Regulation of protein phosphatase 2A by direct interaction with casein kinase 2alpha.

Timely deactivation of kinase cascades is crucial to the normal control of cell signaling and is partly accomplished by protein phosphatase 2A (PP2A). The catalytic (alpha) subunit of the serine-threonine kinase casein kinase 2 (CK2) bound to PP2A in vitro and in mitogen-starved cells; binding required the integrity of a sequence motif common to CK2alpha and SV40 small t antigen. Overexpression of CK2alpha resulted in deactivation of mitogen-activated protein kinase kinase (MEK) and suppression of cell growth. Moreover, CK2alpha inhibited the transforming activity of oncogenic Ras, but not that of constitutively activated MEK. Thus, CK2alpha may regulate the deactivation of the mitogen-activated protein kinase pathway.

Characterization of a cyclic nucleotide-independent protein kinase highly active in human adrenocortical carcinoma.

Study of the protein kinase activity pattern of four human adrenocortical carcinoma showed that in all the samples examined a histone kinase (HK III) activity was present at high level, whereas it was barely detectable in normal tissue. HK III was separated from other known adrenocortical protein kinases by diethylaminoethyl cellulose chromatography. Isolated HK III exhibited a histone (H2B) protamine-phosphotranferase selectivity and used adenosine triphosphate but not guanosine triphosphate as phosphate donor. Serine was identified as the only target amino acid phosphorylated in the protein substrate. HK III showed an apparent molecular weight of 65,000 upon gel filtration and an apparent sedimentation coefficient of 3.7S. HK III activity was cyclic adenosine 3':5'-monophosphate independent and was not influenced by calcium, calmodulin, polyamines, and heparin. The significance of HK III activity in adrenocortical carcinoma extracts at a high level as compared to that of normal tissue remains to be clarified with regard both to its possible relationship with tumoral cell growth and differentiation processes and to its potential interest as a marker of human tumoral tissue activity.

Protein kinase CK2alpha is a target for the Abl and Bcr-Abl tyrosine kinases.

Protein kinase CK2 is a ubiquitous serine-threonine kinase in which a catalytic alpha subunit often associates with a beta subunit. CK2alpha is required for cell survival in yeast and has been proposed to be involved in cell growth control; however, its regulation in cells remains unclear. Here we present evidence that CK2alpha may be an associated substrate for the normal and oncogenic forms of the Abl tyrosine kinase. By probing CK2alpha with anti-phosphotyrosine antibodies, we found that CK2alpha can be phosphorylated on tyrosine in quiescent cells. In vitro phosphorylation of CK2alpha-containing immunoprecipitates showed that CK2alpha is substrate of an associated tyrosine kinase activity. Immunoprecipitation experiments revealed that CK2alpha is associated with normal c-Abl in mouse NIH3T3 fibroblasts and with the Bcr-Abl fusion protein in K562 human myeloid leukemia cells. Coexpression of Bcr-Abl and CK2alpha in NIH3T3 cells also leads to the formation of a Bcr-Abl/CK2alpha complex and to the inhibition of CK2alpha activity. Bcr-Abl-induced inhibition of CK2alpha could be reverted by incubating CK2alpha with a tyrosine phosphatase. These observations clearly support the idea that a signal transduction pathway contributes to CK2 regulation and point to CK2alpha as a possible mediator of Bcr-Abl effects.

A role for protein kinase CK2 in cell proliferation: evidence using a kinase-inactive mutant of CK2 catalytic subunit alpha.

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits generally combined to form alpha(2)beta(2), alphaalpha'beta(2), or alpha'(2)beta(2) heterotetramers. To gain more insight into the role of CK2 in the control of proliferation in mammalian cells, overexpression of isolated CK2 subunits alpha, alpha', or beta was carried out in two fibroblast cell lines: NIH3T3 and CCL39. To interfere with CK2 cellular functions, cells were also transfected with a kinase-inactive mutant of CK2alpha catalytic subunit: CK2alpha-K68A. In NIH3T3 cells, overexpression of either wild-type subunit (alpha, alpha' or beta) had no effect on cell proliferation. In contrast, overexpression of the CK2alpha kinase-deficient mutant induced a marked inhibition of cell proliferation. This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments in both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2alpha-K68A mutant by growth curve analysis. We demonstrated that the kinase-negative mutant has the capacity to integrate the endogenous CK2 subunit pool both as an isolated kinase-inactive alpha subunit and as associated to the beta subunit in a kinase-inactive tetramer. Finally we showed that expression of the kinase-inactive mutant interferes with phosphorylation of an endogenous CK2 substrate; we speculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.

Polyamine uptake by bovine adrenocortical cells.

Bovine adrenocortical cells of fasciculo-reticulata origin in primary culture actively accumulate polyamines from the extracellular medium in an energy-dependent process. At low extracellular concentration (e.g., 1 microM putrescine), the transport system resulted in a several-hundred-fold concentration of polyamine in the cellular compartment within 1-2 h of incubation. Putrescine uptake appeared to be the sum of a sodium-dependent, saturable process, with an apparent Km of about 10 microM and of a non-saturable, sodium-independent component. By contrast, spermine was taken up by the cells mostly in a sodium-independent manner. Cross-competition experiments suggested that both polyamines were at least partly transported by the same system. Using specific corresponding probes, it was shown that the polyamine uptake was independent of the amino acid transport systems of the A, L and N types known in a number of cell systems. Adrenocortical cell polyamine content is known to be modulated by adrenocorticotropin through induction of ornithine decarboxylase activity. The existence of a specific uptake system in these cells opens the possibility of a more rapid pathway for the regulation of cellular polyamine levels. It remains to be examined whether this polyamine transport system is under hormonal control, and whether this can support the suggestion that polyamines may represent a form of intracellular messengers in the mechanism of hormone action.

Glucocorticoid binding in the chicken liver cytosol. Characterization of five macromolecular binding components.

The heterogeneity of the glucocorticoid binding in the chicken liver cytosol, previously suggested by the results obtained with crude preparations, was confirmed using different techniques such as stepwise ammonium sulfate precipitation, hydroxyapatite chromatography and gel filtration on Sephadex G-200. The latter method permitted the separation of five glucocorticoid binding macromolecules respectively named binders S1, S2, S3, S4 and S5, according to their decreasing apparent molecular size upon gel filtration. Apparent molecular weight, binding affinity, capacity and specificity of these five moieties were examined. In addition, S-aryl-transferase activity using glutathione as co-substrate was studied and found to coincide mostly with the fractions containing binder S4, which might represent an avian liver ligandin.

Stimulation of phosphatidylinositol turnover by acetylcholine, angiotensin II and ACTH in bovine adrenal fasciculata cells.

The effect of acetylcholine, angiotensin II and adrenocorticotropin (ACTH) on phosphatidylinositol (PI) metabolism was examined using bovine adrenocortical fasciculata cell suspensions. The three agents, which acutely stimulate glucocorticoid production by these cells, were all able to increase [32P]Pi incorporation into cellular PI. However, whereas the relative steroidogenic potency (at maximally active concentrations) was ACTH greater than or equal to angiotensin II greater than acetylcholine, the effect on PI labeling was in the order angiotensin II greater than acetylcholine greater than ACTH. The dose-response curves for steroidogenesis and that for PI labeling were superimposable in the case of angiotensin II (ED50 = 1 X 10(-8) M) and of acetylcholine (ED50 = 5 X 10(-7) M), while the two responses were dissociated under graded ACTH challenge. Both steroidogenic response and increased PI labeling elicited by angiotensin II and acetylcholine were respectively inhibited by (Sar1-Ala8)-angiotensin II and muscarinic antagonists. Time-course study showed that in the case of angiotensin II and acetylcholine, the sequence of events was: increased phosphatidic acid labeling, increased PI labeling, activated steroidogenesis. By sharp contrast, under ACTH stimulation, increased steroidogenesis was detected well before activation of PI metabolism. These data suggest that in bovine adrenocortical fasciculata cell, steroidogenesis may be activated by two different pathways. The first one would act mainly through cyclic AMP-dependent intracellular events and is usually accepted in the mechanism of action of ACTH. The other, cyclic AMP-independent pathway, as in the case of angiotensin II and acetylcholine actions, may involve phospholipid-mediated intracellular processes.

Cholinergic muscarinic stimulation of steroidogenesis in bovine adrenal cortex fasciculata cell suspensions.

Acetylcholine was found to acutely stimulate cortisol production by bovine fasciculata adrenocortical cell suspensions. This effect was maximal at 10(-4) M acetylcholine concentration, resulted in a 5-fold increase in cortisol production over the control after 1 h incubation, and represented about one fifth of the ACTH maximal stimulation under the same conditions. Acetylcholine-stimulated steroidogenesis was concentration-dependent (10(-8)-10(-5) M), proportional to the cell number (5 X 10(5)-1 X 10(6)) and reached a plateau after 30 min incubation. Use of various cholinergic specific agonists and antagonists showed that the steroidogenic action of acetylcholine was a typical muscarinic effect. This character is in agreement with the previously demonstrated presence of muscarinic receptors in bovine adrenocortical tissue. The steroidogenic effect of acetylcholine required the presence of extracellular calcium in the medium and was impaired upon addition of tetracaine and procaine. No change in cyclic AMP nor cyclic GMP levels could be detected in the system under acetylcholine stimulation. Acetylcholine appeared to exhibit a synergistic effect in combination with ACTH, and exogenous cyclic AMP; these observations suggest a different mechanism of action for acetylcholine and ACTH and point to a possible cholinergic participation in the regulation of adrenocortical differentiated functions in vivo.

Catalytic and molecular properties of a highly purified G type casein kinase from bovine lung tissue.

A preparation procedure has been worked out to obtain a highly purified G type (using GTP as well as ATP) casein kinase from large quantities of bovine lung tissue. It included ion-exchange (DEAE and phosphocellulose) and affinity (casein and ATP-Sepharose) chromatography combined with a flocculation step, and yielded an apparently homogeneous preparation with a 16% yield and a purification factor of more than 1400. The purified lung casein kinase used GTP (Km 16 microM) almost as well as ATP (Km 6.7 microM) and exhibited the major catalytic properties of the casein kinase G previously described in bovine adrenal cortex (Cochet, C., Job, D., Pirollet, F. and Chambaz, E.M. (1981) Biochim. Biophys. Acta 658, 191-201). Mg2+ (30-50 mM) and spermine (2 mM) were potent activators of lung casein kinase G activity, whereas the enzyme was inhibited by heparin and quercetin. The purified enzyme underwent self-phosphorylation in the presence of ATP or GTP, serine being the only target amino acid under these conditions, whereas both serine and threonine were phosphorylated by the enzyme in casein. Lung casein kinase G exhibited an apparent molecular weight between 140 000-160 000 upon gel filtration and appeared formed by the association of two different subunits upon SDS-polyacrylamide gel electrophoresis. The two subunits of Mr 38 000 (alpha) and 27 000 (beta) exhibited a 2:1 ratio upon quantitative scanning, suggesting an alpha 3 beta 2 combination in the oligomeric native enzyme structure. Peptide mapping of the two isolated subunits following 125I-labeling and papain digestion did not disclose any common fragment. The casein kinase catalytic activity was found associated with the alpha (38 kDa) enzyme subunit after recovery from gel electrophoresis in the presence of SDS, whereas the 27 kDa (beta) subunit was the major target of the enzyme self-phosphorylation reaction. alpha and beta subunits appeared strongly associated in the oligomeric enzyme and the possible role of the beta subunit in the casein kinase G activity remains to be examined. The purified casein kinase G, which can be obtained by the present procedure, should facilitate the study of the biological significance of this phosphorylation system in the intact cell.

Reversibility of the phosphate transfer between ATP and phosphoproteins catalysed by a cyclic nucleotide independent (G type) casein kinase.

Purified casein kinase G was found able to catalyse the synthesis of [gamma-32P]ATP in the presence of ADP, phosphocasein (previously 32P-labeled by the forward kinase reaction) and magnesium. Apparent Km values of approx. 0.5 mM for phosphocasein and 7.5 mM for ADP were calculated, these values indicating low affinities for the substrates as compared to those exhibited for casein and ATP in the forward reaction. The reverse casein kinase G activity appeared to prefer ADP and GDP as phosphate acceptors. Whereas the casein kinase G reverse reaction could be supported by casein, phosvitin and histone previously phosphorylated by the enzyme, the same proteins could not serve as a phosphate source when previously phosphorylated by the cAMP-dependent protein kinase. Forward and reverse casein kinase G reactions exhibited different optimal pH values (8.5 and 7.2, respectively) and a different sensitivity to Mg2+. Spermine, which activated the kinase activity, blocked the reverse reaction at millimolar concentrations. Although the biological significance of the casein kinase G reverse activity remains to be assessed in intact cell, the process may be useful as a tool in the characterization of phosphorylatable sites in phosphoproteins.

Study of steroid-proteininteractions by electron spin resonance spectroscopy. Binding of a spin-labelled dihydrotestosterone to bovine serum albumin.

The interaction of bovine serum albumin with dihydrotestosterone bearing a spin label at C-3 was studied using electron spin resonance (ESR) spectroscopy. Quantitative binding parameters (Ka approximately 10(5) M-1; maximum binding capacity; two sites/mol albumin) obtained by ESR were in good agreement with those given by equilibrium dialysis. ESR study at various temperatures allowed the calculation of the thermodynamic parameters of the steroid-protein interaction: deltaG=-6.8 kcal/mol; deltaH=-7.9 kcal/mol; deltaS=-3.2 cal/mol per degree and confirmed a transition temperature of about 65 degrees C for albumin. Na, Liland Ca salts had a generally favorable effect on the interaction whereas other ions (e.g. Hg, Cu) impaired the binding process. Study of the width of the ESR spectra of the protein-bound spin-labelled steroid and extrapolation of a 2 T value to infinite viscosity (Azz coupling constant) indicated a non-polar binding site, which became increasingly hydrophobic as the temperature was raised. Since this methodology can give both pertinent quantitative and qualitative data, ESR spectroscopy should be of value in the study of steroid-protein interactions of biological significance.

Cyclic nucleotide independent casein kinase (G type) in bovine adrenal cortex: purification and properties of two molecular forms.

Two soluble cyclic nucleotide independent protein kinase (ATP: protein-phosphotransferase, EC 2.7.1.37) activities have been purified from bovine adrenal cortex cytosol. Both purified enzymes exhibit the best affinity for acidic substrates such as casein and can use GTP as well as ATP as phosphoryl donor. They can thus be classified as casein kinase of the G type as previously proposed (Cochet C. et al., (1980) Endocrinology 106, 750-757). Whereas the two moieties could be separated using their different affinities toward a phosphocellulose resin, both purified enzymes appeared indistinguishable on the basis of several molecular and catalytic properties. Both G type casein kinase moieties have an identical sedimentation behavior (5.5 S in the presence of 0.5 M NaCl), yield similar patterns upon electrophoresis under denaturing conditions with three major protein components (42 000, 38 000 and 27 000), and show an ability to undergo self-phosphorylation mostly on the 27 000 component. Both enzymes have the same protein and nucleotide (ATP and GTP) substrate specificity, show similar increases in activity in the presence of polyamines and Mg2+ (optimum at 50 mM) and similar inhibition by NaCl above 0.2 M. The only difference between the two forms of casein kinase (i.e., affinity for phosphocellulose) could not be explained by a different degree of self-phosphorylation or by a limited proteolytic process during handling and purification. These results suggest that the two active moieties may represent isoenzymatic forms of the G type casein kinase activity in bovine adrenal cortex cytosol.

Electron spin resonance study of human transcortin: Thiol groups and binding site topography.

A series of cortisol analogs bearing a nitroxide free radical on C-17 side chains with a variation of distance between the steroid D-ring and the spin label from 7.4 to 17.6 A has been synthesized. These analogs were found to retain a good affinity for the specific corticosteroid binding site of purified human transcortin. The spin-labeled cortisol analogs were used to probe the human transcortin binding site structure by electron spin resonance (ESR) spectroscopy. A total depth of approx. 25 A was estimated for the binding site crevice. Use of sulfhydryl reagents (N-ethylmaleimide, p-chloromercuribenzoate) showed that a maximum of two sulfhydryl groups were titratable after reduction and denaturation of the protein. One of these thiol groups appeared to be involved in the cortisol binding site and could not be detected in the presence of bound steroid. ESR study of its environment, using spin-labeled N-ethylmaleimide reagents of various side-chain lengths, led to the conclusion that this thiol was at a depth of approx. 15 A or more in the binding site cavity. The second sulfhydryl group may be present in an oxidized form in the purified native transcortin, since it became titratable only after reductive treatment of the protein. ESR study showed that this thiol may be located in a crevice at approx. 15 A from the protein surface. These findings are compatible with a structural organization of the transcortin cortisol binding site, taking into account tentative models previously proposed by others.

The tight association of protein kinase CK2 with plasma membranes is mediated by a specific domain of its regulatory beta-subunit.

Previous immunocytochemical studies have shown that protein kinase CK2 is mostly detected both in the cytoplasm and the nucleus of most cells. In the present study, CK2 was detected in highly purified plasma membrane preparations from rat liver. The protein kinase could be released from the membranes by high salt extraction (>1 M NaCl). Plasma membranes prepared from SF9 insect cells expressing the alpha- and beta-subunits of CK2 also contained a significant amount of oligomeric CK2. Furthermore, it was demonstrated in this cell system as well as in rat liver plasma membranes, that the beta-subunit of the kinase is the targeting subunit which mediates the tight association of the enzyme to plasma membrane components. Binding studies using membranes and recombinant proteins corresponding to different regions of the beta-subunit suggest that a functional domain previously shown to be involved in the binding of polyamines may also participate to the binding of CK2 to membranes. Modification of membranes by trypsin and phospholipases indicated that the binding process may require both membrane protein(s) and phospholipids. Interestingly, it was observed that the amount of membrane-bound CK2 in liver of embryos and new born rats increases dramatically after birth and persists during the postnatal stages of development.

Molecular organization (topography) of cytochrome P-450(11)beta in mitochondrial membrane and phospholipid vesicles as studied by trypsinolysis.

Cytochrome P-450(11)beta from adrenal cortex is an intrinsic membrane protein embedded in the inner mitochondrial membrane. Topography of the protein inside a phospholipid bilayer was examined using controlled proteolysis of purified cytochrome P-450(11)beta following its integration into artificial liposomes. Inclusion of the protein into phospholipid vesicles led to a marked stabilization of the cytochrome activity. Trypsin treatment of the liposome-integrated cytochrome resulted in the rapid disappearance of the native protein moiety (47 kDa), while a major 34 kDa peptide component was formed. This peptide core retained the heme moiety and part of the cytochrome steroid-11 beta hydroxylase activity. Very similar observations were obtained when inside-out vesicles prepared from isolated adrenocortical mitoplasts were examined with the same approach. It is thus suggested that adrenocortical cytochrome P-450(11)beta is embedded in the inner mitochondrial membrane as well as in artificial liposomes by a major hydrophobic domain associated with the heme moiety while a limited domain remains accessible on the matrix side of the membrane surface. The previous described phosphorylation of the cytochrome P-450(11)beta on a serine residue, by the cAMP-dependent protein kinase is suggested to occur in the protein domain oriented toward the membrane surface, the phosphorylation site being lost under mild proteolytic digestion of the membrane-integrated protein.

Phospholipid independent phosphorylation of protamine by protein kinase C: effects of polyanions.

The ability of purified protein kinase C (PKC) to phosphorylate protamine sulphate was found to be totally independent of phospholipid cofactors, whereas the phosphorylation of protamine free base was markedly increased by the presence of phosphatidylserine (PS). The hypothesis of an activation of PKC by the sulphate groups of protamine sulphate was confirmed by the high phosphorylation of protamine free base in the presence of non-peptide polyanionic compounds, such as glycoaminoglycans or polynucleotides. The catalytic fragment of PKC supported protamine base phosphorylation with the same polyanionic dependency. Light scattering intensity measurements showed that this phosphorylation correlated to the substrate/cofactor aggregation. These data support the view that apparent phospholipid-independent activation of PKC results from the formation of aggregates in the assay and this could result in the non-specific activation of this enzyme through its catalytic domain.

Characterization of the 3 beta-hydroxysteroid dehydrogenase activity associated with bovine adrenocortical mitochondria.

The enzymatic activity of 3 beta-hydroxysteroid dehydrogenase (3 beta HSD/I) constitutes an essential step in the biosynthesis of active steroid hormones such as progesterone, mineralo- and gluco-corticoids, estrogens, and androgens. Its subcellular localization in steroidogenic tissues is usually considered to be mainly microsomal; however, 3 beta HSD/I activity is also present in mitochondrial preparations. In the present study, the distribution of 3 beta HSD/I in bovine adrenocortical subcellular preparations has been reexamined, and the catalytic properties of the enzyme present in the various cell compartments have been characterized. About 30% of the total 3 beta HSD/I was found to remain tightly associated with highly purified mitochondrial preparations. The preferred substrate of the mitochondrial enzyme was pregnenolone. Examination of submitochondrial preparations revealed that 3 beta HSD/I was associated with both the inner membrane and a particulate fraction that sediments in a density gradient between inner and outer membranes. The specific activity of the enzyme was at its highest in this intermediate density fraction, which exhibited the properties of mitochondrial intermembrane contact sites. Taken together, these observations suggest that these contact sites may represent a supramolecular organization of biological significance in adrenocortical cell steroidogenic functions. Such intermembrane fusion sites would facilitate the access of cholesterol to the inner membrane in which cholesterol side-chain cleavage cytochrome P-450 is located as well as the rapid transformation of its reaction product (i.e. pregnenolone) to progesterone by 3 beta HSD/I. Such a submitochondrial organization opens new possibilities in the understanding of the regulation of adrenocortical differentiated functions.

Hormonal control of polyamine levels in bovine adrenocortical cells.

The implication of polyamines in cellular growth and differentiation processes and the existence of a polyamine-mediated protein phosphorylation system in adrenocortical cells suggest that polyamines may be examined as potential intracellular messengers in the pleiotypic action of ACTH. Bovine adrenocortical cells in culture exhibit a specific, energy-dependent, partly sodium-supported, inward polyamine transport system, independent of the A, L, and N aminoacid uptake systems. Steroidogenic concentrations of ACTH (10(-12) to 10(-9)M) induced a rapid activation of the polyamine uptake, resulting in a 2- to 3-fold increase in intracellular polyamine content, over 1 h. The ACTH dose-response curves for steroidogenic activity and for polyamine uptake were similar. Other adrenocortical effectors such as angiotensin II, acetylcholine, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) activated polyamine uptake with a pattern parallel to their steroidogenic potency (i.e. ACTH greater than angiotensin II greater than acetylcholine, TPA). Steroidogenic concentrations of 8-bromo-cAMP displayed no effect on the adrenocortical polyamine uptake, suggesting that cAMP does not mediate this action of ACTH. On the other hand, ACTH induced a large increase in ornithine decarboxylase (ODC) activity in bovine adrenocortical cells, after a 6- to 8-h lag period, resulting in an average 2-fold increase in cell putrescine, spermidine, and spermine content. However, when the cells were previously polyamine loaded, ACTH-dependent ODC induction was suppressed. Adrenocortical cell polyamine content thus appears to be under hormonal control. ACTH may act through two possible pathways: 1) the rapid activation of the cell polyamine accumulation from an extracellular source and 2) the delayed increase in polyamine biosynthesis secondary to induction of ODC activity, when the cells are relatively depleted of the polyamines. These observations suggest that polyamines may function as intracellular messengers for some of the ACTH effects in bovine adrenocortical cells.

Thrombospondins selectively activate one of the two latent forms of transforming growth factor-beta present in adrenocortical cell-conditioned medium.

Transforming growth factor-beta (TGF beta) has been shown previously to be a potent inhibitor of bovine adrenocortical cell steroidogenic functions. However, it is present in the culture medium of these cells in a latent form. In this study, we analyzed in detail the biochemical composition of this latent TGF beta. Two distinct complexes could be separated chromatographically by gel filtration on Sephacryl S-300, and their composition was studied using immunochemical methods. The results indicate that one form (peak I) is a complex between alpha 2-macroglobulin (alpha 2M) and either the unprocessed TGF beta precursor or the mature form of TGF beta. In a major fraction of this complex, TGF beta is covalently linked to alpha 2 M, whereas in a minor fraction, it is noncovalently bound and, therefore, activatable. The second form of latent TGF beta (peak II) is a complex among latent TGF beta-binding protein (LTBP), latency-associated protein, and mature TGF beta and a complex between LTBP and unprocessed TGF beta. We investigated the ability of thrombospondins (TSP1 and TSP2) to activate these latent forms of TGF beta. TSP1 and TSP2 were equally potent at activating the LTBP-latency-associated protein-TGF beta complex in the absence of cell contact, but were ineffective on the alpha 2M-TGF beta complex. Therefore, TGF beta may act as an autocrine regulator of adrenocortical steroidogenic functions. Its activity appears to be controlled by TSPs, the local production of which is regulated by systemic ACTH.

Oxygen availability as a regulatory factor of androgen synthesis by adrenocortical cells.

Molecular oxygen is an obligatory substrate of all cytochrome P-450 (cyt P-450) hydroxylases involved in the steroid biosynthetic pathways. However, oxygen-derived free radicals are highly destructive species resulting from cyt P-450-catalyzed steroid hydroxylation reactions. Cells in culture are usually exposed to an atmospheric pO2 that is well above the estimated in situ pO2 in vivo. It has been suggested that lowering pO2 might prevent the loss of biosynthetic enzymatic activities in bovine adrenocortical or Leydig cells in culture. The present study was performed to examine the effect of low pO2 pressure (1% oxygen in the gas phase) compared to routinely employed conditions (19% oxygen) on bovine adrenocortical cell steroidogenic activities under both basal and stimulated conditions. Lowering the pO2 showed no significant effect on the cultured adrenocortical cell proliferation rate or their ability to produce cortisol. By contrast, it resulted in a dramatic drop in androgen secretion and a slight increase in corticosterone synthesis. The mechanism involved in the qualitative modulation of steroid biosynthesis by oxygen availability was examined in some detail using purified steroid hydroxylase components. We found that cyt P-450(17 alpha, which can catalyze both the steroid 17 alpha-hydroxylation and the 17-20-lyase reaction is probably the major target explaining the oxygen effect. Indeed, cyt P-450(17 alpha) hydroxylase activity exhibits a clearly higher affinity for oxygen (Km, 22 microM) than its lyase activity (Km, 66 microM). These observations suggest that 1) oxygen availability is able to modulate the balance between androgen and corticosteroid pathways in bovine adrenocortical cell; and 2) adrenocortical cell functions studied in vitro under relatively high pO2 do not exactly reflect the in vivo situation.