Non-stereo-selective cytosolic human brain tissue 3-ketosteroid reductase is refractory to inhibition by AKR1C inhibitors

Cerebral 3α-hydroxysteroid dehydrogenase (3α-HSD) activity was suggested to be responsible for the local directed formation of neuroactive 5α,3α-tetrahydrosteroids (5α,3α-THSs) from 5α-dihydrosteroids. We show for the first time that within human brain tissue 5α-dihydroprogesterone and 5α-dihydrotestosterone are converted via non-stereo-selective 3-ketosteroid reductase activity to produce the respective 5α,3α-THSs and 5α,3β-THSs. Apart from this, we prove that within the human temporal lobe and limbic system cytochrome P450c17 and 3β-HSD/Δ5–4 ketosteroid isomerase are not expressed. Thus, it appears that these brain regions are unable to conduct de novo biosynthesis of Δ4-3-ketosteroids from Δ5-3β-hydroxysteroids. Consequently, the local formation of THSs will depend on the uptake of circulating Δ4-3-ketosteroids such as progesterone and testosterone. 3α- and 3β-HSD activity were (i) equally enriched in the cytosol, (ii) showed equal distribution between cerebral neocortex and subcortical white matter without sex- or age-dependency, (iii) demonstrated a strong and significant positive correlation when comparing 46 different specimens and (iv) exhibited similar sensitivities to different inhibitors of enzyme activity. These findings led to the assumption that cerebral 3-ketosteroid reductase activity might be catalyzed by a single enzyme and is possibly attributed to the expression of a soluble AKR1C aldo-keto reductase. AKR1Cs are known to act as non-stereo-selective 3-ketosteroid reductases; low AKR1C mRNA expression was detected. However, the cerebral 3-ketosteroid reductase was clearly refractory to inhibition by AKR1C inhibitors indicating the expression of a currently unidentified enzyme. Its lack of stereo-selectivity is of physiological significance, since only 5α,3α-THSs enhance the effect of GABA on the GABAA receptor, whereas 5α,3β-THSs are antagonists.

Research Highlights
► In the human brain 5α-dihydrosteroids are converted via 3α/β-HSD activity.
► The human brain is unable to conduct de novo biosynthesis of Δ4-3-ketosteroids.
► The local formation of tetrahydrosteroids depends on the uptake of Δ4-3-ketosteroids.
► Cytosolic cerebral 3α/β-HSD activity might be catalyzed by a single enzyme.
► AKR1Cs are expressed.
► The cerebral 3α/β-HSD activity is not affected by AKR1C inhibitors.
► The lack of stereo-selectivity is of physiological significance.
► Only 5α,3α-tetrahydrosteroids enhance the effect of GABA on the GABAA receptor.
► 5α,3β-tetrahydrosteroids are antagonists.