A previous study identified the
peroxisome proliferator-activated receptor alpha (
PPARalpha) activation
biomarkers 21-steroid
carboxylic acids 11beta-hydroxy-3,20-dioxopregn-4-en-21-oic
acid (HDOPA) and 11beta,20-dihydroxy-3-oxo-pregn-4-en-21-oic
acid (DHOPA). In the present study, the molecular mechanism and the metabolic pathway of their production were determined. The
PPARalpha-specific time-dependent increases in HDOPA and 20alpha-DHOPA paralleled the development of adrenal cortex
hyperplasia,
hypercortisolism, and spleen
atrophy, which was attenuated in adrenalectomized mice.
Wy-14,643 activation of
PPARalpha induced hepatic
FGF21, which caused increased
neuropeptide Y and
agouti-related protein mRNAs in the hypothalamus, stimulation of the
agouti-related protein/
neuropeptide Y neurons, and activation of the hypothalamic-pituitary-adrenal (HPA) axis, resulting in increased adrenal cortex
hyperplasia and
corticosterone production, revealing a link between
PPARalpha and the HPA axis in controlling energy homeostasis and immune regulation.
Corticosterone was demonstrated as the precursor of 21-carboxylic
acids both in vivo and in vitro. Under
PPARalpha activation, the classic reductive metabolic pathway of
corticosterone was suppressed, whereas an alternative oxidative pathway was uncovered that leads to the sequential oxidation on
carbon 21 resulting in HDOPA. The latter was then reduced to the end product 20alpha-DHOPA. Hepatic
cytochromes P450,
aldehyde dehydrogenase (ALDH3A2), and 21-hydroxysteroid
dehydrogenase (AKR1C18) were found to be involved in this pathway. Activation of
PPARalpha resulted in the induction of Aldh3a2 and Akr1c18, both of which were confirmed as target genes through introduction of promoter
luciferase reporter constructs into mouse livers in vivo. This study underscores the power of mass spectrometry-based metabolomics combined with genomic and physiologic analyses in identifying downstream metabolic
biomarkers and the corresponding upstream molecular mechanisms.