of the Max - MDC

Cytochrome P450-dependent
Eicosanoids in the Regulation
of Cellular and Organ Function
Wolf-Hagen Schunck
The major focus of our laboratory has been the role of arachidonic acid (AA) metabolizing cytochrome P450 (CYP)
enzymes in the regulation of vascular, renal and cardiac function. CYP enzymes convert AA to epoxyeicosatrienoic
acids (EETs) and 20-hydroxyeicosatetraenoic acid (20-HETE). These eicosanoids function as second messengers
of various hormones and growth factors. Previously, we demonstrated in animal models that hypertension and endorgan
damage is associated with decreased EET-levels and that the disease state can be ameliorated by inducing
EET-generation. Our recent studies with recombinant human, rat and mouse enzymes revealed that various CYP isoforms
do not only metabolize AA but also fish oil omega-3 fatty acids (n-3 PUFA). Omega-3 epoxidation resulted in
novel metabolites regulating the contractility of vascular smooth muscle cells and of cardiomyocytes. Therefore, we
are now interested to test the hypothesis that clinically established beneficial effects of n-3 PUFAs such as improvement
of vascular function and protection against cardiac arrhythmia may rely in part on a shift of CYP-dependent
eicosanoids from AA- to n-3 PUFA-derived metabolites.
Molecular properties of eicosanoid-generating
CYP enzymes
We analyzed the substrate specificity of recombinant EET-
(CYP2C and CYP2J isoforms) and 20-HETE synthases (CYP4A
and CYP4F isoforms). All CYP isoforms were able to metabolize
AA (20:4n n-6) as well as n-3 PUFAs like eicosapentaenoic
acid (EPA; 20:5, n-3). Chemical analysis revealed
that the omega-3 double bound was a preferred site of
epoxidation yielding 17,18-epoxy-EPA as a novel metabolite.
Each CYP isoform displayed a unique stereoselectivity in
terms of producing the R,S- or the S,R-enantiomer during
omega-3 epoxidation. Providing an example, Cyp4a12a,
identified by us as the major renal AA hydroxylase in male
mice, converted AA to 20-HETE but EPA mainly to
17(R),18(S)-epoxy-EPA.
Role of CYP-dependent eicosanoids in
vasodilatation
In collaboration with Maik Gollasch , we showed that 17,18-
epoxy-EPA is a highly potent activator of calcium-dependent
potassium (BK) channels. Based on this property, the
EPA metabolite was an efficient vasodilator. Cerebral artery
BK-channel activation was strictly stereoselective: only the
R,S-but not the S,R-enantiomer of 17,18-epoxy-EPA was
efficient. These findings provide an important link to the
stereoselectivity of enzymes generating this metabolite
(CYP isoforms) and subsequently inactivating it (soluble
epoxide hydrolase). Further studies are in progress to get
direct evidence that these mechanisms contribute to the
improvement of vascular function by n-3 PUFAs.
Role of CYP-dependent eicosanoids in cardiac
function
In collaboration with Dominik Müller and Friedrich Luft, we
demonstrated that n-3 PUFA treatment protects against
angiotensin II-induced end-organ damage in rats. The most
impressive effect was a strong reduction of mortality. Work
in progress with Robert Fischer and Alexander Schirdewan
explains this effect by a reduction of sudden cardiac death
based on antiarrhythmic properties of n-3 PUFAs. As a first
step to understand the mechanisms, we found that EPA protects
isolated rat hearts (with Ingo Morano) and even isolated
cardiomyocytes (with Gerd Wallukat) against calcium
overload, a condition that is linked to electrical and
mechanical heart abnormalities. In isolated cardiomyocytes,
the effect of EPA was reproduced by 17(R),18(S)-
epoxy-EPA. Based on these findings, we are now interested
to test the hypothesis that CYP enzymes contribute to cardiac
protection by converting n-3 PUFAs to antiarrhythmic
metabolites.
Selected Publications
Muller, DN, Schmidt, C, Barbosa-Sicard, E, Wellner, M, Gross, V,
Hercule, H, Markovic, M, Honeck, H, Luft, FC, Schunck WH.
(2007). Mouse Cyp4a isoforms: enzymatic properties, genderand
strain-specific expression, and role in renal 20-hydroxyeicosatetraenoic
acid formation. Biochem J. 403,109-18.
Hercule, H, Salanova, B, Essin, K, Honeck, H, Falck, JR,
Sausbier, M, Ruth, P, Schunck, WH, Luft, FC, Gollasch, M.
(2007). The vasodilator 17,18-epoxyeicosatetraenoic acid
targets the pore-forming BKa channel subunit. Exp Physiol.
in press.
18 Cardiovascular and Metabolic Disease Research