Properties of detergent-solubilized adenylate cyclase and gonadotropin receptors of testis and ovary

Rethinking receptor-G protein-effector interactions

Hundreds of different receptors regulate the activity of effector proteins with the assistance of heterotrimeric guanine nucleotide-binding proteins (G proteins). The hypothesis that G protein-coupled receptors (R) govern their effectors (E) indirectly via a shuttling mechanism involving the exchange of heterotrimeric G proteins (G[alpha betagamma]) or parts thereof (G[alpha], G[betagamma]) between ephemeral R-G and G-E complexes has become firmly established. While there is no direct evidence for the cyclical formation and dissociation of these complexes during signalling, experimental changes in second messenger production, GTPase activity, and the binding characteristics of agonists, antagonists, and guanine nucleotides commonly are believed to reflect perturbations in the equilibria between G protein and the other two components. However, a growing body of evidence seems to argue against the shuttling model. The random, transient association of G protein and receptor is largely inconsistent with the binding of agonists to receptors and the allosteric regulation of that binding by guanine nucleotides. Also, the prevailing paradigm does not readily account for receptor-effector coupling specificity, as the promiscuous interaction of most G proteins with both receptors and effectors in vitro is at odds with the general failure of G proteins to be shared among ostensibly congruous signal transduction pathways in vivo. The latter paradox would be obviated by the simultaneous interaction of G protein with both receptor and effector. Indeed, various findings indicate that R-G-E complexes do occur. How and where in the cell such complexes are assembled and disassembled should provide important clues to the true mechanism of G protein-linked transduction.

Characterization of gonadotropin-sensitive adenylate cyclase activity in human testis: uncoupling of the receptor-cyclase complex by specific hormonal antagonist

Basal and gonadotropin stimulated adenylate cyclase activity was assessed in testicular tissues obtained from men (20-80 years). A disparity was observed in the gonadotropin responsiveness of the human testicular adenylate cyclase system to hFSH and hCG stimulation. Of the tissues analyzed, 61% were FSH responsive and 22% showed low response to hCG. Forskolin, a diterpene which activates adenylate cyclase by a receptor independent mechanism, stimulated adenylate cyclase activity in the gonadotropin unresponsive tissues. This suggests that the tissue unresponsiveness is due to an uncoupling of the catalytic subunit of the adenylate cyclase. Several functional properties of the FSH responsive human testicular adenylate cyclase were investigated. hFSH and oFSH stimulated the enzyme activity in a concentration dependent manner. However, the hormone (DG-oFSH) in which 80% of the carbohydrate residues had been removed was inactive, despite its good binding ability to the FSH receptor. hFSH stimulated adenylate cyclase activity was inhibited by DG-oFSH but not by DG-hCG (deglycosylated hCG). The data demonstrates the existence of specific FSH and LH(hCG) receptors in human testicular membranes. The FSH receptors in some tissues are coupled to adenylate cyclase. The link between the FSH receptor and adenylate cyclase may be uncoupled in the presence of the deglycosylated form of oFSH resulting in a loss of hormone response.

Isolation and characterization of plasma membranes containing LH sensitive adenylate cyclase from a Leydig cell tumour

An LH sensitive adenylate cyclase from a tumour Leydig cell has been investigated. The plasma membranes, prepared by a 2 phase (dextran-polyethylene glycol) centrifugation method were found to have the following properties: In the presence of LH plus p(NH)ppG (guanosine 5'-beta, gamma-imido triphosphate) or fluoride ions, maximum adenylate cyclase activity was obtained in the plasma membranes with 4 to 6 mM Mg2+ plus 0.33 to 2 mM ATP. LH alone stimulated adenylate cyclase activity 2-fold when compared with basal activity and the time course of cyclic AMP production was linear up to 45 min. With GTP (10(-5)M) and GTP plus LH, adenylate cyclase activity was increased 3 and 6-fold, respectively, for up to 20 min and thereafter declined. In contrast p(NH)ppG (10(-5)M) and p(NH)ppG plus LH increased adenylate cyclase activity 7 and 14-fold which was maintained for at least 45 min. Fluoride ions increased the enzyme activity linearly over 45 min approx 18-fold. When GTP or p(NH)ppG were added alone there was a lag time of activation of approximately 10 min which was abolished by the addition of LH. GTP but not p(NH)ppG at concentrations greater than 10(-4) inhibited basal and LH stimulated adenylate cyclase when compared with 10(-5)M GTP. The tumour Leydig cell adenylate cyclase is thus essentially similar to other hormone sensitive somatic cells. The present study makes it feasible to prepare plasma membranes by a simple method from large quantities of pure Leydig cells.

FSH and testosterone effects in seminiferous tubules of immature hypophysectomized rats

The effects of follicle-stimulating hormone (FSH) and testosterone on the development of the cytosolic germ cell adenylate cyclase and germ cell morphology in rats hypophysectomized at 29 days of age were studied. Following hypophysectomy, the adenylate cyclase content fell to marginal levels and germ cell development ceased at the late pachytene stage. Testosterone treatment led to a moderate increase in the cytosolic enzyme content and to progression of spermatid cell development to stages 8-12. FSH treatment with doses of 80-100 micrograms/day restored enzyme content to levels seen in control rats, as well as progression of germ cell development up to stages 15-16, i.e., to the same stages present in age-matched control (sham-operated) rats. The results indicate that in immature rats FSH is essential for spermatid cell maturation as is evidenced by its ability to stimulate the formation of cytosolic germ cell adenylate cyclase to quantitatively normal levels, as well as to stimulate the development of spermatid cells.

Hormone-induced guanyl nucleotide binding and activation of adenylate cyclase in the Leydig cell

The adenylate cyclase activity of Leydig cell homogenates and membrane fractions is highly dependent on guanyl nucleotides, and enzyme responses to luteinizing hormone or human chorionic gonadotropin are small in the absence of guanyl nucleotides. However, in the presence of 10 microM guanosine 5'-[beta, gamma-imido]triphosphate Gpp[NH]p, both hormones consistently stimulated testicular adenylate cyclase activity by up to 200%. Leydig cell membranes bound [3H]Gpp[NH]p at 30 degrees C with high affinity (Ka = 1.5 X 10(7) M-1) and binding capacity of 60 pmol/mg of protein. During kinetic studies, the association rate constant was 1.7 X 10(6) M-1 min-1, and the dissociation constant was 0.038 min-1. In the presence of gonadotropin (10 pM to 10 nM), concentration-dependent increases of 40% to 100% in Gpp[NH]p binding were observed in Leydig cell membranes. Kinetic studies showed that gonadotropin decreased the association rate constant to 0.73 X 10(6) M-1 min-1 and the dissociation rate constant to 0.017 min-1, with no effect on the equilibrium binding constant. Thus, the increase in Gpp[NH]p binding was not due to a change in receptor affinity but was attributable to increased availability of nucleotide binding sites. The 50% effective dose for adenylate cyclase activation by gonadotropin in the presence of Gpp[NH]p was identical with that observed for gonadotropin-induced binding of the GTP analog (50 nM). Gonadotropin-induced binding of Gpp[NH]p in Leydig cell membranes may represent interaction with the guanyl nucleotide regulatory site during hormonal activation of adenylate cyclase.

PGE1 and hCG responsive adenylyl cyclases in interstitial cells from rat testis: biphasic effects of calcium

The effects of PGE1 (10 micrograms/ml), hCG (10 mIU/ml) and Ca2+ (0-10 mM) on rat interstitial cell adenylyl cyclase (AC) activity have been investigated. Like hCG, PGE2 also stimulated membrane bound AC, and both hormonal effects proved to be influenced by Ca2+. Low concentrations of Ca2+ (0.3 mM) enhanced hormonal stimulation, while high concentrations (up to 10 mM) suppressed AC activity. Relative stimulations caused by PGE1 and hCG were maximal in the presence of 0.6 mM Ca2+. Ovine FSH (NIH-S12), at concentrations causing maximal activation of AC in seminiferous tubules (12.5 micrograms/ml), had no effect on AC in interstitial cells. Germinal cell membranes displayed insignificant AC activity. We therefore conclude that the AC activity stimulable by PGE1 is located on Leydig cell membranes and that both PGE1 and hCG activities are susceptible to regulation by Ca2+.

Interactions of gonadotropins with corpus luteum membranes. VIII. The different properties of rat luteal cell light and heavy membranes cannot be explained by fractionation of inside-out and outside-out plasma-membrane vesicles

Fractions enriched in luteal cell plasma-membranes were isolated from superovulated rat ovaries. Cell surface-membrane marker enzyme activities were measured in both light and heavy membrane fractions, with or without treatment with various detergents, to assess whether these fractions consisted of resealed vesicles with different orientations (i.e. inside-out or outside-out). Control and digitonin-treated membranes were also fractionated on sucrose-density gradients to monitor the buoyant density perturbation of each surface-membrane marker induced by this detergent. In another experiment, hCG-binding sites of superovulated rat ovaries were labelled in vivo by injecting [131I]hCG intravenously 2 h prior to fractionation and isolation of ovarian membranes. Membranes were then tested for the ability to bind [125I]hCG and [131I]hCG in vitro. These experiments indicated that light and heavy rat luteal cell membrane fractions had similar levels of inside-out membrane vesicles (ranging from 0 to 34%), and that the hCG-binding sites of the 2 fractions appeared to be equally freely accessible to hormone in vivo. It is concluded that differences in membrane orientation cannot account for the distinct properties and marker enzyme profiles of rat ovarian light and heavy membranes, providing further support that these membrane are derived from different regions of the luteal cell surface-membrane.

Hormonal regulation of peptide receptors and target cell responses

Regulation of plasma membrane receptors for peptide hormones by the prevailing ligand concentration often causes altered target cell function. Receptor number is determined by hormone-induced changes in membrane conformation, irreversible ligand binding, and processing of ligand-receptor complexes during hormone action.

Effects of phospholipase A2 and filipin on the activation of adenylate cyclase

Rat liver plasma membranes were incubated with phospholipase A2 (purified from snake venom) or with filipin, a polyene antibiotic, followed by analysis of the binding of glucagon to receptors, effects of GTP on the glucagon-receptor complex, and the activity and responses of adenylate cyclase to glucagon + GTP, GTP, Gpp(NH)p, and F-. Phospholipase A2 treatment resulted in concomitant lossess of glucagon binding and of activation of cyclase by glucagon + GTP. Greater than 85% of maximal hydrolysis of membrane phospholipids was required before significant effects of phospholipase A2 on receptor binding and activity response to glucagon were observed. The stimulatory effects of Gpp(NH)p or F- remained essentially unaffected even at maximal hydrolysis of phospholipids, whereas the stimulatory effect of GTP was reduced. Detailed analysis of receptor binding indicates that phospholipase A2 treatment affected the affinity but not the number of glucagon receptors. The receptors remain sensitive to the effects of GTP on hormone binding. Filipin also caused marked reduction in activation by glucagon + GTP. However, in contrast to phospholipase A2 treatment, the binding of glucagon to receptors was unaffected. The effect of GTP on the binding process was also not affected. The most sensitive parameter of activity altered by filipin was stimulation by GTP or Gpp(NH)p; basal and fluoride-stimulated activities were least affected. It is concluded from these findings that phospholipase A2 and filipin, as was previously shown with phospholipase C, are valuable tools for differentially affecting the components involved in hormone, guanyl nucleotide, and fluoride action on hepatic adenylate cyclase.

The regulation of adenylate cyclase of the adrenal cortex

This short review summarizes some of the data concerning the regulation of adrenocortical adenylate cyclase by ACTH and other putative effectors, such as guanosine and nucleotides, divalent cations and adenosine. The available information on ACTH-sensitive adenylate cyclase of the adrenal cortex is discussed in comparison to other cyclase systems and the possible biochemical mechanisms of action of ACTH on the adrenal cortex.

Gel-filtration analysis of soluble adenylate cyclase from bovine corpus luteum

1. Sepharose 6B gel-filtration analysis of soluble adenylate cyclase from bovine corpus luteum is described. Both zonal and frontal techniques of analysis were used. 2. Under conditions of zonal analysis recoveries of activity were low. It was concluded that dissociation of two or more components of the adenylate cyclase complex was occurring on the column and that the maintenance of the complex was essential for the high-activity state of the catalytic unit. Two peaks of adenylate cyclase activity, of approximate mol. wts. 45,000 and 160,000 were detected. 3. The theory of frontal analysis (or steady-state gel filtration), applied to the study of the interacting components of the adenylate cyclase complex is discussed, and activity profiles are predicted. Activity profiles obtained experimentally be frontal analysis compared well with the theoretically predicted profile and provide evidence that dissociation of a high-activity complex, with concomitant loss of activity, does occur. Recoveries of activity under conditions of frontal analysis were higher than with zonal analysis. 4. The effects of concentration and removal of detergent on the activity of the soluble enzyme are discussed.

Properties of particulate and detergent-solubilized adenylate cyclase of rat testis. Effects of follitropin stimulation

Basal, fluoride and follitropin stimulated activities of adenylate cyclase have been studied in the testes of immature rats. The enzyme was maximally activated (about twice the basal activity) by low concentrations of follitropin, with an apparent Km of about 9 . 10(-10) M. Both Mg2+ and Mn2+ activate the enzyme but the apparent Ka for Mg2+ is about 10 times that for Mn2+. However, the apparent Km values for MgATP2- and MnATP2- are nearly the same (1.4 . 10(-4) M) and the cation activation of the enzyme is mainly through an increase in V. Ca2+ inhibited all expressions of testicular adenylate cyclase activity. Follitropin and fluoride stimulated adenylate cyclase activity at all Mg2+ concentrations but did not significantly affect the apparent Ka for Mg2+ or the Km for the substrate (MgATP2-). The stimulatory effect of the hormone or fluoride is mainly through an increase in V. Testicular adenylate cyclase could be solubilized with Triton X-100 or Lubrol-PX. The detergent-solubilized enzyme exhibited Km for substrate and Ka values for divalent cations similar to those of the membrane-bound enzyme and remained responsive to stimulation by fluoride. The stimulatory effect of follitropin, however, was lost. Responsiveness to follitropin was also lost by membrane-bound adenylate cyclase after treatment with phospholipase, although the fluoride effect was unchanged. These results reflect the essential role of lipids in the regulation of the follitropin-specific response.

Solubilization of bovine corpus-luteum adenylate cyclase in lubrol-PX, triton X-100 or digitonin and the stabilizing effect of sodium fluoride present in the solubilization medium

1. Adenylate cyclase activity of the washed 600g sediment of bovine corpus-luteum homogenate was solubilized by Lubrol-PX, Triton X-100 and digitonin. Digitonin was the least destructive of NaF-stimulated activity. 2. NaF, present in the solubilization medium together with MgSO4, increased the percentage yields of soluble activity from untreated 600g sediment and 600g sediment which had been preincubated with p[NH]ppG (guanosine 5'-[betagamma-imido]triphosphate). The stabilizing influence of NaF was most marked with digitonin. However, the highest specific activities of soluble enzyme were obtained with Lubrol-PX as solubilizing agent, since digitonin solubilized more membrane protein than does Lubrol-PX, and less of the activity of the digitonin-dispersed 600g sediment was recovered in the 105000g supernatant. 3. p[NH]ppG also has a stabilizing effect when present during the solubilization, but less so than NaF. 4. Both NaF and MgSO4 alone have a stabilizing effect during solubilization. The greatest amounts of soluble activity were obtained with both agents present in the solubilization medium, there being a synergistic effect.