Pregnenolone

Pregnenolone (P5), or pregn-5-en-3β-ol-20-one, is an endogenous steroid and a precursor/metabolic intermediate in the biosynthesis of most steroid hormones, including progestogens, androgens, estrogens, glucocorticoids, and mineralocorticoids. Additionally, pregnenolone is biologically active on its own, functioning as a neurosteroid.

Besides its role as a natural hormone, pregnenolone has been utilized as a medication and supplement; for more details on pregnenolone as a medication or supplement.

Biological function

Pregnenolone and its 3β-sulfate, pregnenolone sulfate, like dehydroepiandrosterone (DHEA), DHEA sulfate, and progesterone, are part of the neurosteroids group found in high concentrations in specific brain regions and synthesized there. Neurosteroids influence synaptic functioning, are neuroprotective, and enhance myelinization. Pregnenolone and its sulfate ester may improve cognitive and memory function. Moreover, they may offer protective effects against schizophrenia.

Biological activity

Neurosteroid activity

Pregnenolone acts as an allosteric endocannabinoid, serving as a negative allosteric modulator of the CB1 receptor. It is part of a natural negative feedback mechanism against CB1 receptor activation in animals, preventing full activation by CB1 receptor agonists like tetrahydrocannabinol, the primary active component in cannabis. A related compound, AEF0117, derived from pregnenolone, is more specific for this activity.

Pregnenolone has been found to bind with high nanomolar affinity to microtubule-associated protein 2 (MAP2) in the brain. Unlike pregnenolone, pregnenolone sulfate did not bind to microtubules. However, progesterone did, with a similar affinity to pregnenolone, but did not enhance MAP2 binding to tubulin. Pregnenolone was shown to induce tubule polymerization in neuronal cultures and stimulate neurite growth in PC12 cells treated with nerve growth factor. Thus, pregnenolone may regulate microtubule formation and stabilization in neurons, impacting both neural development during prenatal development and neural plasticity during aging.

Although pregnenolone itself lacks these activities, its metabolite, pregnenolone sulfate, is a negative allosteric modulator of the GABAA receptor and a positive allosteric modulator of the NMDA receptor. Additionally, pregnenolone sulfate has been shown to activate the transient receptor potential M3 (TRPM3) ion channel in hepatocytes and pancreatic islets, leading to calcium entry and subsequent insulin release.

Nuclear receptor activity

Pregnenolone acts as an agonist of the pregnane X receptor.

Pregnenolone does not exhibit progestogenic, corticosteroid, estrogenic, androgenic, or antiandrogenic activity.

Biosynthesis

Pregnenolone is synthesized from cholesterol. This conversion involves hydroxylation of the side chain at the C20 and C22 positions, with cleavage of the side chain. The responsible enzyme is cytochrome P450scc, located in the mitochondria, and regulated by anterior pituitary trophic hormones, such as adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone, in the adrenal glands and gonads.

There are two intermediates in cholesterol's transformation into pregnenolone: 22R-hydroxycholesterol and 20α,22R-dihydroxycholesterol. All three steps in the transformation are catalyzed by P450scc. Pregnenolone is mainly produced in the adrenal glands, gonads, and brain. Although pregnenolone is also synthesized in the gonads and brain, most circulating pregnenolone originates from the adrenal cortex.

To assay cholesterol to pregnenolone conversion, radiolabeled cholesterol has been utilized. Pregnenolone product can be separated from cholesterol substrate using Sephadex LH-20 minicolumns.

Distribution

Pregnenolone is lipophilic and easily crosses the blood–brain barrier, unlike pregnenolone sulfate, which does not.

Metabolism

Pregnenolone undergoes further steroid metabolism in several ways:

• Pregnenolone can be converted into progesterone. This involves two critical enzyme steps using 3β-hydroxysteroid dehydrogenase and Δ5-4 isomerase. The latter shifts the double bond from C5 to C4 on the A ring. Progesterone enters the Δ4 pathway, leading to 17α-hydroxyprogesterone and androstenedione, precursors to testosterone and estrone. Aldosterone and corticosteroids also derive from progesterone or its derivatives.

  
  

• Pregnenolone can be converted to 17α-hydroxypregnenolone by the enzyme 17α-hydroxylase (CYP17A1). In this Δ5 pathway, the next step is conversion to dehydroepiandrosterone (DHEA) via 17,20-lyase (CYP17A1). DHEA is a precursor of androstenedione.

  
  

• Pregnenolone can be transformed into androstadienol by 16-ene synthase (CYP17A1).

  
  

• Pregnenolone can be converted to pregnenolone sulfate by steroid sulfotransferase, and this conversion can be reversed by steroid sulfatase.

Levels

Normal circulating levels of pregnenolone are:

• Men: 10 to 200 ng/dL

• Women: 10 to 230 ng/dL

• Children: 10 to 48 ng/dL

• Adolescent boys: 10 to 50 ng/dL

• Adolescent girls: 15 to 84 ng/dL

Mean levels of pregnenolone do not significantly differ in postmenopausal women and elderly men (40 and 39 ng/dL, respectively).

Studies indicate that pregnenolone levels remain largely unchanged after surgical or medical castration in men, aligning with the fact that pregnenolone primarily originates from the adrenal glands. Conversely, medical castration has been found to partially suppress pregnenolone levels in premenopausal women. Similarly, an adrenalectomized premenopausal woman showed only partially reduced circulating pregnenolone levels.

Chemistry

Pregnenolone, chemically known as pregn-5-en-3β-ol-20-one, like other steroids, consists of four interconnected cyclic hydrocarbons. It contains ketone and hydroxyl functional groups, two methyl branches, and a double bond at C5 in the B cyclic hydrocarbon ring. Like many steroid hormones, it is hydrophobic. The sulfated derivative, pregnenolone sulfate, is water-soluble.

3β-Dihydroprogesterone (pregn-4-en-3β-ol-20-one) is an isomer of pregnenolone, with the C5 double bond replaced by a C4 double bond.