The role of microfilaments in the response of adrenal tumor cells to adrenocorticotropic hormone

STARD1 Functions in Mitochondrial Cholesterol Metabolism and Nascent HDL Formation. Gene Expression and Molecular mRNA Imaging Show Novel Splicing and a 1:1 Mitochondrial Association

STARD1 moves cholesterol (CHOL) from the outer mitochondrial membrane (OMM) to the inner membrane (IMM) in steroidogenic cells. This activity is integrated into CHOL trafficking and synthesis homeostasis, involving uptake through SR-B1 and LDL receptors and distribution through endosomes, ER, and lipid droplets. In adrenal cells, STARD1 is imported into the mitochondrial matrix accompanied by delivery of several hundred CHOL molecules. This transfer limits CYP11A1-mediated generation of pregnenolone. CHOL transfer is coupled to translation of STARD1 mRNA at the OMM. In testis cells, slower CHOL trafficking seems to be limiting. STARD1 also functions in a slower process through ER OMM contacts. The START domain of STARD1 is utilized by a family of genes, which includes additional STARD (forms 3-6) and GRAMD1B proteins that transfer CHOL. STARD forms 2 and 7 deliver phosphatidylcholine. STARD1 and STARD7 target their respective activities to mitochondria, via N-terminal domains (NTD) of over 50 amino acids. The NTD is not essential for steroidogenesis but exerts tissue-selective enhancement (testis>>adrenal). Three conserved sites for cleavage by the mitochondrial processing protease (MPP) generate three forms, each potentially with specific functions, as demonstrated in STARD7. STARD1 is expressed in macrophage and cardiac repair fibroblasts. Additional functions include CHOL metabolism by CYP27A1 that directs activation of LXR and CHOL export processes. STARD1 generates 3.5- and 1.6-kb mRNA from alternative polyadenylation. The 3.5-kb form exclusively binds the PKA-induced regulator, TIS11b, which binds at conserved sites in the extended 3'UTR to control mRNA translation and turnover. STARD1 expression also exhibits a novel, slow splicing that delayed splicing delivery of mRNA to mitochondria. Stimulation of transcription by PKA is directed by suppression of SIK forms that activate a CRTC/CREB/CBP promoter complex. This process is critical to pulsatile hormonal activation in vivo. sm-FISH RNA imaging shows a flow of single STARD1 mRNA particles from asymmetric accumulations of primary transcripts at gene loci to 1:1 complex of 3.5-kb mRNA with peri-nuclear mitochondria. Adrenal cells are similar but distinguished from testis cells by appreciable basal expression prior to hormonal activation. This difference is conserved in culture and in vivo.

Cofilin is a cAMP effector in mediating actin cytoskeleton reorganization and steroidogenesis in mouse and human adrenocortical tumor cells

cAMP pathway plays a major role in the pathogenesis of cortisol-producing adrenocortical adenomas (CPA). cAMP-induced steroidogenesis is preceded by actin cytoskeleton reorganization, a process regulated by cofilin activity. In this study we investigated cofilin role in mediating cAMP effects on cell morphology and steroidogenesis in adrenocortical tumor cells. We demonstrated that forskolin induced cell rounding and strongly reduced phosphorylated (P)-cofilin/total cofilin ratio in Y1 (-52 ± 16%, p < 0.001) and human CPA cells (-53 ± 18%, p < 0.05). Cofilin silencing significantly reduced both forskolin-induced morphological changes and progesterone production (1.3-fold vs 1.8-fold in controls, p < 0.05), whereas transfection of wild-type or S3A (active), but not S3D (inactive) cofilin, potentiated forskolin effects on cell rounding and increased 3-fold progesterone synthesis with respect to control (p < 0.05). Furthermore, cofilin dephosphorylation by a ROCK inhibitor potentiated forskolin-induced cell rounding and steroidogenesis (2-fold increase vs forskolin alone). Finally, we found a reduced P-cofilin/total cofilin ratio and increased cofilin expression in CPA vs endocrine inactive adenomas by western blot and immunohistochemistry. Overall, these results identified cofilin as a mediator of cAMP effects on both morphological changes and steroidogenesis in mouse and human adrenocortical tumor cells.

Regulation of aldosterone production from zona glomerulosa cells by ANG II and cAMP: evidence for PKA-independent activation of CaMK by cAMP

Aldosterone production in zona glomerulosa (ZG) cells of adrenal glands is regulated by various extracellular stimuli (K(+), ANG II, ACTH) that all converge on two major intracellular signaling pathways: an increase in cAMP production and calcium (Ca(2+)) mobilization. However, molecular events downstream of the increase in intracellular cAMP and Ca(2+) content are controversial and far from being completely resolved. Here, we found that Ca(2+)/calmodulin-dependent protein kinases (CaMKs) play a predominant role in the regulation of aldosterone production stimulated by ANG II, ACTH, and cAMP. The specific CaMK inhibitor KN93 strongly reduced ANG II-, ACTH-, and cAMP-stimulated aldosterone production. In in vitro kinase assays and intact cells, we could show that cAMP-induced activation of CaMK, using the adenylate cyclase activator forskolin or the cAMP-analog Sp-5,6-DCI-cBIMPS (cBIMPS), was not mediated by PKA. Activation of the recently identified cAMP target protein Epac (exchange protein directly activated by cAMP) by 8-pCPT-2'-O-Me-cAMP had no effect on CaMK activity and aldosterone production. Furthermore, we provide evidence that cAMP effects in ZG cells do not involve Ca(2+) or MAPK signaling. Our results suggest that ZG cells, in addition to PKA and Epac/Rap proteins, contain other as yet unidentified cAMP mediator(s) involved in regulating CaMK activity and aldosterone secretion.

Association of globular beta-actin with intracellular lipid droplets in rat adrenocortical cells and adipocytes

Proteins located on the surface of lipid droplets may mediate intracellular lipid metabolism. In the present study, immunofluorescent staining and polyacrylamide gel electrophoresis demonstrated that actin (43 kD) is associated with isolated intracellular lipid droplets of rat adrenocortical cells and adipocytes. Two-dimensional gel electrophoresis and immunoblot analysis further confirmed that the lipid droplet-associated actin is the beta isoform. In cultured adrenocortical cells, stress fibers and the surface of intracellular lipid droplets were labeled with anti-beta-actin monoclonal antibody, whereas FITC-phalloidin staining did not mark the rim of lipid droplets. The present results provide the first morphological evidence that globular beta-actin is associated with intracellular lipid droplets. This significant association of actin with the surface of lipid droplets suggests that beta-actin might be involved in the regulation of intracellular lipid metabolism, particularly providing insight into the important transport of lipid constituents.

Partial characterization of mitochondrial G proteins in adrenal cells

Four low molecular mass G proteins have been identified in mitochondrial membranes from bovine adrenal cortex. These proteins (referred to as proteins 1 to 4) showed molecular masses of 28, 27, 26 and 24 kDa with isoelectric points (pI) of 8.1, 5.6, and 6.3 respectively for proteins 1, 2 and 4. Protein 3 was shown to be heterogeneous, with isoelectric points of 5.0-6.1. Proteins were identified by binding of [alpha-(32)P]guanosine triphosphate (GTP) after separation by 12% SDS-polyacrylamide gel electrophoresis and transfer to nitrocellulose. Competitive binding by unlabelled competing nucleoside phosphate ligands showed specificity for guanosine triphosphate (GTP) and guanosine diphosphate (GDP) with little binding of guanosine monophosphate and no detectable binding with adenosine nucleoside phosphates. Binding was less than 10% with 100-fold excess GDP and GTP which showed equal intensities of binding. Inhibition of binding by 1000-fold cytidine triphosphate and uridine triphosphate was approx. 10%. Magnesium (Mg(2+)) stimulated binding of GTP by all four proteins. The effect of Mg(2+) was essentially the same for proteins 1, 2 and 3, while protein 4 was less sensitive to Mg(2+) at concentrations <10(-3) M. Centrifugation of sonicated mitochondrial membranes through sucrose density gradients showed the presence of all four proteins in contact points. The presence of lower concentrations (expressed per mg protein) of the proteins in inner and outer membranes suggests that either small amounts of these membranes are part of contact points as presently prepared or that the proteins occur in contact points and to a much smaller extent in inner and outer membranes. It is proposed to examine a possible role for these proteins in transport of cholesterol from outer to inner mitochondrial membranes.

Effects of angiotensin II and adrenocorticotropic hormone on myristoylated alanine-rich C-kinase substrate phosphorylation in glomerulosa cells

Angiotensin II (AngII) is thought to stimulate aldosterone secretion from bovine adrenal glomerulosa cells in part via activation of protein kinase C (PKC), while adrenocorticotropic hormone (ACTH) functions through increases in intracellular cAMP levels and calcium influx. Rather than using invasive homogenization techniques as in previous studies, we chose to monitor PKC activity in intact glomerulosa cells in situ by measuring the phosphorylation of the endogenous PKC substrate, myristoylated alanine-rich C-kinase substrate (MARCKS). AngII enhanced MARCKS phosphorylation in a rapid, sustained manner; whereas ACTH induced a rapid and sustained inhibition of MARCKS phosphorylation. Studies using pharmacological agents to mimic various signals indicated that the AngII-induced MARCKS phosphorylation was due to PKC activation, and the ACTH-elicited decrease was mediated by increases in calcium influx rather than cAMP production. We propose that changes in the phosphorylation state of MARCKS, an actin-binding protein, may contribute to cytoskeletal rearrangements involved in steroidogenesis.

Induction of ectopic corticotropic tumor in mouse embryos by exo utero cell transplantation and its effects on the fetal adrenal gland

To establish an in vivo experimental system for developmental endocrinology research, AtT-20 cells, a corticotropic tumor cell line, were transplanted by exo utero manipulation into mouse embryos on embryonic day 14. The induced tumor secreted ACTH in situ, and the circulating ACTH level was elevated. This was the first model for studying the regulation of ACTH in the mouse fetal adrenal in vivo and the first continuous ACTH treatment model in rodent fetuses. The changes in the adrenal gland from the tumor-induced embryos were analyzed by light microscopic morphometry, immunohistochemistry for steroidogenic enzymes, and electron microscopy. In the treated adrenal, the volume of the inner cortical zone was significantly larger than that in controls. In the inner zone, cell density was decreased, and average cell size was increased, whereas bromodeoxyuridine-incorporation was not increased. The enlarged inner zone cells expressed an enhanced level of cytochrome P45011beta, the corticosterone-synthesizing enzyme, and the serum corticosterone level was increased. Electron microscopy showed an active form of the organelles involved in steroidogenesis. These findings indicate that ACTH stimulates both adrenocortical hypertrophy and steroidogenesis in fetal mice. Potential perspectives of the novel paradigm in this research for molecular developmental endocrine study are discussed.

A novel splicing junction mutation in the gene for the steroidogenic acute regulatory protein causes congenital lipoid adrenal hyperplasia

Congenital lipoid adrenal hyperplasia (lipoid CAH) is a relatively common genetic disorder of adrenal and gonadal steroidogenesis and is the most severe form of CAH. As typical affected individuals cannot produce any steroid hormones or can only produce low levels of steroid hormones in the adrenals and gonads, including glucocorticoids, mineralcorticoids, and sex steroids, a genetic defect in the cholesterol side-chain cleavage enzyme, cytochrome P450scc (CYPXIA1), has been postulated to be the cause of their insufficient production to date. Recently, Lin and co-workers proved a link between mutations of the steroidogenic acute regulatory protein (StAR) gene and the lipoid CAH phenotype. Therefore, we investigated both the cytochrome P450scc and StAR genes in a Korean family with a fairly mild form of lipoid CAH to identify the mutation(s) causing this disease. The result was that no mutations could be found in the two genes, except for a thymine (T) insertion into intron 2 of the StAR gene, 3 bp from the splice donor site of exon 2. PCR-amplified StAR genes from a normal subject and the patient were cloned into an expression vector and then introduced into COS-7 cells. Northern blot and reverse transcriptase-PCR analyses indicated that the StAR messenger ribonucleic acid derived from the vector with the normal StAR gene spliced exons 2 and 3 correctly, whereas most, but not all, StAR messenger ribonucleic acid derived from the vector with the T-inserted StAR gene could not remove intron 2. We concluded from these results that the T insertion into the StAR gene accounts for the lipoid CAH phenotype in this patient.

Roles of microfilaments and intermediate filaments in adrenal steroidogenesis

The problem for the steroidogenic cell if it is to accelerate steroid synthesis in response to trophic stimulation, consists in moving cholesterol from the sites of synthesis and storage to mitochondria at an accelerated rate. The most intensely studied situation is that in which the sterol is stored as ester in lipid droplets. Cholesterol ester must be de-esterified and transported to mitochondria where steroid synthesis begins. Since droplets and mitochondria are now known to be attached to intermediate filaments and since these structures are not contractile, it appears to be necessary to invoke the actions of other cytoskeletal elements. Actin microfilaments are involved in cholesterol transport so that it is tempting to propose that the contractile properties of actomyosin are used in this process. It is known that an energy-dependent contractile process involving actin is capable of disrupting intermediate filaments. Since the intermediate filaments appear to act by keeping lipid droplets and mitochondria apart, disruption of the filaments accompanied by a contractile process would be expected to allow these two structures to come together. This would open the way for the transfer of cholesterol to the steroidogenic pathway. This should be regarded as a first step. The events necessary for entry of cholesterol from droplets into the mitochondria remain to be clarified. In addition, the transport process for newly synthesized cholesterol that is not stored in droplets, is still not understood. At least four protein kinase enzymes have been identified in the cytoskeletons of adrenal cells, namely, Ca2+/calmodulin-dependent kinase, protein kinase (Ca2+ and phospholipid-dependent), myosin light chain kinase, and protein kinase A (cyclic AMP-dependent). The Ca2+/calmodulin kinase promotes transport of cholesterol to mitochondria and does so under conditions in which phosphorylation of vimentin and myosin light chain occurs. Phosphorylation of vimentin results in disruption of intermediate filaments while phosphorylation of light chain promotes contraction of the actomyosin ring. It now appears that intermediate filaments are cross-linked by actin filaments so that such contraction would be expected to produce significant structural changes in the cytoskeleton and the attached organelles. Although the details of the changes taking place in the organ in vivo are not known, the potential for interaction between droplets and mitochondria as the result of these changes in intermediate filaments and actomyosin, is clear. Protein kinase C is activated by ACTH and cyclic AMP, although this activation does not appear to be directly involved in the regulation of steroid synthesis. Nevertheless, vimentin is a substrate for this enzyme, and changes in the organisation of vimentin filaments and the attached organelles under the influence of protein kinase C have been reported in other cells. Presumably these changes represent part of the response to ACTH because when protein kinase C is activated by phorbol ester, the cytoskeletal changes necessary for rounding up take place but such changes are not accompanied by increased steroid synthesis. Protein kinase A causes rounding of adrenal cells. and cytoskeletons. This kinase also causes increased cholesterol transport and, hence, stimulation of steroid synthesis. The enzyme also causes phosphorylation of vimentin but with a different cytoskeletal reorganisation from that seen with the other three kinase enzymes. Clearly phosphorylation plays a major role in these responses. Phosphorylation alters the morphology and the functions of the cytoskeleton and this, in turn, is associated with accelerated cholesterol transport. It is now necessary to define the details of the specific phosphorylation reactions that occur during the response to ACTH, that is, which amino acids are phosphorylated and to what extent by each of the kinase enzymes.

The roles of calmodulin, actin, and vimentin in steroid synthesis by adrenal cells

The rate of steroid synthesis is regulated by the rate of transport of cholesterol to mitochondria. The transport process involves two elements of the cytoskeleton (microfilaments and intermediate filaments) and Ca2+/ calmodulin. Electron microscopy and immunofluorescence reveal that lipid droplets in which steroidogenic cholesterol is stored in the cytoplasm are tightly attached to vimentin intermediate filaments. Mitochondria are also attached to intermediate filaments. Ca2+/calmodulin is known to be essential for the steroidogenic response to ACTH and acts to increase transport of cholesterol to mitochondria. Ca2+/ calmodulin promotes phosphorylation of two important adrenal proteins: vimentin via its protein kinase and myosin light chain via the calmodulin-dependent light-chain kinase. In permeabilized adrenal cells Ca2+/calmodulin causes an ATP-dependent contraction of the cells. Phosphorylation of vimentin is known to cause breakdown of intermediate filaments. Electron microscopy reveals that actin filaments cross-link intermediate filaments in adrenal cells. It is proposed that ACTH has at least two second messengers, Ca2+/calmodulin and cAMP. Ca2+/calmodulin causes breakdown of vimentin filaments and activates a contractile event dependent on ATP and myosin light chain. These changes reorganize the cytoskeleton in such a way as to facilitate the interaction of lipid droplets with mitochondria, resulting in transport of cholesterol to these organelles and hence increased steroid synthesis.

Involvement of microtubules in prothoracicotropic hormone-stimulated ecdysteroidogenesis by insect (Manduca sexta) prothoracic glands

Secretion of ecdysteroid molting hormones by insect prothoracic glands is stimulated by neuropeptide prothoracicotropic hormones (PTTH). Studies reported here were conducted to assess the effects of microfilament and microtubule inhibitors on in vitro ecdysteroidogenesis by prothoracic glands of Manduca sexta. Microfilament inhibitors (cytochalasins B and D) had no effect on basal or big PTTH-stimulated ecdysteroidogenesis. Microtubule inhibitors (colchicine, podophyllotoxin, nocodazole) had no effect on basal ecdysteroid secretion, but suppressed PTTH-stimulated secretion in a concentration-dependent manner. The effect of nocodazole was partially reversible, suggesting it was not due to nonspecific toxicity. Colchicine had no effect on glandular ecdysteroid levels, indicating that inhibition was not due solely to blockage of secretion. The combined results are consistent with the hypothesis that microtubule-mediated transport of ecdysteroid precursors plays a critical role in stimulation of ecdysteroidogenesis by PTTH.

Role of the steroidogenic acute regulatory protein (StAR) in steroidogenesis

The rate-limiting, hormone-regulated, enzymatic step in steroidogenesis is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme system (CSCC), which is located on the matrix side of the inner mitochondrial membrane. However, it has long been observed that hydrophilic cholesterol-like substrates capable of traversing the mitochondrial membranes are cleaved to pregnenolone by the CSCC in the absence of any hormone stimulation. Therefore, the true regulated step in the acute response of steroidogenic cells to hormone stimulation is the delivery of cholesterol to the inner mitochondrial membrane and the CSCC. It has been known for greater than three decades that transfer of cholesterol requires de novo protein synthesis; however, prior to this time the regulatory protein(s) had yet to be identified conclusively. It is the purpose of this commentary to briefly review a number of the candidates that have been proposed as the acute regulatory protein. As such, we have summarized the available information that describes the roles of transcription, translation, and phosphorylation in this regulation, and have also reviewed the supporting cases that have been made for several of the proteins put forth as the acute regulator. We close with a comprehensive description of the Steroidogenic Acute Regulatory protein (StAR) that we and others have identified and characterized as a family of proteins that are synthesized and imported into the mitochondria in response to hormone stimulation, and for which strong evidence exists indicating that it is the long sought acute regulatory protein.

Hormones and the cytoskeleton of animals and plants

It is often overlooked that a cell can exert its specific functions only after it has acquired a specific morphology: function follows form. The cytoskeleton plays an important role in establishing this form, and a variety of hormones can influence it. The cytoskeletal framework has also been shown to function in a variety of cellular processes, such as cell motility (important for behavior), migration (important for the interrelationship between the endocrine and immune systems, e.g., chemotaxis), intracellular transport of particles, mitosis and meiosis, maintenance of cellular morphology, spatial distribution of cell organelles (e.g., nucleus and Golgi system), cellular responses to membrane events (e.g., endocytosis and exocytosis), intracellular communication including conductance of electrical signals, localization of mRNA, protein synthesis, and--more specifically in plants--ordered cell wall deposition, cytoplasmic streaming, and spindle function followed by phragmoplast function. All classes of hormones seem to make use of the cytoskeleton, either during their synthesis, transport, secretion, degradation, or when influencing their target cells. In this review special attention is paid to cytoskeleton-mediated effects of selected hormones related to growth, transepithelial transport, steroidogenesis, thyroid and parathyroid functioning, motility, oocyte maturation, and cell elongation in plants.

The roles of microfilaments and intermediate filaments in the regulation of steroid synthesis

Much of the cholesterol used in steroid synthesis is stored in lipid droplets in the cytoplasm of steroid-forming cells. The cholesterol ester in these droplets is transported to the inner mitochondrial membrane where it enters the pathway to steroid hormones as free cholesterol--the substrate for the first enzyme, namely P450scc. It has been shown that this transport process governs the rate of steroid synthesis and is specifically stimulated by ACTH and its second messenger. The stimulating influence of ACTH on cholesterol transport is inhibited by cytochalasins, by monospecific anti-actin and by DNase I demonstrating that the steroidogenic cell must possess a pool of monomeric actin available for polymerization to F actin if it is to respond to ACTH and cyclic AMP. It has been shown that the two structures involved in cholesterol transport (droplets and mitochondria) are both bound to vimentin intermediate filaments in adrenal and Leydig cells. In addition these filaments are closely associated with the circumferential actomyosin ring in which they are crosslinked by actin microfilaments. In permeabilized adrenal cells Ca2+/calmodulin phosphorylates vimentin and this change is known to disrupt intermediate filaments and to cause contraction of actomyosin by phosphorylating myosin light chain kinase. Ca2+/calmodulin stimulated cholesterol transport and steroid synthesis and causes rounding of the responding cells by contraction of the actomyosin, if ATP is also added at the same time. Other agents that disrupt intermediate filaments include anti-vimentin plus ATP in permeabilized cells which also results in rounding of the cell. Acrylamide exerts a similar effect in intact adrenal cells and in addition causes rounding of the cells and increase in steroid synthesis without increase in cyclic AMP. It is also known that if adrenal cells are grown on surfaces treated with poly(HEMA), the cells grow in rounded form and steroid synthesis is increased in proportion to the degree of rounding (r = 0.92). This response does not involve increase in cellular levels of cyclic AMP. It is proposed that in vivo where the cell is always round and cannot show more than strictly limited change in shape, ACTH activates Ca2+/calmodulin possibly by redistributing cellular Ca2+. Ca2+/calmodulin in turn promotes phosphorylation of vimentin and myosin light chain. The first of these phosphorylations shortens intermediate filaments and the second promotes contraction of the actomyosin ring with internal shortening and approximation of lipid droplets and mitochondria. Details of the earlier events (activation of Ca2+/calmodulin) and later changes (transfer of cholesterol to the inner membrane) remain to be elucidated. It is clear however that the action of ACTH requires increase in cellular cyclic AMP. These experimental responses bypass this step in the response to ACTH.

GTP-binding proteins in adrenocortical mitochondria

We have identified two GTP-binding proteins in mitochondria from bovine adrenal cortex (fasciculata). Sub-mitochondrial particles were fractionated into inner membrane, contact point and outer membrane vesicles on sucrose density gradients. These sub-mitochondrial fractions were identified by the presence of enzyme markers and electron microscopy. Photoaffinity labelling with [gamma-32P]GTP identified a 45 kDa GTP-binding protein in outer mitochondrial membranes and a 19 kDa protein in the contact points. The molecular weight of 45 kDa and requirement for Mg2+ ions raise the possibility that this protein is an alpha subunit of a heterotrimeric GTP-binding protein or a novel GTP-binding protein. The specificity of nucleotide binding, the requirement for low concentrations of Mg2+ (0.1 mM) and molecular weight of 19 kDa suggest that this protein is a typical member of the so-called small GTP-binding protein family. The location of 45 kDa in the outer membrane and that of 19 kDa in the contact points suggest roles for these proteins in the interaction with the extramitochondrial environment and in the regulation of mitochondrial membranes, respectively.

The role of endozepine in the regulation of steroid synthesis

Endozepine has recently been isolated from various steroid-forming organs. The following article explores the role of endozepine in the regulation of steroid synthesis. Steroid hormone synthesis from cholesterol begins in the inner mitochondrial membrane, where cytochrome P450 converts cholesterol to pregnenolone. Scientists thought that ACTH would stimulate this conversion, but experiments showed no such stimulation. However, addition of aminoglutethimide to block side-chain cleavage caused the expected reaction of ACTH to take place. Next the role of protein synthesis on the actions of ACTH was explored. Then endozepine was isolated from bovine fasciculata based on stimulation of pregnenolone production by freshly prepared mitochondria. After further experimentation it was concluded that endozepine is a peptide with at least two groups of actions: It binds GABAA receptors in the central nervous system, and it increases the mitochondrial synthesis of pregnenolone.

The requirement of phosphorylation on a threonine residue in the acute regulation of steroidogenesis in MA-10 mouse Leydig cells

In the present study we have used several non-phosphorylatable analogs of the amino acids threonine and serine to determine the role of phosphorylation in the acute regulation of steroidogenesis in MA-10 mouse Leydig tumor cells. Our results indicate that substitution of the threonine analog into protein results in a inhibition of hormone stimulated steroid production in these cells while none of the serine analogs employed displayed a similar inhibition. Strikingly, only the threonine analog resulted in the inhibition of the synthesis of several 30 kDa mitochondrial proteins which we have previously shown to be induced by hormone stimulation of MA-10 cells. Thus, it is apparent that phosphorylation of a threonine residue is obligatory for the acute production of steroids in MA-10 Leydig cells and also for the synthesis of a series of previously described mitochondrial proteins. However, a causal relationship between the 30 kDa mitochondrial proteins and steroid regulation cannot be made unequivocally at this time.

The role of the cytoskeleton in the regulation of steroidogenesis

The slow step in steroid synthesis involves the transport of cholesterol from lipid droplets in the cytoplasm to the first enzyme in the pathway-the cytochrome P450 that converts cholesterol to pregnenolone (P450scc) which is located in the inner mitochondrial membrane. ACTH stimulates this intracellular transport of cholesterol in adrenal cells (Y-1 mouse adrenal tumour cells and cultured bovine fasciculata cells) and this effect of the trophic hormone is inhibited by cytochalasins, by anti-actin antibodies and DNase I suggesting that the response to ACTH requires a pool of monomeric (G-) actin that can be polymerized to F-actin. Recent studies have shown that lipid droplets and mitochondria of adrenal cells are both attached to intermediate filaments. Moreover ACTH reorganizes the cytoskeleton and changes the shape of the cell. These observations suggest a mechanism for transport of cholesterol that involves reorganization and contraction of actin microfilaments which may, in turn, cause movement of droplets and mitochondria together through their common attachment to intermediate filaments.

Regulation of cholesterol movement to mitochondrial cytochrome P450scc in steroid hormone synthesis

Transfer of cholesterol to cytochrome P450scc is generally the rate-limiting step in steroid synthesis. Depending on the steroidogenic cell, cholesterol is supplied from low or high density lipoproteins (LDL or HDL) or de novo synthesis. ACTH and gonadotropins stimulate this cholesterol transfer prior to activation of gene transcription, both through increasing the availability of cytosolic free cholesterol and through enhanced cholesterol transfer between the outer and inner mitochondrial membranes. Cytosolic free cholesterol from LDL or HDL is primarily increased through enhanced cholesterol ester hydrolysis and suppressed esterification, but increased de novo synthesis can be significant. Elements of the cytoskeleton, probably in conjunction with sterol carrier protein(2) (SCP(2)), mediate cholesterol transfer to the mitochondrial outer membranes. Several factors contribute to the transfer of cholesterol between mitochondrial membranes; steroidogenesis activator peptide acts synergistically with GTP and is supplemented by SCP(2). 5-Hydroperoxyeicosatrienoic acid, endozepine (at peripheral benzodiazepine receptors), and rapid changes in outer membrane phospholipid content may also contribute stimulatory effects at this step. It is suggested that hormonal activation, through these factors, alters membrane structure around mitochondrial intermembrane contact sites, which also function to transfer ADP, phospholipids, and proteins to the inner mitochondria. Cholesterol transfer may occur following a labile fusion of inner and outer membranes, stimulated through involvement of cardiolipin and phosphatidylethanolamine in hexagonal phase membrane domains. Ligand binding to benzodiazepine receptors and the mitochondrial uptake of 37 kDa phosphoproteins that uniquely characterize steroidogenic mitochondria could possibly facilitate these changes. ACTH activation of rat adrenals increases the susceptibility of mitochondrial outer membranes to digitonin solubilization, suggesting increased cholesterol availability. Proteins associated with contact sites were not solubilized, indicating that this part of the outer membrane is resistant to this treatment. Two pools of reactive cholesterol within adrenal mitochondria have been distinguished by different isocitrate- and succinate-supported metabolism. These pools appear to be differentially affected in vitro by the above stimulatory factors.

Further evidence that the mitochondrial proteins induced by hormone stimulation in MA-10 mouse Leydig tumor cells are involved in the acute regulation of steroidogenesis

In previous studies we and others have described several mitochondrial proteins which are synthesized in response to acute hormone stimulation in several steroidogenic tissues. In both MA-10 mouse Leydig tumor cells and primary cultures of rat adrenal cortex cells, these proteins consist of a family of 37 kilodalton (kDa) and 32 kDa precursor forms and fully processed forms which are 30 kDa in molecular weight. The nature of the appearance of these proteins and their subcellular localization to the mitochondria, the site of the rate limiting step in steroidogenesis, has led to the speculation that they may be involved in the acute regulation of steroidogenesis. In the present study we have taken advantage of another steroidogenic cell, the R2C rat Leydig tumor cell, to perform studies which further indicate that these mitochondrial proteins are involved in the regulation of steroidogenesis. Unlike the MA-10 cell which requires hormone stimulation for steroid production, the R2C cell is a constitutive progesterone producer whose steroid production cannot be further increased with hormone stimulation. We have shown that the R2C cell line is less sensitive to the inhibition of steroid production by the metal chelator orthophenanthroline (OP) than is the MA-10 cell. We have demonstrated that progesterone production and the 30 kDa mitochondrial proteins remain present in the R2C cells at a concentration of OP which completely inhibits progesterone production and totally eliminates the 30 kDa proteins in MA-10 cells. As further evidence for the role of these proteins in steroidogenic regulation, we have isolated several revertants of the R2C parent (P) cell line which have lost the ability to synthesize progesterone constitutively, but which can be stimulated to synthesize this steroid by trophic hormone and cAMP analog. In these revertants, designated (R), the normally constitutively present 30 kDa proteins are greatly decreased compared to controls, but reappear in large amounts following hormone stimulation. Taken together, these data provide further evidence that the 30 kDa mitochondrial proteins are involved in the acute regulation of steroidogenesis in Leydig cells.

Attachment of mitochondria to intermediate filaments in adrenal cells: relevance to the regulation of steroid synthesis

The rate of steroid synthesis is regulated by the rate of transport of cholesterol from lipid droplets to mitochondria. We have previously demonstrated that lipid droplets in adrenal cells are tightly attached to intermediate filaments. Here we now show that mitochondria colocalize with intermediate filaments in modified double indirect immunofluorescence and by electron microscopy of extracted adrenal cells. Direct contact between mitochondria and intermediate filaments was established by examination of stereo pairs of electron micrographs from extracted cells. The attachment of both droplets and mitochondria to intermediate filaments suggests possible mechanisms for this form of intracellular transport of cholesterol to mitochondria and hence for the regulation of steroid synthesis.

Introduction of large molecules into viable fibroblasts by electroporation: optimization of loading and identification of labeled cellular compartments

Access to the cell cytoplasm in viable cells may permit direct labeling or manipulation of intracellular molecules and metabolic processes. One method to gain access to the cell cytoplasm is by electroporation, a technique that transiently creates pores in cell membranes by means of applied electrical fields. We used electroporation to introduce large-molecular-mass dextrans and proteins as probes of the cytoplasmic compartment in human gingival fibroblasts. Electrical field strength and pulse decay time were optimized to obtain cellular viability greater than 80%. Analysis by confocal microscopy and by fluorescence spectrophotometry demonstrated that a large proportion of high-molecular-mass probe was membrane-bound after electroporation. Trypsinization did not affect membrane-bound FITC-dextran but eliminated protein probe incorporated into the membrane, thereby permitting measurement of only intracellular, cytoplasmic label. Proteins of up to 66 kDa were incorporated at intracellular concentrations of 10(-15) M. After electroporation under optimal conditions, incorporated anti-vimentin antibodies were capable of binding to vimentin. Cells electroporated in the presence of RNase A exhibited significant reductions of cellular RNA. Electroporation appears to be a useful approach to probe or perturb specific cellular processes by introduction of functional molecular species into the cytoplasm of viable cells.

Effect of ACTH on steroidogenic enzymes in guinea pig fasciculata-glomerulosa cells: changes in activity and mRNA levels

Adrenocorticotropin (ACTH) is known to exert an acute effect on adrenal steroidogenesis as well as long-term effects by regulation of gene expression. In order to further study the long-term action of ACTH, guinea pig fasciculata-glomerulosa (FG) cells in primary culture were treated for up to 72 h with ACTH. The effects of this treatment on steroid secretion, enzyme activity and mRNA levels for steroid enzymes were measured. While the rate of 17-deoxy C-21 steroid secretion decreased over the 72-h period of incubation with ACTH, the 17-hydroxy C-21 steroid secretion rate remained constant for the first 24 h of incubation and declined thereafter; the rate of 4-ene C-19 steroid secretion increased over the 72-h incubation period. ACTH treatment increased 17-hydroxylase and 17,20-lyase activities and the maximal stimulation was reached after 48 h. In contrast, the activity of 21-hydroxylase (P450c21) steadily declined over the 72-h incubation period. ACTH also caused an increase in mRNA levels for P450c21, 17-hydroxylase and 17,20-lyase (P450c17), 3 beta-hydroxysteroid dehydrogenase 4-ene-5-ene-isomerase (3 beta-HSD) and cholesterol side-chain cleavage enzyme (P450scc). The maximal stimulation for the four mRNAs was observed after 18 h of incubation with ACTH, decreasing afterwards except for P450c17 mRNA levels which remained elevated over the 72-h incubation period. Despite the increase in mRNA levels for 3 beta-HSD and P450c21, no increase in their respective enzyme activities was observed and 21-hydroxylase activity even declined over the 72-h incubation period with ACTH, thus suggesting that mechanism(s) other than gene expression alone regulate steroid secretion in FG cells. In conclusion ACTH caused major changes in steroid distribution due to increased 17-hydroxylase and 17,20-lyase activities and decreased 21-hydroxylase activity in FG cells in culture. Moreover, our data revealed major differences in the induction of mRNAs for steroidogenic enzymes and their activities following ACTH treatment.

The role of diazepam binding inhibitor in the regulation of steroidogenesis

It has been well established that one factor influencing the rate of side-chain cleavage of cholesterol is the rate of delivery of the substrate, cholesterol, from depots in the cytoplasm to the inner mitochondrial membrane within the organelle. This process of intracellular transport consists of two steps: transport to the mitochondria and transport from outer to inner membrane. Transport to mitochondria requires the cytoskeleton and Ca(2+)-calmodulin but not newly synthesized protein. Transport within the mitochondria requires newly synthesized protein. Both steps are stimulated by the trophic hormones ACTH and LH, in their respective target organs. Since the steroidogenic responses to these hormones are inhibited by cycloheximide, it is proposed that the new protein(s) required for these responses are synthesized in the cytoplasm. Using an assay based on the production of pregnenolone by mitochondria from bovine adrenal cortex, a protein of mol. wt approximately 8200 (temporarily referred to as 8.2 K) was isolated. The protein was purified to homogeneity and found to possess the following properties: (i) 8.2 K accelerated the synthesis of pregnenolone by isolated mitochondria (ii) it promoted entry of cholesterol from the incubation medium into mitochondria (iii) 8.2 K accelerated the transport of cholesterol from outer to inner membrane (iv) it also promoted loading of P-450scc with cholesterol, when either mitoplasts of the inner mitochondrial membranes were incubated with 8.2 K in vitro. Moreover, (v) the synthesis of 8.2 K was increased by ACTH and (vi) the half-life of the protein was less than 2 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of the electroinjection method for introducing proteins into living cells

The introduction of impermeant probes such as antibodies and other proteins into living cells without compromising physiological function is an important approach for studying cellular regulatory mechanisms. Many techniques including direct microinjection, liposome-mediated delivery, fusion of red cell ghosts, and osmotic lysis of pinocytic vesicles have been used to introduce proteins into intact cells. We have used a modification of the voltage-discharge technique to introduce antibodies and other proteins into living physiologically responsive pheochromocytoma and other cultured cells. In this technique, called electroinjection, a single discharge of relatively low field strength is used to transiently permeabilize the plasma membrane. Our experiments demonstrate that electroinjection permits the introduction of large amounts (microM) of probe into 2-5 x 10(6) cells simultaneously without compromising cell viability or physiological responsiveness when performed under carefully defined conditions. They also demonstrate that electroinjection results in a single population of loaded cells and that protein incorporation is a function of field strength, capacitance, molecular weight of the protein, and the concentration of the protein in the electroinjection buffer. Interestingly, a significant fraction of the protein electroinjected into cells is trapped in the plasma membrane when cells are shocked at high capacitance. These results demonstrate that electroinjection appears to be an efficient method for loading exogenous proteins into cells while maintaining the integrity of the physiological properties of the cell.

Time-dependent biphasic effect of cytochalasin D on luteal progesterone release in the pregnant rat

Numerous studies have examined the effects of cytoskeletal disruption on steroidogenesis; while some report an inhibition, other studies show a stimulation of steroid hormone production. In the present study, the possibility of a biphasic effect of cytoskeletal inhibitors on steroidogenesis was examined. Luteal tissue from day 12 pregnant rats was incubated for either 3.5 h (short-term) or 12.5 h (long-term) with cytochalasin D or colchicine at 10(-4) M in Medium 199 (medium). Controls were incubated in medium alone. After the incubation, the tissues were separated from the medium, and either processed for electron microscopy, or weighed and snap-frozen for subsequent homogenization and steroid hormone measurements. Progesterone, testosterone, and 17 beta-estradiol levels in the medium were measured by radioimmunoassay. After the short-term incubation, progesterone release decreased with cytochalasin D treatment, while cells became more rounded in shape with a loss of microfilaments. Upon long-term incubation, progesterone release increased and cell contact lessened. Colchicine had no effect at either incubation time, and estradiol and testosterone production remained unchanged throughout the experiments. These results demonstrate that cytochalasin D has a biphasic effect on luteal progesterone release in the rat and provides an explanation for the dichotomy of results thus far reported. In addition, the effects of cytochalasin D on rat luteal progesterone production appear to be the result of changes in cell shape or cell-to-cell contact.

Distinctive properties of adrenal cortex mitochondria

The mitochondria in cells that synthesize steroid hormones not only have enzymes not present in mitochondria of non-steroidogenic cells but also have unique mechanisms for regulating the steroid substrate availability for certain of these enzymes. We have considered in detail the cytochrome P-450scc system that is located in the inner mitochondrial membrane and that catalyzes the initial and rate-determining step in the steroid hormone biosynthetic pathway. The flux through this pathway is regulated both by the levels of these catalysts themselves and by the availability of the substrate cholesterol for conversion to pregnenolone. These two levels of regulation occur in different time frames but are both controlled externally by the action of tissue-specific peptide hormone. We have used the adrenal cortex fasciculata cells as our paradigmatic cell type. The overall picture seems closely similar for mitochondria in other such steroidogenic cells when analogous data are available. Thus, in adrenal cortex fasciculata cells ACTH triggers several long-term (trophic) and short-term (acute) effects upon and within mitochondria that influence the initial and rate-determining step in the steroid hormone biosynthetic pathway. The only second messenger for both effects characterized thus far is cAMP. An increase in membrane-associated cAMP rapidly activates cAMP-dependent protein kinase, which in turn phosphorylates several cellular proteins, e.g., cholesterol ester hydrolase (vide supra). The trophic action, i.e., that produced by exposure of the cells to increased levels of ACTH or cAMP for a prolonged period (minutes to hours), increases the amounts of the steroid hormone synthesizing proteins in the mitochondria by increasing the transcription of the relevant nuclear genes. This latter process is not needed for the acute increase in the rate of steroid hormone biosynthesis. Whether induction of steroidogenic enzymes requires activation of a kinase has not been determined. However, the postulated SHIP proteins provide a mechanism by which cAMP levels and protein synthesis itself may regulate this induction. Mitochondria in steroidogenic tissues exert control over this process by their ability to recognize, import and process correctly the nuclear encoded precursors of the steroidogenic enzymes. Whether control at this level is ultimately dictated by nuclear or mitochondrial gene products or by an interplay between them is still unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-calmodulin-dependent phosphorylation of cytoskeletal proteins from adrenal cells

We have identified a highly active Ca2+ calmodulin-dependent protein kinase in the cytoskeletons of normal (bovine fasciculata) and transformed (Y-1 mouse tumor) adrenal cells. In view of evidence for the involvement of calmodulin and microfilaments in the regulation of cholesterol transport and hence steroidogenesis, it is likely that this kinase is important in this process. The kinase activity was examined for its capacity to phosphorylate endogenous proteins analyzed by one- and two-dimensional gel electrophoresis, in the presence of saturating amounts of Ca2+ (5 mM) and calmodulin (5 microM). Three inhibitors of calmodulin (trifluoperazine, pimozide and W-7) inhibit steroidogenesis and Ca2(+)-calmodulin-dependent phosphorylation kinase activity with similar values for EC50 for the two processes. All three inhibitors inhibit the increased transport of cholesterol to mitochondria in response to ACTH. Two substrates for the kinase (alpha-spectrin and beta-tubulin) were identified and two others (51,000 and 60,000 molecular weight) were tentatively identified as the subunits of the kinase itself in cytoskeletons of both cell types. Calmodulin-binding proteins analyzed by [125I]iodocalmodulin overlay and calmodulin-Sepharose affinity chromatography were also identified in the same cytoskeletons including alpha-spectrin, the Ca2+ calmodulin-dependent phosphatase calcineurin and three that were tentatively identified as the two subunits of the kinase itself and myosin light chain kinase. It is concluded that calmodulin, by binding to the kinase and phosphatase, is capable of influencing the degree of phosphorylation of specific substrates in the cytoskeleton and of forming complexes with spectrin, actin and tubulin. These events may be involved in the regulation of the rate-limiting step of steroidogenesis, i.e. transport of cholesterol to mitochondria.

Further characterization of the inhibitory effect of monensin on adrenal steroidogenesis

We have previously reported that treatment of cultured mouse adrenal tumor cells with 0.6-1.2 microM monensin, a monovalent carboxylic ionophore, results in disruption of the organized structure of the Golgi complex. This is associated with an inhibition of adrenocorticotropic hormone (ACTH) or dibutyryl cAMP-stimulated steroidogenesis and impairment of mitochondrial cholesterol side-chain cleavage activity. The present report describes further investigations regarding possible mechanisms for the inhibition. Monensin inhibits both synthesis of fluorogenic steroids and incorporation of [14C]acetate into the end-product steroid 11 beta,20 alpha-dihydroxy-4-pregnen-3-one. Supplementation of monensin-treated cells with 25-hydroxycholesterol, a readily available substrate for steroidogenesis, does not reverse the inhibitory effect on the reaction. The incorporation of L-[35S]methionine into trichloroacetic acid precipitable proteins in the isolated mitochondria of monensin-treated cells is inhibited approximately by 40%, whereas the inhibitory effect on the proteins in the cell homogenate is marginal. These findings suggest that a deficiency of newly synthesized proteins in mitochondria, rather than the availability of the substrate cholesterol, may be the primary factor causing impairment of steroidogenesis.

Intracellular trafficking of sterols

Cavalier-Smith (1981) has identified 22 characters that are universally present in eukaryotes but absent in prokaryotes. Of these, he argues that one, exocytosis, might have been the driving force behind the evolution of modern eukaryotic cells. Bloom and Mouritsen (1988) further argue that sterols may have removed an evolutionary bottleneck to cytosis. Therefore, the advent of sterols in membranes might have been the single feature that led to eukaryote evolution. The evolutionary advantage conferred by cholesterol is associated primarily with plasma membrane function, since the majority of cellular free cholesterol resides in that membrane. However, sterol synthesis occurs in the ER; therefore, the cell must have a mechanism for transporting sterol to the plasma membrane and its regulation. As has been pointed out in this review, much remains to be elucidated in the study of intracellular sterol trafficking. To date, neither diffusion nor vesicle-mediated transport can be fully confirmed or ruled out. Microtubule and microfilament involvement appears important in some routes (e.g., mitochondria) but not in others. In addition, trafficking roles of cytoplasmic lipoproteinlike particles have not been addressed. Finally, although some "sterol carrier proteins" demonstrate the ability to mediate intervesicular transfer of cholesterol in vitro, the true physiological role of these proteins remains obscure. Future research in this field awaits the refinement of available techniques. Particularly valuable would be cytochemical methods for detection of sterol at the ultrastructural level. Possibly, direct microscopic visualization of radiolabeled components in cells represents the necessary approach. Purification of elements carrying newly synthesized sterols would allow the proteins mediating transport to be identified. Continued analysis of mutants defective in transport, such as in type C Niemann-Pick disease, will shed light on this complex problem. The importance of extracellular trafficking of cholesterol owing to its involvement in the progression of atherosclerosis, has been emphasized in recent years. Little emphasis has been placed on intracellular trafficking of sterol; however, it can be argued that such transport also plays a major role in atherosclerosis, possibly by fueling retrotransport of cholesterol to the liver and secretion in the bile. Therefore, we hope this review will serve to stimulate research interest in this area.

Isolation and characterisation of calcineurin from adrenal cell cytoskeleton: identification of substrates for Ca2+-calmodulin-dependent phosphatase activity

Ca2+-calmodulin-dependent protein phosphatase activity is found in cytoskeletons of Y-1 mouse adrenal and bovine fasciculata cells. The activity is inhibited by three inhibitors of calmodulin (trifluoperazine, W-7 and pimozide) with EC50 in the low micromolar range. Protein phosphatase activity is inhibited by vanadate, fluoride, Zn2+ and pyrophosphate, stimulated by Mn2+ and found to be tightly bound to the cytoskeleton. Substrates for endogenous phosphatase activity were defined by one- and two-dimensional polyacrylamide gels. Phosphatase activity was seen with proteins that are substrates for both cyclic AMP-dependent and cyclic AMP-independent kinase enzymes. One specific Ca2+-calmodulin-dependent phosphatase, namely calcineurin, was purified to near homogeneity from cytoskeletons of Y-1 cells. The enzyme was found to be a heterodimer (MW 61,000 and 16,000) and the smaller subunit was shown to cross-react with antibodies raised against calcineurin from bovine brain. The purified enzyme catalyzes dephosphorylation of proteins (phosphorylase kinase and casein), phosphoamino acids (tyr greater than thre greater than ser) and a synthetic substrate (p-nitrophenyl phosphate). In addition, a new application of membrane transfer was devised by which the purified enzyme was incubated with a Western blot of cytoskeleton following incubation with [32P]ATP. This method defined four specific substrates of the enzyme (MW 150,000, 55,000, 35,000 and 30,000). Anti-calcineurin revealed that only a single Ca2+-calmodulin-dependent phosphatase is found in adrenal cell cytoskeleton.(ABSTRACT TRUNCATED AT 250 WORDS)

Exogenous glycosaminoglycans modulate chondrogenesis, cyclic AMP level and cell growth in limb bud mesenchyme cultures

Effects of hyaluronate, heparin and chondroitin-6-sulfate were studied on micromass cultures of chick limb bud mesenchyme (Hamburger and Hamilton stages 23-24). Histochemical, electron microscopical, biochemical and radiochemical investigations of day 4 cultures revealed dose-dependent inhibitory effects of these glycosaminoglycans on chondrogenesis, cyclic AMP level and growth of cells. In addition, hyaluronate with 100 micrograms/ml dose caused a displacement of newly formed proteoglycan from cultures into the medium. It is supposed that exogenous glycosaminoglycans influence ionic equilibrium in the immediate vicinity of cells and disturb the organization of the prechondrogenic extracellular matrix resulting in alterations of cell membrane--cytoskeleton associations. These alterations may provoke a reduction in cyclic AMP level and DNA synthesis. It is suggested that a reduction in cyclic AMP level preceding the expression of cartilage phenotype results in the inhibition of chondrogenesis.

Effect of anticalmodulin drugs on testosterone synthesis in hCG stimulated mouse Leydig cells

The effect of three anticalmodulin drugs, prepared in this laboratory and a commercially available drug Mastoparan, was tested on the secretion (or synthesis) of testosterone in hCG stimulated Leydig cells. The results of the use of drugs RN-IV A, RN-IV B and RN-IV C indicated that hCG (10 ng/ml), DbcAMP (0.1 mM) and cholera toxin (2 micrograms/ml)-stimulated testosterone production was inhibited in Leydig cells in a dose dependent manner. In hCG stimulated cells, the ID50 for drug RN-IV A was 2 microM, for drug RN-IV B was 25 microM and for drug RN-IV C was 130 microM. Based on ID50 the most effective drug was RN-IV A. Maximum inhibition of testosterone production was obtained at a concentration of 20 microM for drug RN-IV A, 150 microM for drug RN-IV B and 200 microM for drug RN-IV C. Further extensive experiments with drug RN-IV B showed that (a) at 100 microM concentration the drug does not impair the binding of receptor with 125I hCG, (b) the cAMP accumulation was prevented in a dose dependent manner reaching a minimal of 1.1 pM at 50 microM, compared with 3.5 pM in hCG (10 ng/ml) stimulated cells. The drug RN-IV B at a concentration of 100 microM, which failed to prevent conversion of exogenous pregnenolone or progesterone to testosterone, otherwise caused complete inhibition of testosterone production in hCG stimulated cells. Mastoparan also inhibited testosterone production in hCG stimulated cells in a dose dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular matrix changes PC12 cell shape and processing of newly synthesized dopamine

PC12 cells on extracellular matrix (ECM) or plastic were incubated with 3H-tyrosine (3H-TY) in the presence and absence of serum or cold tyrosine. 3H-Dopamine (3H-DA) was determined in medium and cells from 1 to 48 h later with Dowex cation exchange chromatography. In serum-free and tyrosine-free medium, PC12 cells on ECM released significantly more 3H-DA, whereas cells on plastic had a significantly higher cellular content of 3H-DA, but total 3H-DA (medium plus cells) was equal in ECM and plastic cultures. When 3H-TY was added to tyrosine-containing medium, there was a significant decrease in the levels of 3H-DA detected and the differences between ECM and plastic cultures were attenuated, but the patterns of secretion and storage were similar to those observed with tyrosine-free medium and total synthesis did not decline at 48 h. Serum decreased the efficiency of the resin to retain 3H-DA from culture medium, attenuated the difference in dopamine release between ECM and plastic cultures, and contributed to variations in 3H-TY uptake. The morphometric relationship between the cell membrane and the internal compartment in PC12 cells of different shapes was also characterized. The perimeter length and area of the midsection of cells were determined with a modular system for quantitative digital analysis. The perimeter length of cells on ECM was significantly greater than cells on plastic, whereas the internal areas were similar. The ratio of perimeter length to area (P/A) for all cells on ECM was 30% higher than the P/A ratio for cells on plastic. The ratio of P/A for a subpopulation of very flat cells on ECM was 70% higher than the ratio for round cells on plastic. Immunocytochemistry for tyrosine hydroxylase revealed a more diffuse distribution of this enzyme in cells on ECM. These data suggest that there is an increase in the ratio of cell surface area to cell volume as PC12 cells spread on ECM which could facilitate secretory vesicle fusion with the cell membrane, and hence, exocytosis. Although there is a concomitant increase in the secretion of dopamine and a decrease in the storage of dopamine, the change in cell shape does not appear to immediately alter the synthesis of dopamine.

Glucocorticoid stimulation of dopamine production in PC12 cells on extracellular matrix and plastic

PC12 cells are spherical when cultured on plastic, but flatten and spread extensively when cultured on extracellular matrix (ECM) produced by bovine corneal endothelial cells. We previously demonstrated that cells which have spread on ECM release more dopamine and contain less intracellular dopamine than cells which are rounded on plastic cultureware. Glucocorticoids increase dopamine production in PC12 cells presumably via an increase in tyrosine hydroxylase gene transcription. We questioned whether cell shape as determined by ECM would change the response of PC12 cells to glucocorticoids. Dopamine release and cell content were measured by a radioenzymatic assay in serum-free cultures of PC12 cells on ECM and plastic treated with various glucocorticoids in a dose-response fashion for 24 or 48 h. In addition, the response of cells on both substrata to one dose of dexamethasone was examined from 3-48 h. PC12 cells on ECM and plastic exhibited a dose-related increase in dopamine release and content when treated for 24 h with corticosterone or for 48 h with dexamethasone, corticosterone or cortisol. The ED50s for dexamethasone- and corticosterone-stimulated release and content were similar for cells on ECM and plastic as were the doses at which the first significant increases were detected. The average times at which the first significant increase in release and content occurred were also similar for cells on ECM and plastic. PC12 cells on ECM continue to release more dopamine and store less dopamine than cells on plastic with glucocorticoid treatment. However, glucocorticoid-treated cells on ECM showed a greater percent increase over ECM controls in the cell content of dopamine, whereas glucocorticoid-treated cells on plastic showed a greater percent increase over controls in the release of dopamine. It is hypothesized that glucocorticoids increase dopamine production to a similar extent in cells on ECM and plastic, but that storage is facilitated in cells on ECM.

Characterization of filaments within Leydig cells of the rat testis

Rat Leydig cells were permeabilized and the cytoplasm partially extracted to visualize, describe, and characterize filamentous elements of the cytoskeleton. It was demonstrated by immunofluorescence microscopy that vimentin is abundant within Leydig cells. Ultrastructurally, intermediate filaments in Leydig cells were concentrated at perinuclear sites and comprised bundles that coursed through the cytoplasm. Actin was identified in Leydig cells with the F actin probe, NBD-phallacidin. Fluorescence was strongest at the cortex of the cell. With myosin S-1 subfragments, sparse actin was found positioned almost exclusively in cortical regions of the cell associated with coated pits and in Leydig cell processes.

Immunogold cytochemistry of cytochromes P-450 in porcine adrenal cortex. Two enzymes (side-chain cleavage and 11 beta-hydroxylase) are co-localized in the same mitochondria

In order to study the distribution of mitochondrial cytochromes P-450 in porcine adrenal glands, the glands of anesthetized pigs were fixed in situ. Polyclonal antibodies against two cytochromes P-450, i.e., C27 side-chain cleavage enzyme and 11 beta-hydroxylase, were used to study the distribution of these enzymes in cryosections of the adrenal cortex. Ultrathin cryosections were evaluated by both protein-A/gold/silver immunocytochemistry and immunoelectron microscopy using double labeling with protein-A/colloidal-gold. At light microscopy, the two cytochrome P-450 enzymes were found to be broadly distributed in both the fasciculata and glomerulosa zones of the adrenal cortex. Quantitative immunoelectron microscopy revealed that both enzymes were localized only in mitochondria, in which they were present on the inner aspects of the inner mitochondrial membrane. Both cytochromes P-450 were demonstrable in all of the mitochondria examined, and statistical evaluation of the ratios of the two enzymes present in individual mitochondria yielded a normal distribution curve. Since no evidence was found for the preferential localization of either enzyme in a special population of mitochondria, we conclude that all mitochondria of the adrenal cortex contain both enzymes. We discuss implications of these findings with respect to the regulation of steroidogenesis.

Structure of genes encoding steroidogenic enzymes

Synthesis of adrenal steroid hormones from cholesterol entails the actions of only five enzymes, four of which are specific forms of cytochrome P450. These cytochrome P450 enzymes have all been isolated and their activities reconstituted in vitro, showing that each enzyme catalyses multiple steroidal conversions. Genes or complementary DNAs have been cloned for human P450scc (the cholesterol side-chain cleavage enzyme), P450c17 (17 alpha-hydroxylase/17,20 lyase) and P450c21 (21-hydroxylase). The sequences for microsomal P450c17 and P450c21 are much more closely related to one another than either is to the sequence for mitochondrial P450scc. Each of these P450 enzymes is encoded by a single human gene; the gene for P450scc lies on chromosome 15, that for P450c17 lies on chromosome 10, and that for P450c21 lies on chromosome 6. The human, mouse and bovine genomes each have two P450c21 genes. While only one of these is active in mouse and man, both genes may be active in cattle. A wide variety of lesions in the human P450c21(B) gene causes congenital adrenal hyperplasia, a common genetic disorder.

Preparation and characterization of submitochondrial fractions from adrenal cells

A method for preparing submitochondrial fractions from adrenocortical cells was developed by adapting a procedure that has been successful with yeast mitochondria. The method is based upon osmotic swelling, sonication and centrifugation in sucrose. The preparation yields highly purified fractions of outer membrane, intermembrane space, inner membrane and a less purified fraction of matrix. Recoveries are good so that 10(7) cells yield approximately 170 micrograms of inner membrane protein and 12 micrograms of outer membrane protein. Electron microscopy shows that the outer membrane fraction consists of vesicles (0.2-0.6 micron diameter) while inner membrane appears as densely packed sheets of membranous material. Two-dimensional polyacrylamide gels (isoelectric focusing followed by electrophoresis) of all the fractions give highly reproducible patterns of protein spots with Coomassie staining. Steroidogenic proteins were found only in inner membrane fractions which were shown to contain cytochrome P-450 C27 side-chain cleavage and P-450 11 beta-hydroxylase together with adrenodoxin and adrenodoxin reductase. Inner membrane catalyzes side-chain cleavage of cholesterol (conversions to pregnenolone) and 11 beta-hydroxylation (DOC----corticosterone) when substrate and NADPH are added. The preparation yields highly purified submitochondrial fractions from Y-1 mouse adrenal tumor cells and from porcine and bovine adrenocortical mitochondria. The method has the virtue of yielding highly purified intermembrane fluid which is not true of other methods for fractionation of adrenal mitochondria. The procedure also yields cleaner preparations of the two membranes than two other published methods currently used to prepare submitochondrial fractions from adrenocortical cells.