Effects of suramin on hormone release by cultured rat anterior pituitary cells

Modulation of the growth hormone-insulin-like growth factor (GH-IGF) axis by pharmaceutical, nutraceutical and environmental xenobiotics: an emerging role for xenobiotic-metabolizing enzymes and the transcription factors regulating their expression. A review

The growth hormone-insulin-like growth factor (GH-IGF) axis has gained considerable focus over recent years. One cause of this increased interest is due to a correlation of age-related decline in plasma GH/IGF levels with age-related degenerative processes, and it has led to the prescribing of GH replacement therapy by some practitioners. On the other hand, however, research has also focused on the pro-carcinogenic effects of high GH-IGF levels, providing strong impetus for finding regimes that reduce its activity. Whereas the effects of GH/IGF activity on the action of xenobiotic-metabolizing enzyme systems is reasonably well appreciated, the effects of xenobiotic exposure on the GH-IGF axis has not received substantial review. Relevant xenobiotics are derived from pharmaceutical, nutraceutical and environmental exposure, and many of the mechanisms involved are highly complex in nature, not easily predictable from existing in vitro tests and do not always predict well from in vivo animal models. After a review of the human and animal in vivo and in vitro literature, a framework for considering the different levels of direct and indirect modulation by xenobiotics is developed herein, and areas that still require further investigation are highlighted, i.e. the actions of common endocrine disruptors such as pesticides and phytoestrogens, as well as the role of xenobiotic-metabolizing enzymes and the transcription factors regulating their expression. It is anticipated that a fuller appreciation of the existing human paradigms for GH-IGF axis modulation gained through this review may help explain some of the variation in levels of plasma IGF-1 and its binding proteins in the population, aid in the prescription of particular dietary regimens to certain individuals such as those with particular medical conditions, guide the direction of long-term drug/nutraceutical safety trials, and stimulate ideas for future research. It also serves to warn athletes that using compounds touted as performance enhancing because they promote short-term GH release could in fact be detrimental to performance in the long-run.

Identification and characterization of a selective, nonpeptide follicle-stimulating hormone receptor antagonist

The glycoprotein hormones (LH, FSH, and TSH) are critical to the maintenance of physiological homeostasis and control of reproduction. However, despite an obvious utility for synthetic pharmacological agents, there are few reports of selective, nonpeptide agonists or antagonists to receptors for these hormones. We have identified and characterized a novel synthetic molecule capable of inhibiting the action of FSH. This compound, 7-[4-[Bis-(2-carbamoyl-ethyl)-amino]-6-chloro-(1,3,5)-triazin-2-ylamino)-4-hydroxy-3-(4-methoxy-phenylazo)-naphthalene]-2-sulfonic acid, sodium salt (compound 1), is a selective, noncompetitive inhibitor of the human (h) and rat (r) FSH receptors (FSHRs). Compound 1 selectively inhibited binding of [(125)I]hFSH with an IC(50) value of 5.4 +/- 2.3 micro M. Radioligand-binding assays were performed using the baculovirus expressed extracellular domain of hFSHR (BV-tFSHR) to demonstrate site-specific interaction. Compound 1 competed for [(125)I]hFSH binding to BV-tFSHR with an IC(50) value of 10 +/- 2.8 micro M. Functionally, compound 1 inhibited hFSH-induced cAMP accumulation and steroidogenesis in vitro with an IC(50) value of 3 +/- 0.6 micro M. Competition of compound 1 for binding to other glycoprotein hormone receptors and other G protein-coupled receptors demonstrated select activity for FHSRs. Compound 1 inhibited ovulation in immature and cycling adult rats. These data provide proof of concept that selective, small molecule antagonists can be designed for glycoprotein hormone receptors.

Mineralocorticoid insufficiency due to suramin therapy

BACKGROUND

During a Phase I trial of suramin, a novel antineoplastic agent with activity against hormone-refractory prostate carcinoma, the authors observed two patients with clinical mineralocorticoid insufficiency in spite of hydrocortisone replacement therapy.

METHODS

The authors retrospectively assessed adrenal cortical function in 20 such patients via adrenocorticotropic stimulation testing, measuring both cortisol and aldosterone responses, either at the time or treatment of immediately after discontinuation of treatment.

RESULTS

Two of 9 patients (22%) treated at relatively low dose levels (< or = 1200 mg/m2 on Day 1) demonstrated adrenal cortical insufficiency, as compared with 9 of 11 patients (32%) treated with relatively high doses (> 1200 mg/m2 on Day 1) (P = 0.03 by 1-tailed Fisher's exact test). There appeared to be a cumulative dose-response relationship to the development of glucocorticoid insufficiency, with no instances being observed at doses < 4.8 g/m2 and uniform toxicity occurring at doses > 7.6 g/m2. Long term glucocorticoid insufficiency was present in 1 of 5 patients (20%) tested at an interval of > 90 days after discontinuation of suramin treatment. All instances of glucocorticoid insufficiency were associated with mineralocorticoid insufficiency. Suramin did not affect the absorption or excretion of exogenously administered glucocorticoid in one patient.

CONCLUSIONS

Suramin causes both primary mineralocorticoid and primary glucocorticoid insufficiency. This may occur in a dose-dependent manner. Long term glucocorticoid insufficiency appears to occur in a minority of patients treated with low doses of suramin. Patients receiving high doses of suramin for treatment of advanced carcinoma should receive at least physiologic replacement doses of both mineralocorticoid and glucocorticoid. Higher doses of glucocorticoid may be required in selected patients.

Characterization of ATP receptor which mediates norepinephrine release in PC12 cells

PC12 cells, a rat pheochromocytoma cell line, has been reported to release norepinephrine in response to extracellular ATP in the presence of extracellular Ca2+. The potency order of ATP analogues was adenosine 5'-O-(3-thiotriphosphate) greater than ATP greater than adenosine 5'-O-(1-thiotriphosphate) = 2-methylthioadenosine 5'-triphosphate (MeSATP) greater than 2'- and 3'-O-(4-benzoyl-benzoyl)ATP (BzATP) greater than ADP greater than 5-adenylylimidodiphosphate. Adenosine 5'-O-(2-thiodiphosphate), beta, gamma-methyleneadenosine 5'-triphosphate, AMP and adenosine were inactive. The ATP action in the absence of extracellular Ca2+, suggests a small but appreciable contribution of intracellular Ca2+ mobilization, for norepinephrine release. However, for some ATP derivatives, like BzATP, almost no contribution of the phospholipase C-Ca2+ pathway is suggested, based on their low activity in inositol phosphates production. To identify the ATP-receptor protein, PC12 cell membranes were photoaffinity-labeled with [32P]BzATP. SDS-PAGE analysis showed that a 53-kDa protein labeling was inhibited by ATP and its derivatives, as well as by P2-antagonists, suramin and reactive blue 2, which inhibit the nucleotide-induced norepinephrine release. The inhibitory activity of the nucleotides was, in parallel with their potency, to induce norepinephrine release. Despite their inability to release norepinephrine, GTP and GTP gamma S inhibited the BzATP labeling, suggesting the participation of a putative G protein in the ATP-receptor-mediated actions. We suggest that the 53-kDa protein on the PC12 cell surface is an ATP receptor, which mediates the norepinephrine release, depending, mainly, on extracellular Ca2+ gating.

Discrimination between two types of P2 purinoceptors by suramin in rat hepatocytes

Suramin, currently reported as a P2 purinoceptor antagonist, competitively inhibited P2 purinoceptor agonist-induced phospholipase C (PLC) stimulation, but not P2 purinoceptor agonist-induced adenylate cyclase (AC) inhibition in rat hepatocytes. Suramin did not inhibit vasopressin-induced PLC activity. We conclude that there are two types of P2 purinoceptors; one is suramin-sensitive and coupled to PLC in a stimulatory manner, and the other is suramin-insensitive and coupled to AC in an inhibitory manner.

Extracellular ATP stimulates three different receptor-signal transduction systems in FRTL-5 thyroid cells. Activation of phospholipase C, and inhibition and activation of adenylate cyclase

In FRTL-5 thyroid cells, extracellular ATP, a P2-agonist, not only stimulates phospholipase C but also inhibits forskolin- or thyrotropin (TSH)-induced stimulation of adenylate cyclase in a pertussis toxin-sensitive manner [Okajima, Sato, Nazarea, Sho, & Kondo (1989) J. Biol. Chem. 264, 13029-13037]. We have now found that, in pertussis toxin-treated cells, ATP can directly stimulate adenylate cyclase. Although adenylate cyclase modulation occurs through ATP metabolites such as AMP and adenosine, we show that extracellular ATP itself also regulates cyclic AMP production, based on the following: (1) the actions of ATP were imitated by hydrolysis-resistant ATP analogues, (2) the elimination of adenosine by adenosine deaminase decreased the effect of ATP only partially, at least at concentrations greater than 10 microM-ATP, and (3) the amount of AMP produced from ATP was too low to account for the ATP effects. To identify the respective receptors for the three different actions of ATP, we established an antagonist profile. Suramin, which has been reported to be a P2-receptor antagonist, inhibited ATP-induced phospholipase C activation in a competitive fashion, but did not affect ATP-induced adenylate cyclase modulation. On the other hand, 8-cyclopentyl-1,3-diphenylxanthine competitively antagonized both the stimulatory and inhibitory ATP actions on cyclic AMP levels, but did not influence the activation of phospholipase C by ATP. The order of potency for various xanthine derivatives was clearly different with respect to their antagonistic effects on the stimulation and inhibition of adenylate cyclase induced by ATP. We conclude that ATP activates three receptors, each of which is coupled to a different signal transduction system in FRTL-5 cells, i.e. phospholipase C activation, and adenylate cyclase activation and inhibition.

Suramin inhibits gonadotropin action in rat testis: implications for treatment of advanced prostate cancer

Suramin is being evaluated for the treatment of metastatic prostate cancer based on its inhibition of growth factor action. In addition, suramin may inhibit the endocrine control of androgen production, which was explored herein. Adult Sprague-Dawley rats were injected (i.p.) daily with varying doses of suramin. At a cumulative dose of 200 mg., suramin significantly depressed serum testosterone (p less than 0.05), and follicle stimulating hormone (p less than 0.002) levels. In vitro studies showed that suramin-mediated suppression of androgen production might be secondary to inhibition of gonadotropin action. In MA-10 cell cultures, suramin inhibited a maximum stimulatory dose of human chorionic gonadotropin with an ED50 of 4.4 microM. Studies in rat Sertoli cell cultures showed that follicle stimulating hormone action was also inhibited by suramin, with an ED50 of 8.6 microM. Using receptor binding assays with calf testis membrane, we showed that suramin inhibited 125I-hFSH binding to receptor in a dose dependent fashion with an ED50 of 10.4 microM; comparable to the ED50 of suramin inhibition of follicle stimulating hormone action in Sertoli cell culture cells. Thus the mechanism of suramin's suppression of androgen production may involve multiple sites of action, including inhibition of gonadotropin binding to its receptor and suppression of pituitary gonadotropin levels in serum. This inhibition of androgen production may be useful in the treatment of advanced prostate cancer.