Oxidative Stress-Mediated Brain Dehydroepiandrosterone (DHEA) Formation in Alzheimer's Disease Diagnosis

Abstract

Neurosteroids are steroids made by brain cells independently of peripheral steroidogenic sources. The biosynthesis of most neurosteroids is mediated by proteins and enzymes similar to those identified in the steroidogenic pathway of adrenal and gonadal cells. Dehydroepiandrosterone (DHEA) is a major neurosteroid identified in the brain. Over the years we have reported that, unlike other neurosteroids, DHEA biosynthesis in rat, bovine, and human brain is mediated by an oxidative stress-mediated mechanism, independent of the cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1) enzyme activity found in the periphery. This alternative pathway is induced by pro-oxidant agents, such as Fe(2+) and β-amyloid peptide. Neurosteroids are involved in many aspects of brain function, and as such, are involved in various neuropathologies, including Alzheimer's disease (AD). AD is a progressive, yet irreversible neurodegenerative disease for which there are limited means for ante-mortem diagnosis. Using brain tissue specimens from control and AD patients, we provided evidence that DHEA is formed in the AD brain by the oxidative stress-mediated metabolism of an unidentified precursor, thus depleting levels of the precursor in the blood stream. We tested for the presence of this DHEA precursor in human serum using a Fe(2+)-based reaction and determined the amounts of DHEA formed. Fe(2+) treatment of the serum resulted in a dramatic increase in DHEA levels in control patients, whereas only a moderate or no increase was observed in AD patients. The DHEA variation after oxidation correlated with the patients' cognitive and mental status. In this review, we present the cumulative evidence for oxidative stress as a natural regulator of DHEA formation and the use of this concept to develop a blood-based diagnostic tool for neurodegenerative diseases linked to oxidative stress, such as AD.

Keywords: Alzheimer’s disease; dehydroepiandrosterone; diagnostic tool; neurosteroids.

Figure 1

Fenton’s reaction occurs between hydrogen peroxide and Ferrous Iron(II). Ferrous Iron(II) oxidized by hydrogen peroxide to ferric iron(III), a hydroxyl radical, and a hydroxyl anion. Iron(III) is then reduced back to iron(II), a peroxide radical, and a proton by the same hydrogen peroxide.

Figure 2

Mechanism of ketone formation from hydroperoxides. This scheme illustrates how the addition of FeSO₄ could reduce the 17-hydroperoxide of PREG to an intermediate alkoxy radical. Cleavage of the two-carbon side-chain by b-fragmentation results in the formation of DHEA.

Figure 3

Samples were extracted, and DHEA was purified by HPLC and measured using a specific radioimmunoassay. DHEA levels (pg/mg protein) in brain samples from AD and age-matched control patients with and without treatment with 30 mM FeSO₄ (adapted from Brown et al., 2003).

Figure 4

Outline of the methodology used to determine serum DHEA levels and formation in response to FeSO₄ treatment (A). Variation of DHEA levels before and after Fe²⁺ oxidation of human sera from AD and age-matched control patients (adapted from Rammouz et al., 2011).