The rate-limiting step of adrenal steroidogenesis and adenosine 3':5'-monophosphate

Benzo[a]pyrene reduces testosterone production in rat Leydig cells via a direct disturbance of testicular steroidogenic machinery

BACKGROUND

Benzo[a]pyrene (B[a]P), a polycyclic aromatic hydrocarbon (PAH), is a ubiquitous environmental pollutant that is currently suspected of being an endocrine disruptor. The testis is an important target for PAHs, yet insufficient attention has been paid to their effects on steroidogenesis in Leydig cells.

OBJECTIVE

We hypothesized that long-term exposure to low concentrations of B[a]P might disrupt testosterone production in Leydig cells via an alteration of steroidogenic proteins.

RESULTS

Oral exposure to B[a]P reduced serum and intratesticular fluid testosterone levels in rats. However, we did not observe serious testicular atrophy or azoospermia, although spermatogonial apoptosis was significantly increased. Compared with control cells, Leydig cells primed with B[a]P in vivo produced less testosterone in response to human chorionic gonadotropin (hCG) or dibutyl cyclic adenosine monophosphate in vitro. Of note, the reduction of testosterone levels was accompanied by decreased expression of steroidogenic acute regulatory protein (StAR) and 3β-hydroxysteroid dehydrogenase (3β-HSD), as well as increased levels of cytochrome P450 side chain cleavage (P450scc), in Leydig cells. The up-regulation of P450scc expression after exposure to B[a]P appears to be associated with a compensatory mechanism for producing the maximum amount of pregnenolone with the minimum amount of transported cholesterol by StAR; the down-regulation of 3β-HSD may occur because B[a]P can negatively target 3β-HSD, which is required for testosterone production.

CONCLUSIONS

B[a]P exposure can decrease epididymal sperm quality, possibly by disturbing testosterone levels, and StAR may be a major steroidogenic protein that is targeted by B[a]P or other PAHs.

Involvement of protein kinase C and cyclic adenosine 3',5'-monophosphate-dependent kinase in steroidogenic acute regulatory protein expression and steroid biosynthesis in Leydig cells

This study investigated the roles of the protein kinase C (PKC) and protein kinase A (PKA) pathways in regulating constitutive steroidogenesis and steroidogenic acute regulatory (STAR; herein designated by its common name, StAR) protein in R2C Leydig tumor cells. Inhibition of PKC and phospholipase C resulted in significant decreases in steroid production, phosphorylation of cAMP-responsive element binding (CREB) protein, and Star gene transcription under basal conditions in R2C cells. These observations were corroborated in MA-10 and mLTC-1 Leydig tumor cell lines, in which activation of PKC by phorbol-12-myristate-13-acetate (PMA, 10 nM) increased CREB phosphorylation and total StAR (tot-StAR) protein expression. However, induction of StAR protein by PMA did not result in the expected concomitant increase in steroids because PKC failed to phosphorylate StAR, the biologically active form of the protein. However, in conjunction with PMA, minor increases in PKA activity using submaximal doses of (Bu)2cAMP (0.05-0.1 mM; a concentration range insufficient for induction of StAR), were able to stimulate dramatic increases in both phospho-StAR (P-StAR) and steroid production. Human chorionic gonadotropin stimulation also resulted in a further enhancement in P-StAR and progesterone production when added to PMA-treated MA-10 cells. Similar results for tot-StAR and P-StAR expression were observed in primary cultures of immature rat Leydig cells treated with PMA and submaximal doses of (Bu)2cAMP. In summary, the present study demonstrates that basal activities of both PKC and PKA play important roles in the constitutive steroidogenic characteristics of R2C cells. This study also demonstrates for the first time a role for PMA-induced PKC in StAR protein regulation and the requirement for submaximal doses of cAMP to produce steroids in Leydig cells.

Controle de dopagem de anabolizantes: o perfil esteroidal e suas regulações

O conceito de perfil esteroidal é discutido neste artigo. As principais vias biossintéticas são apresentadas. A importância do monitoramento do perfil esteroidal é demonstrada dentro da clínica médica e da medicina esportiva. Parâmetros da literatura para a identificação de dopagem por esteróides endógenos são apresentados, assim como os fatores que acarretam alterações no perfil esteroidal normal. É dada atenção especial a essa última abordagem.

Cholesterol sulfate: a naturally-occurring inhibitor of cholesterol side-chain cleavage which functions at the level of intramitochondrial cholesterol translocation

Cholesterol sulfate is a naturally occurring compound which is widely distributed among various tissues including the adrenal cortex. When added to adrenal mitochondria from ether-stressed rats, it inhibits pregnenolone synthesis from exogenous but not endogenous cholesterol. Evidence supports a locus of action at the level of an intramitochondrial cholesterol translocation system with no effect on the side-chain cleavage enzymatic system (cytochrome P-450scc). Levels of endogenous cholesterol sulfate in the adrenal are similar to the Ki for inhibition by this compound, suggesting a possible physiological role. Moreover, quantities of cholesterol sulfate present in isolated mitochondria from ether-stressed animals are variable, and levels correlate inversely with rates of pregnenolone production by these preparations. In the presence of malate, mitochondrial cholesterol sulfate is metabolized slowly to pregnenolone sulfate, and its removal correlates with activation of cholesterol side-chain cleavage. Cholesterol sulfate levels are not regulated acutely by stress, but can be decreased significantly after several weeks of daily injection of ACTH (a model of chronic stress). Such treatments result in an increased capacity of isolated adrenal mitochondria to synthesize pregnenolone, and increased activity correlates with increased levels of circulating corticosterone. Thus, we propose that cholesterol sulfate is a new physiological regulator of steroid hormone biosynthesis which may function to regulate the magnitude of the steroidogenic response of the adrenal cortex to ACTH.

Compartmentalization of corticotropin-dependent steroidogenic factors in adrenal cortex: evidence for a post-translational cascade in stimulation of the cholesterol side-chain split

A sensitive cell-free assay was developed for the analysis of corticotropin-dependent factors that stimulate the rate-limiting step of adrenal steroidogenesis. In this assay adrenal post-mitochondrial supernates from corticotropin-stimulated rats caused a 10- to 100-fold increase in the de novo synthesis of pregnenolone and progesterone. A similar stimulation was observed by corresponding fractions from Leydig cells and mouse Y-1 adrenal tumor cells, but not from rat liver. Subcellular fractionation of rat adrenal tissue showed several steroidogenic factors to be present in various compartments. Recombination of them produced highly synergistic effects. The activation of some components could also be demonstrated in vitro, suggesting a cascade of events possibly linking the cAMP-dependent phosphorylation pathway with the rate-limiting step. Cycloheximide prevented the production of these steroidogenic factors in vivo upon stimulation but had no effect in vitro, suggesting a post-translational cascade involved in the activation of the cholesterol side-chain split.

Pregnenolone efflux from mitochondria of bovine adrenal cortex

Pregnenolone efflux from bovine adrenal cortex mitochondria was measured with the aid of rapid separation of the organelles from the incubation by centrifugation through silicon oil. In the absence of bovine serum albumin or cytosolic supernatant (100 000 x g), an equilibrium distribution of newly synthesised pregnenolone was attained after 2 min at 30 degrees C; 20% of total pregnenolone was located outside the mitochondria. The presence of 13 mg albumin/ml or 13 mg supernatant protein/ml increased the exogenous proportion of pregnenolone to over 90% and 75% respectively without affecting the rate of cholesterol side-chain cleavage. The binding of pregnenolone by both albumin and supernatant was non-specific and non-saturable. It is suggested that in vivo the mitochondrial membranes do not present a significant barrier to pregnenolone efflux from the organelle.

Regulation of cholesterol metabolism in rat adrenal mitochondria

Addition of cholesterol to rat adrenal mitochondria resulted in a stimulation of pregnenolone synthesis. The slow step of the mitochondrial cholesterol side-chain cleavage reaction could be the interaction of the sterol with cytochrome P-450. The rate of cholesterol binding to this enzyme as observed spectroscopically correlated with the equilibration period (20 min) of the mitochondria and exogenous cholesterol required for maximal rates of pregnenolone synthesis. It is suggested that translocation of cholesterol between different sterol pools occurs within the mitochondria. Potential intracellular effectors that could be of importance in the movement or regulation of mitochondrial cholesterol include bivalent metallic ions, prostaglandins, cyclic nucleotides, polyamines and polylysine. Of the effectors studied, only calcium ions and polylysine markedly stimulated pregnenolone synthesis. These effectors might stimulate steroidogenesis by lateral displacement of cholesterol in the mitochondrial membrane into a compartment easily accessible to the cholesterol side-chain cleavage enzyme complex.

Effect of prostaglandins on steroidogenesis by bovine adrenal cortex mitochondria

Prostaglandins (PGA1, PGE2, PGF2 alpha) were found to increase cholesterol side-chain clevage activity in isolated bovine adrenal cortex mitochondria, provided calcium was present in the incubation medium. Optimal stimulation was observed at low PG concentrations (10-7 to 10-9 M), with malate or malate-NADPH supported side-chain cleavage. Under the same conditions, two endoperoxide analogs and several fatty acids were ineffective. The PG action was not observed with a mitochondrial acetone powder preparation. These observations suggest that primary PG may act by interfering with calcium distribution at the mitochondrial level, leading to the activation of cholesterol side-chain cleavage. Thus, an intracellular action of endogenous PG may be considered in the regulation of adrenal cortex steroidogenic functions.

Insect prothoracic glands: a role for cyclic AMP in the stimulation of alpha-ecdysone secretion

The prothoracic glands of the tobacco hornworm, Manduca sexta, were studied to determine if cyclic AMP is involved in the regulation of alpha-ecdysone secretion. Culturing glands in the presence of the phosphodiesterase inhibitors, aminophylline and 1-methyl-3-isobutylxanthine, caused a greater than 2-fold stimulation of ecdysone secretion while cyclic AMP alone was ineffective. Based on a dose-response analysis, 1-methyl-3-isobutylxanthine was 200 times more potent than aminophylline. Measurements of endogenous prothoracic gland cyclic AMP during the fifth larval instar demonstrated that dramatically increased levels preceded the increase in in vitro ecdysone-secretory ability. The data suggest that cyclic AMP may act as a second messenger in the stimulation of prothoracic gland alpha-ecdysone secretion by the prothoracicotropic brain hormone.

Effect of Ca2+, ruthenium red and ageing on pregnenolone production by mitochondrial fractions from normal and luteinizing hormone treated rat testes

The effect of Ca2+ in vitro on pregnenolone production rates under various incubation conditions by mitochondrial fractions isolated from testes of normal rats and of rats after in vivo treatment with luteinizing hormone has been investigated. Concentrations of Ca2+ in the range of 0.1-0.5 mM stimulated succinate supported pregnenolone production in mitochondrial fractions from both control and luteinizing hormone treated testes. When mitochondrial fractions were isolated in 0.25 M sucrose without additions, Ca2+ in vitro increased succinate supported pregnenolone production rates in mitochondrial fractions isolated from control testes to a greater extent than in mitochondrial fractions, from luteinizing hormone treated testes. Production rates in control mitochondrial fraction, incubated in the presence of initial Ca2+ concentrations of 0.7 mM and higher were almost similar to production rates in relevant luteinizing hormone treated mitochondria.