism of ATP transport in rat liver RER.

Salivary histatin 5 induces non-lytic release of ATP from Candida albicans leading to cell death

Salivary histatins are potent in vitro antifungal proteins and have promise as therapeutic agents against oral candidiasis. We performed pharmacological studies directed at understanding the biochemical basis of Hst 5 candidacidal activity. Three inhibitors of mitochondrial metabolism: carbonyl cyanide p-chlorophenylhydrazone, dinitrophenol, and azide inhibited Hst 5 killing of Candida albicans, while not inhibiting cellular ATP production. In contrast, Hst 5 caused a drastic reduction of C. albicans intracellular ATP content, which was a result of an efflux of ATP. Carbonyl cyanide p-chlorophenylhydrazone, dinitrophenol, and azide inhibited Hst 5-induced ATP efflux, thus establishing a correlation between ATP release and cell killing. Furthermore, C. albicans cells were respiring and had polarized membranes at least 80 min after ATP release, thus implying a non-lytic exit of cellular ATP in response to Hst 5. Based on evidence that transmembrane ATP efflux can occur in the absence of cytolysis through a channel-like pathway and that released ATP can act as a cytotoxic mediator by binding to membrane purinergic receptors, we evaluated whether extracellular ATP released by Hst 5 may have further functional role in cell killing. Consistent with this hypothesis, purinergic agonists BzATP and adenosine 5'O-(thiotriphosphate) induced loss of C. albicans cell viability and purinergic antagonists prevented Hst 5 killing.

Use of resistant mutants to characterize the target of mycobacillin in Aspergillus niger membranes

The mycobacillin-sensitive Aspergillus niger strain G3Br and resistant mutants of it did not show any differences in their total lipid content, although the amounts of phospholipids and sterols, particularly phosphatidylcholine and cholesterol, were lower in resistant cells. Mycobacillin resistance was accompanied by an increase in the phase-transition temperature of plasma membrane preparations. When exposed to mycobacillin, resistant and sensitive cells did not differ qualitatively with respect to most released materials (lysine, proline, Pi, Na+, K+, Ca2+); however, the release of ATP was completely inhibited in resistant cells unless they were exposed to concentrations of mycobacillin exceeding their respective MIC value. Resistant cells, under steady-state conditions, displayed greater uptake and release of the same specific materials--except ATP--as sensitive cells did under similar conditions. Thus release and uptake of those materials except ATP are not implicated in the mode of action of mycobacillin. The inhibiting action of mycobacillin (at concentrations higher than the MIC) on sensitive or resistant cells was completely antagonized by ATP (which did not form any complex with mycobacillin) but not by any of the releasable components, either alone or in combination. This observation, coupled with the authors' recent findings on ATP release, indicates that the fungistatic action of mycobacillin is due to excessive ATP release, leading to energy starvation. Interestingly, ATP release during the first 2 h of incubation with mycobacillin was minimal, but increased to over 96% during the next 48 h. Release and uptake of ATP via liposomes, prepared with lipid and protein isolated from membranes of the mycobacillin-sensitive parent and resistant mutants, showed that mycobacillin action could be inhibited either by resistant protein or by resistant lipid. The mycobacillin target appears to be a lipid-protein site on the membrane of sensitive A. niger G3Br.