ABSTRACT The synthesis of the vasoconstrictor peptide endothelin-2 (ET-2) is dependent on hydrolysis of the biologically inactive intermediate big ET-2 by an endothelin-converting enzyme (ECE). Here, mechanisms inducing ET-2 synthesis have been investigated using the human renal adenocarcinoma cell line (ACHN). Synthesis of ET-2 by ACHN cells was inhibited by phosphoramidon (IC(50( congruent with11 microM). To determine whether ET-2 synthesis occurs in parallel with the metallopeptidase ECE-1, a putative processing peptidase for big ET-2, changes in the levels of their mRNAs were compared by semi-quantitative RT-PCR under conditions causing the upregulation of ET-2 synthesis. Tumour necrosis factor-alpha (TNFalpha), forskolin and a cell-permeable cAMP analogue (dibutyryl cAMP) caused concentration-dependent increases in ET-2 synthesis. Combination of forskolin or dibutyryl cAMP with TNFalpha produced a significantly greater increase in ET-2 production than these agents alone, indicating that adenylate cyclase and TNFalpha induce ET-2 synthesis by separate signalling pathways. Studies using receptor selective TNFalpha mutants, (125(I-TNFalpha binding and TNF receptor mRNA showed that type-1 TNF receptors mediate the ET-2 response to TNFalpha. PreproET-2 mRNA levels were increased by TNFalpha at 1 h and 2 h, but returned to control levels at 4 h. Treatment with forskolin significantly increased preproET-2 mRNA levels after 1 h and 4 h. ACHN cells expressed ECE-1b and ECE-1c, but not the ECE-1a isoform of this peptidase. RT-PCR for the combined isoforms ECE-1b/c/d showed TNFalpha to increase mRNA levels at 2 h and 4 h. Forskolin had no effect on ECE-1b/c/d mRNA levels. Thus, expression of ET-2 and ECE-1b/c/d mRNAs in ACHN cells do not display the co-ordinated regulation observed with typical peptide prohormone processing enzymes and their substrates.
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GL Lambert, S Barker, DM Lees, and R Corder
M Montiel, S Barker, G P Vinson, and E Jiménez
The angiotensin II (Ang II)-binding sites in rat adrenal gland membranes were characterized using 125I-radiolabelled Ang II. While Scatchard analysis identified a single population of Ang II receptor sites, isoelectric focusing (IEF) on polyacrylamide gels revealed four peaks of specific Ang II binding which migrated to isoelectric points (pI values) 6·8, 6·7, 6·5 and 6·3. In binding assays in the presence of an excess of the Ang II receptor AT1 subtype antagonist DuP 753, a monophasic dose-dependent displacement of 125I-labelled Ang II binding by the Ang II receptor AT2 subtype antagonist CGP42112A was observed, and vice versa. In this system, reduction of disulphide bridges using 1 mmol dithiothreitol (DTT)/l markedly increased the number of binding sites in the adrenal zona glomerulosa without affecting receptor affinity.
Using IEF, it was found that both DuP 753 and CGP42112A were able to reduce specific binding of each of the four peaks to some extent. However, the predominant effect of DuP 753 was to reduce the labelling of the isoform at pI 6·7 substantially, while CGP42112A significantly inhibited the specific 125I-labelled Ang II binding to the pI 6·3 isoform. When DuP 753 and CGP42112A were used together, specific binding of 125I-labelled Ang II to the isoforms of pI values 6·8, 6·7 and 6·3 was completely eliminated. These data suggest that the four peaks of specific binding found may be composed of different isoforms of both AT1 and AT2 receptor subtypes and that the Ang II receptor isoforms which migrated to pI 6·7 and pI 6·3 are predominantly composed of AT1 and AT2 receptor subtypes respectively. Interestingly, in the presence of both antagonists, 8·7 ± 0·9% of the specific binding migrating at pI 6·5 remained unaffected. This finding suggests the presence of an additional subtype, which is neither AT1 nor AT2, in the rat adrenal zona glomerulosa.
In further studies, pretreatment with DTT was found to increase the specific 125I-labelled Ang II binding of all four isoforms. Moreover, DTT also produced a further specific binding component between pI 6·5 and pI 6·7 which exhibited AT2 subtype pharmacology in DTT-treated preparations. Since DTT has been reported to enhance only AT2 subtype binding this also suggests that the different isoforms may contain components related to both AT1 and AT2 receptor subtypes.
S. Kapas, C. D. Orford, S. Barker, G. P. Vinson, and J. P. Hinson
The intracellular mechanisms of action of α-MSH in rat adrenocortical cells were examined. When rat adrenal capsule (largely glomerulosa) cells were stimulated with a range of concentrations of α-MSH there was significant stimulation of aldosterone secretion at 10-10 mol/l, although cyclic AMP was not increased until high concentrations of α-MSH were used (10-6 mol/l and above). However, cells incubated with ACTH showed an increase in aldosterone secretion at 10-11 mol/l and levels of cyclic AMP were elevated at 10-9 mol ACTH/1.
When rat adrenal whole capsules were incubated with α-MSH, membrane-bound protein kinase C (PKC) activity was increased and cytosolic enzyme activity decreased, showing PKC activation. Stimulation with angiotensin II also induced translocation of PKC activity, but ACTH did not.
When [3H]inositol-loaded glomerulosa cells were stimulated with α-MSH there was significant generation of [3H]inositol trisphosphate (IP3) at concentrations of α-MSH which stimulated secretion of aldosterone. Significantly increased levels of [3H]IP3 were also measured when loaded cells were exposed to angiotensin II. ACTH did not cause any significant stimulation of [3H]IP3 production at any concentration used. These results indicate that activation of PKC and phospholipase C is important in modulating the steroidogenic effect of α-MSH.
S. Barker, S. M. Laird, M. M. Ho, G. P. Vinson, and J. P. Hinson
We have previously reported the production of a monoclonal antibody (IZAb) which interacts with an antigen, found predominantly in rat adrenal inner zone tissue, which may have a role in steroidogenesis. Here we describe initial studies on its characterization.
Immunoblot analysis of rat adrenocortical proteins obtained from fresh tissue and separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, showed that the IZAb interacted with a protein with a molecular mass of approximately 30 000 Da (IZAg1). This protein was found predominantly in rat adrenal inner zone tissue. Small amounts were seen in the zona glomerulosa, while no corresponding protein was seen in rat ovary, heart, liver, testis or kidney tissue. Subcellular fractionation of rat adrenocortical inner zone tissue and immunoblot analysis showed that the IZAg1 was present in the microsomal and mitochondrial fractions of the cell, but was absent from the cytosol. Invivo treatment with ACTH (100 μg/day) for more than 5 days also increased the expression of this protein by rat adrenal inner zone tissue, and this was coincident with increased corticosterone and 18-hydroxydeoxycorticosterone (18-OH-DOC) production in incubations of inner zone tissue in vitro.
In experiments involving the short-term culture of rat adrenal inner zone cells, IZAb interacted with two protein bands. IZAg1 was detected as a minor band in untreated control cells, while another protein with a molecular mass of approximately 60 000 Da, designated IZAg2, was present in greater amounts. Treatment of cells for 48 h with either ACTH (1 μmol/l) or dibutyryl-cAMP (100 μmol/l) resulted in apparent increased expression of IZAg1 and diminished levels of IZAg2. As in the in-vivo treatments, the increase in IZAg1 was associated with a corresponding increase in corticosterone and 18-OH-DOC production.
These findings suggest that the IZAb recognizes a protein (IZAg2) which occurs in unstimulated adrenal cells. On stimulation by steroidogenic agents, this protein becomes processed to yield a smaller protein (IZAg1) which is associated with enhanced adrenal steroidogenesis.
E. Jimenez, S. Marsigliante, S. Barker, J. P. Hinson, and G. P. Vinson
Angiotensin II (AII) receptors were identified in rat tissue membranes by specific binding of 125I-labelled AII. Using an isoelectric focusing technique, two forms of the high-affinity AII receptor were identified in rat adrenal zona glomerulosa and liver membranes. These migrated to isoelectric points (pI) 6.8 and 6.7. Two low-affinity forms migrated to pI 6.5 and 6.3. The two high-affinity forms were in greatest abundance in the zona glomerulosa, while the low-affinity pI 6.5 isoform was predominant in liver membranes. In uterine membranes both low-affinity isoforms were observed, but there was only one of the high-affinity forms (pI 6.7).
Concentrations of AII receptor isoforms were increased in the zona glomerulosa of sodium-deprived rats.
Reduction of disulphide bridges with dithiothreitol (DTT) had different effects on the various AII receptor isoforms. Thus 1 mmol DTT/l caused a twofold increase in 125I-labelled AII binding in zona glomerulosa membranes. DTT produced no appreciable differences in specific AII binding in uterine membranes, whereas there was a 50% reduction of binding in liver membranes. At 20mmol/1, DTT greatly decreased AII binding in all tissues.
The data suggest the existence of multiple forms of AII receptors which may have different functions.
S. Marsigliante, V. A. Baker, J. Puddefoot, S. Barker, and G. P. Vinson
The variability in the profile of oestrogen receptor (ER) isoforms in breast tumours has been studied.
Using low-resolution isoelectric focussing (IEF), two major ER isoforms with isoelectric point (pI) values of 6.1 and 6.6 could be identified, with corresponding sedimentation coefficients in sucrose density gradients of 8 S and 4 S respectively.
Using high-resolution IEF or immunoblotting, the pI 6.6 form (4S) was shown to be composed of three different species, with pI values of 6.3, 6.6 and 6.8, while the oligomeric pI 6.1 protein (8 S) did not show charge heterogeneity. Data were obtained on the soluble receptors from supernatants of 42 ER-positive primary breast tumour homogenates using high-resolution IEF to obtain ER isoform profiles. It was found that 54.7% of tumours contained the isoforms at pI 6.6 and 6.1, while only 11.9% contained the full complement of isoforms (pI 6.1, 6.3, 6.6 and 6.8). Of the tumours studied, 11.9% contained isoforms of pI 6.1, 6.6 and 6.8, with 14.3% containing isoforms with pI 6.1, 6.6 and 6.3. Very few tumours contained only one isoform, with 4.8% of tumours containing a single isoform at pI 6.1 and 2.4% of tumours containing only the isoform at pI 6.6.
All four ER isoforms were also shown to be present in some tumours by immunoblotting using antibody H222 and, in addition, high-resolution IEF indicated that all isoforms bind oestradiol, diethylstilboestrol and tamoxifen.
The variability in the ER isoform profile may have a bearing on the known variability of tumour response to endocrine therapy and prognosis.
S Kapas, A Purbrick, S Barker, G P Vinson, and J P Hinson
It is well established that ACTH and angiotensin II (Ang II) stimulate aldosterone secretion from rat adrenal zona glomerulosa cells in vitro and mediate their steroidogenic effects via the cyclic AMP (cAMP) pathway and phosphoinositide turnover respectively. α-MSH also stimulates aldosterone secretion from zona glomerulosa cells in vitro, and recent studies from our laboratory have shown that its steroidogenic effects are mediated by increases in inositol 1,4,5-trisphosphate (IP3) production. α-MSH also stimulates adenylyl cyclase activity, but only at concentrations that are supramaximal for stimulation of steroidogenesis. The observation that α-MSH-stimulated IP3 accumulation declines as the activity of adenylyl cyclase increases prompted further studies on the interactions of cAMP and phosphoinositide production.
The effects of α-MSH and ACTH on Ang II-stimulated steroidogenesis and IP3 accumulation were studied. On addition of increasing concentrations of ACTH, both the aldosterone and IP3 responses to Ang II were significantly inhibited; however, only high concentrations of α-MSH achieved this effect. These results suggest that cAMP or a cAMP-dependent event is able to inhibit phospholipase C activity. This hypothesis was tested by measuring IP3 production in Ang II-stimulated zona glomerulosa cells exposed to two different concentrations of α-MSH: 1 nmol/l, which stimulates the generation of IP3, and 1 μmol/l, which activates adenylyl cyclase. It was found that this high concentration of α-MSH significantly inhibited Ang II-stimulated aldosterone secretion and IP3 levels. In addition, α-MSH reduced 125I-labelled Ang II binding to rat adrenal zona glomerulosa cells. ACTH and cAMP also inhibited Ang II binding, thus supporting the hypothesis that cAMP (or a cAMP-mediated event) inhibits Ang II receptor function.
S Marsigliante, T Verri, S Barker, E Jimenez, G P Vinson, and C Storelli
Previous studies have shown the effects of angiotensin II (Ang II) in teleosts, and Ang II-binding sites have also been localized in tissues from rainbow trout. The purpose of this study was to extend these findings and to provide an analysis of Ang II receptor (Ang II-R) isoforms in three tissues obtained from European eel (Anguilla anguilla).
Ang II-Rs were identified in eel liver, kidney and intestine membranes by the binding of either 0·5 nmol human 125I-labelled Tyr4-lle5-Ang II/l or increasing concentrations (1–120 nmol/l) of [3,5-3H]Tyr4-Ile5-Ang II. Using an isoelectric focusing technique, two Ang II-binding sites were identified in liver membranes. These migrated to isoelectric points (pI values) 6·5 and 6·7. Seventy per cent of binding to both sites was displaced by a 10 000-fold excess of unlabelled human Ang II. In both whole plasma membranes and brush border membranes from intestine, only one form of the Ang II-R was found, with pI 6·5 and high affinity (K d=3·4 nmol/l) for the [3,5-3H]Tyr4-Ile5-Ang II. Similarly, only the isoform focusing at pI 6·5 was observed in renal tubular epithelial brush border membranes. Reduction of disulphide bridges with dithiothreitol significantly enhanced Ang II binding to the isoform at pI 6·5 in liver (P<0·05) and kidney (P<0·01), while in liver the binding to the isoform of pI 6·7 was significantly reduced (P<0·001).
The data suggest the existence in eel liver of multiple forms of Ang II-R, which may have different functions, while one single form appeared to be present in enterocyte plasma membrane and in renal brush border membrane.
J R Puddefoot, V A Baker, B Bakkers, S Marsigliante, S Barker, C Panahy, A W Goode, R Carpenter, and G P Vinson
Oestrogen receptors (ERs) in breast tumours are highly heterogeneous. In previous studies we have shown that at least four isoforms may exist. These migrate in isoelectric focusing (IEF) gels to isoelectric points (pI values) 6·1, 6·3, 6·6 and 6·8. Of these the first (pI 6·1) corresponds to the 8S isoform as detected by sucrose gradient fractionation, while the others all sediment at 4S. In a series of 66 breast tumours it was found that those at pI 6·3 and pI 6·8 were significantly correlated with the presence of progesterone receptors.
To characterize the isoforms more fully, ER isoforms labelled by [3H]oestradiol binding were fractionated by IEF. The results were compared with those obtained after sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting using the H222 anti-ER monoclonal antibody. In other experiments, tumour ER isoforms were covalently labelled with [ring-3H] tamoxifen aziridine and separated by IEF. The individual isoforms were electroeluted from the IEF gel and further analysed by SDS-PAGE and non-denaturing PAGE. In summary, the evidence shows that the isoforms of pI values 6·3, 6·8 and 6·6 have molecular masses of 50, 65 and 70 kDa respectively. In addition, all three of these isoforms, i.e. the pI 6·3, 6·8 and 6·6 isoforms, could form dimers.
We conclude that the three isoforms sedimenting at 4S have the capacity to form dimers and thus may have the potential for binding to oestrogen response elements in the genome.
S Barker, W Marchant, M M Ho, J R Puddefoot, J P Hinson, A J L Clark, and G P Vinson
We have generated hybridomas which secrete monoclonal antibodies to the AT1 subtype of the angiotensin II receptor (AT1 receptor). These were obtained after immunization of Balb C/c mice with synthetic peptides representing sequences from either the extracellular domain (residues 8-17) or the intracellular domain (residues 229-237) of the AT1 receptor.
Hybridoma populations were first screened for the production of antibodies which bound to rat liver cells. Further selection, and cloning by limiting dilution, was carried out for antibodies which bound specifically to rat adrenal glomerulosa cells. Confirmation that the antibody designated 6313/G2 interacted with the angiotensin II receptor was obtained using COS-7 cells transfected with AT1A receptor cDNA. In particular, the initial characterization of 6313/G2 showed specific immunofluorescence of vascular endothelium.