|dc.description.abstract||7-Oxysterols constitute the major component (40%) of oxidized low-density
lipoprotein (oxLDL). They arise in the body via auto-oxidation of cholesterol and are
known to induce endothelial dysfunction, oxidative stress and apoptosis in the
vascular wall, prior to development of atherosclerosis. A novel pathway has been
described for hepatic inter-conversion of 7-ketocholesterol (7-KC) and 7β -hydroxycholesterol
(7β OHC) by the enzyme 11β-hydroxysteroid dehydrogenase type-1
(11β HSD1), better known for metabolizing glucocorticoids. Inhibition of 11βHSD1
is atheroprotective and the potential underlying mechanism for this may involve
altered metabolism and actions of glucocorticoids. However, alterations in the
metabolism of 7-oxysterols may also play an important role in this atheroprotective
The work described here addresses the hypotheses that (i) 7-oxysterols are substrates
for murine 11βHSD1; (ii) inhibition of 11β HSD1 may abolish cellular metabolism of
7-oxysterols; (iii) this route of metabolism may modulate the actions of 7-oxysterols
and glucocorticoids on murine vascular physiology.
Murine 11β HSD1 inter-converted 7-oxysterols (Km=327.6±98ìM,
Vmax=0.01±0.001pmol/ìg/min) but the regulation of reaction direction is different
from that for glucocorticoids. Predominant dehydrogenation of 7β OHC to 7-KC was
quantified in several models (recombinant protein, cultured cells stably transfected
with 11β HSD1), in which predominant reduction of glucocorticoids was measured.
Furthermore, in murine hepatic microsomes, dehydrogenation of 7β OHC occurred
exclusively. In aortic rings in culture, however, both reduction and dehydrogenation
of 7-oxysterols were evident. 7-Oxysterols and glucocorticoid substrates competed
for metabolism by 11β HSD1, with 7β OHC inhibiting dehydrogenation of
The circulating concentrations of 7-oxysterols in the plasma of C57Bl6 and
11β HSD1-/- mice were in the ìM range (0.02 – 0.13ìM). The disruption of
11β HSD1 has resulted in increased ratios of 7-KC and 7β OHC over total plasma
cholesterol levels (*p<0.05). This finding suggested that 11β HSD1 is involved in
metabolizing and determining the plasma levels of 7-KC and 7β OHC.
To assess the consequences of these alterations for vascular function, studies were
undertaken in aortic rings. Prolonged incubation with 7-oxysterols (20-25 ìM)
showed a tendency to attenuate noradrenaline-mediated contractions of C57Bl6
aortae, but had no effect on contractions in response to 5-hydroxytryptamine or KCl.
Similarly, endothelium-dependent and -independent relaxations of murine aortae
were unaltered after exposure to 7-oxysterols.
Thus in the mouse, 11β HSD1 may influence the balance of circulating and cellular
7-oxysterols which may have consequential effects on glucocorticoid action.
Although this work suggests that concentrations present in murine tissues are
unlikely to cause vascular dysfunction, they may influence further cellular events as
yet undescribed. Under pathological conditions where high concentrations of
7-oxysterols occur, 11β HSD1 may influence the extracellular-transport and delivery
of 7-KC and 7β OHC to the plaque. This work therefore proposes that inhibition of
metabolism of 7-oxysterols by 11β HSD1 inhibitors, may contribute to the
atheroprotective effects of these drugs.||en