Role of a disordered steroid metabolome in the elucidation of sterol and steroid biosynthesis

Abstract

In 1937 Butler and Marrian found large amounts of the steroid pregnanetriol in urine from a patient with the adrenogenital syndrome, a virilizing condition known to be caused by compromised adrenal secretion even in this pre-cortisol era. This introduced the concept of the study of altered excretion of metabolites as an in vivo tool for understanding sterol and steroid biosynthesis. This approach is still viable and has experienced renewed significance as the field of metabolomics. From the first cyclized sterol lanosterol to the most downstream product estradiol, there are probably greater than 30 steps. Based on a distinctive metabolome clinical disorders have now been attributed to about seven post-squalene cholesterol (C) biosynthetic steps and around 15 en-route to steroid hormones or needed for further metabolism of such hormones. Forty years ago it was widely perceived that the principal steroid biosynthetic defects were known but interest rekindled as novel metabolomes were documented. In his career this investigator has been involved in the study of many steroid disorders, the two most recent being P450 oxidoreductase deficiency and apparent cortisone reductase deficiency. These are of interest as they are due not to mutations in the primary catalytic enzymes of steroidogenesis but in ancillary enzymes needed for co-factor oxido-reduction A third focus of this researcher is Smith-Lemli-Opitz syndrome (SLOS), a cholesterol synthesis disorder caused by 7-dehydrocholesterol reductase mutations. The late George Schroepfer, in whose honor this article has been written, contributed greatly to defining the sterol metabolome of this condition. Defining the cause of clinically severe disorders can lead to improved treatment options. We are now involved in murine gene therapy studies for SLOS which, if successful could in the future offer an alternative therapy for this severe condition.

Fig. 1

Cortisol biosynthesis and the first reported forms of Congenital Adrenal Hyperplasia (CAH). Identifying urinary metabolites and defining these conditions confirmed the postulated adrenal biosynthetic pathway determined by in vitro experimentation. The first form of CAH investigated was 21-hydroxylase deficiency [1], followed by 11β-hydroxylase deficiency [2] and 3β-hydroxysteroid dehydrogenase deficiency [3]. LAH patients [4] essentially had absence of urinary metabolites, with adrenal cholesterol build-up. 17-Hydroxylase deficiency [5] was the final form defined during the classical era. The major urinary metabolites are shown in italics, steroid hormones and their precursors in conventional script

Fig. 2

Oxido-reductase and 17,20-lyase activity. 17-Hydroxylase and C17,20-lyase activities reside in a single protein encoded by a single gene. This enzyme only works effectively in concert with P450 oxido-reductase which is necessary for providing the electrons required for oxidation. The lyase function of the hydroxylase enzyme also needs cytochrome b5

Fig. 3

Proposed alternative pathway of feto-placental androgen synthesis from 17-hydroxypregnenolone. The lower-left corner represents the conventional pathway of androgen synthesis requiring sequential side-chain removal, oxidation by 3β, 17-keto reduction by 17βHSD and finally 5α-reduction terminating in DHT. The alternative pathway (upper right corner) starts with the formation of 17-hydroxyprogesterone and its reduction to 5α-pregnane-3α,17α-diol-20-one. In spite of attenuated 17,20-lyse activity due to POR deficiency this steroid can be converted to androsterone and likely on to DHT

Fig. 4

Cortisone reductase deficiency. This disorder was discovered in affected patients through the domination in urine of THE and the cortolones, corticosteroids with 11-carbonyl function. The analogous steroids with 11β-hydroxyl function, the epimeric tetrahydrocortisols (THFs) and cortols were severely diminished. This reduction was attributed to attenuated 11βHSD1 activity preventing hepatic regeneration of cortisol from cortisone. Reduced activity was shown to be caused by mutations in H6PDH responsible for NADP oxido-reduction

Fig. 5

Structures of Estriol and Dehydroestriol. Estriol together with pregnanediol are the two dominant steroids excreted during pregnancy, products of feto-placental steroid synthesis. Women with a SLOS affected steroids excrete dehydroestriols, predominantly Δ⁸. Measurement of 8-dehydroestriol can be used for prenatal diagnosis of the condition

Fig. 6

Sterol and steroid synthesis and metabolism. Simplified scheme showing reported deficiencies in steroid synthesis and metabolism. A list of disorders and the causes attributed are given in Table 1