Comparison of changes in inositide and noninositide phospholipids during acute and prolonged adrenocorticotropic hormone treatment in vivo

Functional metabolite reserves and lipid homeostasis revealed by the MA-10 Leydig cell metabolome

In Leydig cells, intrinsic factors that determine cellular steroidogenic efficiency is of functional interest to decipher and monitor pathophysiology in many contexts. Nevertheless, beyond basic regulation of cholesterol storage and mobilization, systems biology interpretation of the metabolite networks in steroidogenic function is deficient. To reconstruct and describe the different molecular systems regulating steroidogenesis, we profiled the metabolites in resting MA-10 Leydig cells. Our results identified 283-annotated components (82 neutral lipids, 154 membrane lipids, and 47 other metabolites). Neutral lipids were represented by an abundance of triacyglycerols (97.1%), and low levels of cholesterol esters (2.0%). Membrane lipids were represented by an abundance of glycerophospholipids (77.8%), followed by sphingolipids (22.2%). Acylcarnitines, nucleosides, amino acids and their derivatives were the other metabolite classes identified. Among nonlipid metabolites, we recognized substantial reserves of aspartic acid, choline, creatine, betaine, glutamine, homoserine, isoleucine, and pantothenic acid none of which have been previously considered as a requirement in steroidogenic function. Individually limiting use of betaine, choline, or pantothenic acid, during luteinizing hormone-induced steroidogenesis in MA-10 cells resulted in substantial decreases to acute steroidogenic capacity, explained by intermediary metabolite imbalances affecting homeostasis. As such, our dataset represents the current level of baseline characterization and unravels the functional resting state of steroidogenic MA-10 Leydig cells. In identifying metabolite stockpiles and causal mechanisms, these results serve to further comprehend the cellular setup and regulation of steroid biosynthesis.

ACTH increases de novo synthesis of diacylglycerol and translocates protein kinase C in primary cultures of calf adrenal glomerulosa cells

Effects of ACTH on the production of diacylglycerol (DAG) and translocation of protein kinase C were studied in primary cultures of calf adrenal glomerulosa cells. To study DAG production two different labeling protocols were used: (a) cells were prelabeled for 3 days with [2-3H]glycerol before ACTH addition; (b) ACTH and [2-3H]glycerol were added simultaneously to cells. In both cases, ACTH provoked rapid increases in the labeling of DAG which were maximal in 2 min, dose-dependent, and paralleled by increases in DAG mass. ACTH also increased the labeling of total glycerolipids including phosphatidic acid (PA), phosphatidylinositol, phosphatidylethanolamine, phosphatidylcholine and triacylglycerol. In both labeling protocols, the rates of increase in the labeling of DAG and PA were greater than those of other glycerolipids. Our results indicate that ACTH rapidly increases DAG, at least partly by stimulating the de novo synthesis of PA. In addition, we found that ACTH, like phorbol esters, stimulated the apparent translocation of immunoreactive protein kinase C from the cytosol to the membrane fraction.

Enhanced 17 alpha-hydroxylation of pregnenolone and increased androgen production by rabbit adrenocortical cells stimulated chronically with corticotropin

The postulated chronic stimulatory effect of corticotropin (ACTH) on pregnenolone production and on 17 alpha-hydroxylase activity was evaluated on adrenocortical cells obtained from control and chronically ACTH-treated rabbits. The cells were incubated with various concentrations of ACTH added alone or together with trilostane, so as to inhibit further conversion of pregnenolone and dehydroepiandrosterone. The maximal steroidogenic effect of ACTH (determined in the absence of trilostane) was increased 2-fold in adrenocortical cells from ACTH-treated animals; furthermore, cortisol production was increased whereas that of corticosterone decreased. While the generation of pregnenolone was of comparable magnitude for cells from both experimental groups, chronic in vivo treatment with ACTH was followed by a 40-fold enhancement in 17-hydroxypregnenolone production. Concomitantly, maximal DHEA production documented in the presence of ACTH and trilostane was enhanced more than 200-fold, from 0.45 +/- 0.20 pmol in control rabbits to 147 +/- 67 pmol in cells from ACTH-treated animals. The corresponding values of DHEA-sulphate production were 0.86 +/- 0.12 and 432 +/- 334 pmol, respectively. Thus, a prolonged stimulatory effect of ACTH on rabbit adrenocortical cells consists in an enhancement of the capacity to generate pregnenolone, and to convert this compound into 17-hydroxylated steroids.

Effect of cholesterol and protein content on membrane fluidity and 3 beta-hydroxysteroid dehydrogenase activity in mitochondrial inner membranes of bovine adrenal cortex

The steroid biosynthetic enzymes in the adrenal cortex are localised in endoplasmic reticulum and mitochondrial membranes. For some of the enzymes in endoplasmic reticulum the activity appears to be modulated by lipid fluidity, (21-hydroxysteroid hydroxylase and 3 beta-hydroxysteroid dehydrogenase). A mechanism for the regulation of corticosteroid biosynthesis mediated by the membrane fluidity has been suggested. Therefore a study of the mitochondrial inner membrane of the bovine adrenal cortex has been undertaken in comparison with a previous study of the endoplasmic reticulum. The kinetic parameters of the 3 beta-hydroxysteroid dehydrogenase were studied as a function of pH and temperature. No thermal transition can be observed in the Arrhenius plot for this enzyme in contrast with the results obtained for the microsomal enzyme. Membrane fluidity using, as fluorescent probes, diphenylhexatriene and a set of n-(9-anthroyloxy)fatty acids has been also studied as a function of temperature with or without addition of cholesterol. No thermal transition in the lipid phase can be observed. The addition of cholesterol to total mitochondrial membrane as to a lipid extract of the membrane decreases fluidity to the same extent as it does with microsomes. The presence of a large amount of protein in mitochondria has an effect which is additive to that of the cholesterol.

Apparent increases in phospholipid degradation and turnover during combined treatment with protein synthesis inhibitors and adrenocorticotropin

Inhibitors of protein synthesis, cycloheximide and puromycin, blocked ACTH (adrenocorticotropin)-induced increases in phospholipid mass, including phosphatidylinositol, but paradoxically increase 32P-labelling (but not [3H]glycerol-labelling) therein. Cycloheximide also provoked an initial rapid decrease in 32P-prelabelled phospholipids, followed by an increase in [32P]Pi incorporation. These effects of cycloheximide and puromycin occurred in ACTH-treated (but not in control) cells. It appears that inhibition of protein synthesis during ACTH action provokes an increase in phospholipid degradation, followed by partial resynthesis of the phospholipid head groups.

The role of the phosphatidate-inositide cycle in the action of steroidogenic agents

Most steroidogenic agents have been found to provoke rapid changes in the metabolism of phospholipids in the phosphatidate-inositide cycle. These changes include: (a) de novo synthesis of phosphatidic acid and its derivatives, phosphatidylglycerol, mono- and polyphosphoinositides and diglyceride; and (b) phosphatidylinositol hydrolysis and consequent generation of diglyceride and phosphatidic acid. The de novo phosphatidate synthesis effect occurs in the action of all tested steroidogenic agents (ACTH, LH, angiotensin, K+, serotonin) and may be induced by either of the "second messengers", cAMP and Ca2+. The phosphatidylinositol hydrolysis effect(s) occurs only in the action of steroidogenic agents that operate via Ca2+ (angiotensin and K+) and may be important in Ca2+ mobilization. The de novo phospholipid effect correlates well with changes in steroidogenesis and, like the latter, requires Ca2+ and protein synthesis; from these and other results, it seems likely that this phospholipid effect plays an important role in the stimulation of steroidogenesis.

The phosphatidate-phosphoinositide cycle: an intracellular messenger system in the action of hormones and neurotransmitters

Many hormones and neurotransmitters provoke rapid and striking changes in the metabolism of phospholipids in the phosphatidate-inositide cycle. These changes appear to occur before and after the generation of other accepted "second messengers" (e.g., Ca++ and cAMP), and seem to be important intracellular effector substances for the elicitation of biological effects. The two major mechanisms for perturbing the phosphatidate-inositide cycle are phosphatidylinositol hydrolysis and de novo phosphatidate-inositide synthesis. Phosphatidylinositol hydrolysis occurs in the action of all agents which operate via Ca++ and appears to be provoked both by Ca++-dependent and Ca++-independent mechanisms. Ca++-independent phosphatidylinositol hydrolysis may be triggered directly by receptor activation and may control Ca++ release or entry into the cytosol. Ca++-dependent phosphatidylinositol hydrolysis may be important for further changes in cellular Ca++ distribution and membrane fusion during exocytosis. The de novo phosphatidate-inositide synthesis effect has been observed in the action of most steroidogenic agents (ACTH, luteinizing hormone, angiotensin II, K+, serotonin), parathyroid hormone and insulin. The de novo effect is inhibited by cycloheximide, requires Ca++, and appears to serve as a post-second messenger mechanism to alter membrane structure and the function of membrane associated substances. Considerable evidence suggests that the de novo effect is important in the control of steroidogenesis by the above-mentioned agents; it may also be important in the action of insulin in adipose tissue.