Therapie mit NO-Donatoren

Zusammenfassung.

Der NO-Donor Pentaerythrityltetranitrat (PETN) besitzt als Langzeitnitrat antiischämische und vasodilatierende Wirkung. Außerdem induziert PETN antiatherogene und antioxidative Effekte, wobei die verantwortlichen zellulären Mechanismen bisher nicht geklärt werden konnten. Daher sind wir in verschiedenen Studien der Frage nachgegangen, ob PETN und sein Metabolit PETriN in der Lage sind, die Expression der antioxidativen Proteine Ferritin und Hämoxygenase-1 (HO-1) zu modulieren. PETriN stimulierte sowohl die endotheliale Expression der HO-1 als auch die Bildung der HO-1-Metaboliten Bilirubin und Kohlenmonoxid. Gleichzeitig führte die Inkubation von Endothelzellen mit PETriN zu einer deutlichen Stimulation der Ferritinexpression. Nach Vorbehandlung mit PETriN war die Sensibilität von Endothelzellen gegenüber oxidativem Stress herabgesetzt. Dieser Effekt war vergleichbar mit der zellprotektiven Wirkung von exogen zugesetztem Bilirubin. Die Langzeitnitrate Isosorbiddinitrat (ISDN) und Isosorbidmononitrat (ISMN) zeigten unter diesen Bedingungen weder einen Effekt auf die Expression von HO-1 und Ferritin im Endothel noch eine signifikante Zellprotektion. Möglicherweise sind zur Aktivierung endogener antioxidativer Signalwege NO-Mengen erforderlich, die in diesem Zellkulturmodell von Mono- und Dinitraten erst in einem wesentlich höheren Konzentrationsbereich generiert werden. In Übereinstimmung mit diesen Beobachtungen haben tierexperimentelle Untersuchungen und Studien an menschlichen Probanden ergeben, dass der HO-1-Induktor PETN im Gegensatz zu anderen Langzeitnitraten die endotheliale Dysfunktion verbessert und atherosklerotische Gefäßveränderungen unterdrückt. Weitere klinische Studien werden nötig sein, um zu klären, ob PETN neben seiner positiven Wirkung auf die Angina-pectoris-Symptomatik auch die Progredienz der koronaren Herzkrankheit und der peripheren Verschlusskrankheit durch Aktivierung vasoprotektiver Signalwege kausal beeinflussen kann.

Abstract.

Nitric acid esters such as glyceryl trinitrate were introduced into therapy more than a century ago and are still widely used for the treatment of myocardial ischemia and its main symptom angina pectoris. The basic mechanisms responsible for the vasodilatory and anti-ischemic action of organic nitrates involve bioactivation of, and nitric oxide (NO) release from, these compounds which have therefore been termed NO donors. The organic nitrate pentaerythritol tetranitrate (PETN) is known to possess antioxidant properties that are thought to be the underlying cause for its specific pharmacological profile. In contrast to other long-acting nitrates, PETN induces tolerance- free vasodilation in humans and was reported to prevent endothelial dysfunction as well as atherogenesis in cholesterol- fed rabbits. However, the exact nature of the vasoprotective signaling pathways triggered by PETN has remained obscure.

The present study demonstrates that the active PETN metabolite PETriN stimulates protein expression of the antioxidant defense protein heme oxygenase-1 (HO-1; Figures 1 and 2). Additionally, PETriN enhanced the enzymatic activity of HO-1 measured as formation of the HO-1 metabolites bilirubin (Figure 3) and carbon monoxide (Figure 4) in lysates from endothelial cells. HO-1 induction subsequently led to a marked increase in protein expression of a second antioxidant protein, ferritin, via the HO-1-dependent release of free iron from endogenous heme sources (Figures 1 and 5). Pretreatment of endothelial cells with PETriN was followed by increased cellular resistance to oxidant injury mediated by hydrogen peroxide (Figure 6). Endothelial protection by PETriN was mimicked by exogenous bilirubin which led to an almost complete reversal of hydrogen peroxide-induced toxicity (Figure 8). Increased HO-1 and ferritin expression as well as endothelial protection occurred at micromolar concentrations of PETriN which are well within the range of plasma or tissue levels that can be expected during oral therapy. The capacity to protect the endothelium in vitro may translate into and explain the previously observed antiatherogenic actions of PETN in vivo.

In this study, another long-acting nitrate, isosorbide dinitrate (ISDN), did not protect endothelial cells from oxidant damage (Figure 6). The absence of significant cytoprotection in the presence of ISDN was paralleled by a lack of HO-1 and ferritin stimulatory capacity (Figures 2 and 5). ISDN had no significant effect on carbon monoxide release or bilirubin formation (Figures 3 and 4). These observations are in agreement with results demonstrating small or nondetectable amounts of NO released from ISDN and its active metabolite isosorbide mononitrate (ISMN) measured as cyclic GMP formation in RFL-6 reporter cells (Figure 7). Interestingly and in contrast to PETN, isosorbide nitrates are known to induce tolerance to their cardiovascular effects, presumably via oxidant stress. Moreover, in earlier investigations aimed at assessing the antiatherogenic potential of nitrates, PETN but not isosorbide nitrates prevented plaque formation and endothelial dysfunction in animal models of atherosclerosis. Thus, the ability to activate HO-1 induction and associated antioxidant pathways apparently distinguishes PETN from other long-acting nitrates and may explain their different patterns of action in vivo (Figure 9).