Distinct Calcium Signaling for Wildtype, Loss-of-Function and Gain-of-Function Human MC4R Variants

There is compelling evidence for human melanocortin-4-receptor (hMC4R) playing a critical function regulating energy balance; yet signal transduction pathways contributing to this are unclear. The hMC4R activates multiple signaling pathways, including induced increases in cAMP and mobilization of intracellular calcium ([Ca2+]i). Recent evidence showed cAMP signaling was not a good predictor for hMC4R variant-associated obesity. We hypothesize that hMC4R mobilization of [Ca2+]i plays an important role in regulating energy balance. To test this, we developed a robust high-throughput Fura-2 ratiometric fluorescent assay to quantitatively measure [Ca2+]i in vitro. We compared basal and α-melanocyte stimulating hormone (α-MSH) activation of [Ca2+]i for hMC4R-wildtype (WT) and hMC4R-variants stably expressed in HEK293 cells. The loss-of-function variants studied were two obesity-associated variants (R7H and R18L) known to exhibit cAMP signaling similar to WT, two obesity-associated variants (H76R and L250Q) known to exhibit cAMP-constitutive activity (CA) compared to WT, and one overweight-associated variant (H158R) known to exhibit cAMP-CA compared to WT. The gain-of-function variants (V103I and I251L) studied are known to exhibit cAMP signaling similar to WT. The data for basal [Ca2+]i were pooled from three independent experiments performed with WT and all variants in each assay. Data (mean ± SEM) were analyzed using one-way ANOVA with Dunnett’s multiple comparisons. The data (mean ± SEM) for α-MSH activation of hMC4R were pooled from three independent experiments and analyzed using non-parametric sum of squares F-test for maximum best-fit values and EC50. The α-MSH activated assays were performed with each hMC4R variant alongside WT. WT hMC4R and non-CA loss-of-function variants exhibited similar basal and α-MSH activated [Ca2+]i (WT: EC50 = 1.44 nM; R7H: EC50 = 1.40 nM; R18L: EC50 = 1.12 nM). The CA loss-of-function variants exhibited significantly (p < 0.0001) increased basal [Ca2+]i compared with WT (WT = 97.6 ± 0.9 nM; H76R = 114.2 ± 1.7 nM; L250Q = 112.1 ± 2.6 nM; H158R = 110.7 ± 1.8 nM) and significantly lower EC50’s compared with WT (H76R: EC50 = 0.07 nM; p = 0.0019; L250Q: EC50 = 0.09 nM; p = 0.0066; H158R: EC50 = 0.14 nM; p = 0.0009). The gain-of-function hMC4R variants exhibited significantly (p < 0.0001) decreased basal [Ca2+]i compared with WT (WT = 97.6 ± 0.9 nM; V103I: = 86.4 ± 0.9 nM; I251L = 87.5 ± 1.0 nM) and significantly (p = 0.0001) increased α-MSH stimulated maximum [Ca2+]i compared with WT (WT = 224.5 ± 13.6 nM; V103I = 288.2 ± 31.5 nM; I251L = 295.6 ± 20.0 nM). To summarize, we show three distinct patterns of hMC4R-associated calcium signaling; (1) WT and non-CA loss-of-function, (2) CA loss-of-function and (3) non-CA gain-of-function. Future studies are required to understand how hMC4R mobilization of [Ca2+]i might contribute to the regulation of energy balance.

Quantitative high-throughput assay to measure MC4R-induced intracellular calcium

The melanocortin-4 receptor (MC4R), a critical G-protein-coupled receptor (GPCR) regulating energy homeostasis, activates multiple signalling pathways, including mobilisation of intracellular calcium ([Ca2+]i). However, very little is known about the physiological significance of MC4R-induced [Ca2+]i since few studies measure MC4R-induced [Ca2+]i. High-throughput, read-out assays for [Ca2+]i have proven unreliable for overexpressed GPCRs like MC4R, which exhibit low sensitivity mobilising [Ca2+]i. Therefore, we developed, optimised, and validated a robust quantitative high-throughput assay using Fura-2 ratio-metric calcium dye and HEK293 cells stably transfected with MC4R. The quantitation enables direct comparisons between assays and even between different research laboratories. Assay conditions were optimised step-by-step to eliminate interference from stretch-activated receptor increases in [Ca2+]i and to maximise ligand-activated MC4R-induced [Ca2+]i. Calcium imaging was performed using a PheraStar FS multi-well plate reader. Probenecid, included in the buffers to prevent extrusion of Fura-2 dye from cells, was found to interfere with the EGTA-chelation of calcium, required to determine Rmin for quantitation of [Ca2+]i. Therefore, we developed a method to determine Rmin in specific wells without probenecid, which was run in parallel with each assay. The validation of the assay was shown by reproducible α-melanocyte-stimulating hormone (α-MSH) concentration-dependent activation of the stably expressed human MC4R (hMC4R) and mouse MC4R (mMC4R), inducing increases in [Ca2+]i, for three independent experiments. This robust, reproducible, high-throughput assay that quantitatively measures MC4R-induced mobilisation of [Ca2+]i in vitro has potential to advance the development of therapeutic drugs and understanding of MC4R signalling associated with human obesity.

SAT-598 Shared Signaling Profile Between Human MRAPα-Induced Human MC4R Constitutive Activity and Obesity-Associated Human MC4R Constitutive Activity

The human melanocortin 4 receptor (hMC4R) plays a critical role in the regulation of energy balance with more than 150 distinct human obesity-associated mutations. Most exhibit defective MC4R functionality but six have been reported to associate with constitutive activity. This represents a conundrum since a lean phenotype is expected for enhanced MC4R signaling. Human melanocortin 2 receptor accessory protein alpha (hMRAPα) induces hMC4R constitutive activity in transfected HEK293 cells (1,2). We do not know whether the hMRAPα-induced gain-in-function for hMC4R would cause, or prevent, obesity because of this conundrum. Here, we hypothesize that wild-type hMC4R, obesity-associated constitutively active hMC4R and hMRAPα-induced constitutive active hMC4R can exist in distinct conformational states and elicit distinct signaling profiles. To test this, we compared transiently expressed HA-hMC4R in HEK293 cells for basal and agonist activation for adenylyl cyclase, Cre driven β-galactosidase reporter transcription, and receptor protein expression. Six previously reported obesity-associated hMC4R constitutively active variants were compared with two hMC4R constitutively active mutations not associated with obesity, two hMC4R variants associated with protection from development of obesity, five non-constitutively active hMC4R mutations associated with obesity, hMRAPα co-expressed with hMC4R, and wild-type hMC4R. Our data confirm hMC4R constitutive activity coupling to both adenylyl cyclase and Cre β-galactosidase reporter for only two hMC4R variants associated with obesity (H76R & L250Q), one hMC4R mutation (H158R) not associated with obesity, and hMRAPα co-expressed with hMC4R. We show α-MSH stimulated concentration curves for wild-type hMC4R, H76R, L250Q & H158R hMC4R variants and hMRAPα co-expressed with hMC4R coupling to adenylyl cyclase. Surprisingly, out of these, only wild-type hMC4R and H158R hMC4R variant exhibited α-MSH-stimulated Cre β-galactosidase reporter concentration curves. Western blotting and ELISA showed ~70% reduced cell surface and total receptor protein expression for hMC4R co-expressed with hMRAPα and obesity-associated constitutively active hMC4R variants, compared to wild-type hMC4R. To summarize, two constitutively active hMC4R variants (H76R and L250Q) associated with obesity, and hMC4R co-expressed with hMRAPα, share a signaling profile comprising protein expression and α-MSH stimulated functional coupling to adenylyl cyclase and Cre-reporter gene expression. We conclude (1) if hMC4R is co-expressed with hMRAPα in vivo it would likely contribute to human obesity, and (2) obesity-associated constitutively active hMC4R variants exhibit a signaling anomaly that may underpin development of anti-obesity therapeutics.

1. Kay EI, et al. J Mol Endocrinol. 2013;50:203-215.

2. Kay EI, et al. PLoS ONE. 2015;10(10):e0140320.

Pro-Opiomelanocortin (POMC) Neurones, POMC-Derived Peptides, Melanocortin Receptors and Obesity: How Understanding of this System has Changed Over the Last Decade

Following the cloning of the melanocortin receptor and agouti protein genes, a model was developed for the central melanocortin system with respect to the regulation of energy and glucose homeostasis. This model comprised leptin regulation of melanocortin peptides and agouti-related peptide (AgRP) produced from central pro-opiomelanocortin (POMC) and AgRP neurones, respectively, as well as AgRP competitive antagonism of melanocortin peptides activating melanocortin 4 receptor (MC4R) to Gαs and the cAMP signalling pathway. In the last decade, there have been paradigm shifts in our understanding of the central melanocortin system as a result of the application of advanced new technologies, including Cre-LoxP transgenic mouse technology, pharmacogenetics and optogenetics. During this period, our understanding of G protein coupled receptor signal transduction has also dramatically changed, such that these receptors are now known to exist in the plasma membrane oscillating between various inactive and active conformational states, and the active states signal through G protein-dependent and G protein-independent pathways. The present review focuses on evidence obtained over the past decade that has changed our understanding of POMC gene expression and regulation in the central nervous system, POMC and AgRP neuronal circuitry, neuroanatomical functions of melanocortin receptors, melanocortin 3 receptor (MC3R) and MC4R, and signal transduction through MC3R and MC4R.

Long-term alterations in body weight do not affect the expression of melanocortin receptor-3 and -4 mRNA in the ovine hypothalamus

The pro-opiomelanocortin-derived peptides and the melanocortin receptors are implicated in various functions within the CNS including the regulation of food intake. In the present study, we used in situ hybridization, with specific 35S-labelled ovine riboprobes to map the expression of melanocortin receptor-3 (MC3-R) and -4 (MC4-R) mRNA in the diencephalon and brainstem of normal female sheep. Furthermore, we examined the effect of long-term alterations in energy balance on the distribution and expression of MC3-R and MC4-R mRNA in food-restricted and ad libitum-fed ovariectomized female sheep. The distribution of melanocortin receptors generally resembled that of the rat. A high number of MC3-R-labelled cells were seen in the ventral division of the lateral septum and the medial preoptic area. In the hypothalamus, a moderate number of MC3-R-labelled cells was observed in the lateral hypothalamic area while other nuclear groups had low to intermediate numbers of MC3-R-labelled cells. The distribution of MC4-R mRNA was generally similar to that of MC3-R mRNA in the septal/preoptic and hypothalamic regions, with a high number of labelled cells present in the intermediate division of the lateral septum. Within the hypothalamus, no MC4-R mRNA expression was observed in the arcuate nucleus. There was more widespread distribution of moderate to low numbers of MC4-R mRNA-expressing cells in the brainstem compared to that of MC3-R mRNA. Unlike findings in the rat, only a low number of cells expressed melanocortin receptor mRNA in the ovine hypothalamic nuclei associated with feeding behavior. The number of melanocortin receptor-labelled cells and the level of expression (silver grains/cell) in the hypothalamic feeding centers was similar in food-restricted and ad libitum-fed animals. These findings suggest that long-term alterations in metabolic status do not change the melanocortin receptor mRNA distribution and/or expression in the sheep hypothalamus.

Mouse melanocortin-4 receptor gene 5'-flanking region imparts cell specific expression in vitro

Weight homeostasis is exquisitely sensitive to changes in the abundance of melanocortin-4 receptor (MC4-R). To begin to understand the factors that regulate MC4-R gene expression, we determined there are no introns in the gene, there are multiple starts of transcription, and a cluster of 3' ends. A series of MC4-R-luciferase gene reporter chimerics was developed and transfected into cell lines expressing (UMR106; GT1-7; HEK293) and not expressing (Neuro 2A) endogenous MC4-R mRNA. The longest construct, which includes approximately 3.3 kb 5'-flanking, 425 bp 5'-untranslated (UTR) and 1852 bp 3'-flanking, significantly increased luciferase reporter gene expression 24-, 13-, and 3-fold compared to pGL3-basic when expressed in HEK293, UMR106, and GT1-7 cells, respectively. Deletion analysis of mMC4-R 5'-flanking cDNA identified full mMC4-R promoter activity within 178 bp upstream of the major start of transcription. The mMC4-R gene structure and reporter chimerics provide a fundamental framework for the identification of specific factors regulating MC4-R gene expression.

Differential expression of cocaine- and amphetamine-regulated transcript and agouti related-protein in chronically food-restricted sheep

Recently, much attention has focused on the role of the melanocortin system in the regulation of energy homeostasis, especially the satiety effects of the pro-opiomelanocortin (POMC)-derived peptide alpha-melanocyte stimulating hormone (alpha-MSH). We have found that POMC mRNA levels are similar in fat and thin sheep and the current study sought to further characterize the effects of nutritional status on the melanocortin system. To this end, we studied the expression of agouti-related peptide (AGRP) (an endogenous antagonist of alpha-MSH) and cocaine- and amphetamine-regulated transcript (CART), which is co-localized within POMC cells of the arcuate nucleus (ARC) in rodents. Twelve ovariectomized ewes were randomly divided into two groups and fed a maintenance (n=6) or restricted diet (n=6). At the time of experimentation, the animals had significantly (P<0.0001) different bodyweights (53.4+/-2.2 kg, ad libitum vs. 30.4+/-1.2 kg, food-restricted), which was largely due to altered body fat deposits. In situ hybridization was used to study the expression of POMC, AGRP and CART. The expression of POMC in the ARC was similar in ad libitum and food-restricted animals but the expression of AGRP was profoundly increased in the food-restricted group. The expression of CART was abundant throughout the hypothalamus but was not found in the ARC. In food-restricted animals, the expression of CART was lower in the retrochiasmatic nucleus (P<0.01), paraventricular nucleus (P<0.001), the dorsomedial nucleus and the lateral hypothalamic area (P<0.05), but was higher (P<0.01) in the posterior hypothalamic area. Thus, long-term changes in nutritional status have profound effects on the expression of AGRP and CART in the hypothalamus.

Melanocortin receptor-mediated mobilization of intracellular free calcium in HEK293 cells

Mouse melanocortin receptors, MC1-R, MC3-R, MC4-R, and MC5-R, when expressed in HEK293 cells and stimulated with either alpha-melanocyte-stimulating hormone (alpha-MSH) or desacetyl-alpha-MSH, mediate increases in intracellular free calcium concentration ([Ca(2+)](i)) with EC(50) values between 0.3 and 4.3 nM. The increase in [Ca(2+)](i) is cholera toxin sensitive and pertussis toxin insensitive. The mechanism involves calcium mobilization from intracellular stores without a transient rise in inositol trisphosphate. Mouse agouti protein (55 nM) is a competitive antagonist of alpha-MSH (6-fold) and desacetyl-alpha-MSH (8-fold), coupling the mMC1-R to increased [Ca(2+)](i). Agouti protein (55 nM) significantly increased the EC(50) for alpha-MSH (3-fold), and 550 nM agouti protein significantly increased the EC(50) for desacetyl-alpha-MSH (4-fold), coupling the mMC4-R to a rise in [Ca(2+)](i). However, agouti protein antagonism of the MC4-R may not be competitive since there was a trend for the maximum response to also increase. There was no significant antagonism of the MC3-R and MC5-R by agouti protein (55 nM). Understanding the physiological relevance of the transduction of a calcium signal by melanocortin peptides may be important for future development of therapeutic targeting of the melanocortin receptors.

Physiological consequences of ectopic agouti gene expression: the yellow obese mouse syndrome

This review summarizes primary and downstream phenotypic manifestations, with emphasis on altered responsiveness to environmental stimuli, of dominant yellow mutations at the mouse agouti locus. Obvious effects include hyperinsulinemia, obesity, stimulation of somatic growth and tumorigenesis, and coat color. Downstream influences of hyperinsulinemia and obesity on the individual's physiology determine important components of the obese yellow agouti mouse syndrome. Collectively, the phenotypic aberrations described support the concept that identical genomes are expressed in a spectrum of physiological phenotypes that reflect the complex interdependence of gene-regulated physiological pathways and processes in the organism throughout extended, but temporally ordered, periods of fetal and neonatal development and aging. This summary identifies important areas for additional research and provides integrated information required for a systematic approach to the development of interventions for common adult human health problems.

Agouti antagonism of melanocortin-4 receptor: greater effect with desacetyl-alpha-melanocyte-stimulating hormone (MSH) than with alpha-MSH

Desacetyl-alpha-MSH is more abundant than alpha-MSH in the brain, the fetus, human blood, and amniotic fluid, but there is little information on its ability to interact with melanocortin receptors. The aim of this study is to compare and contrast the ability of desacetyl-alpha-MSH and alpha-MSH to couple melanocortin receptors stably expressed in HEK293 cells, to the protein kinase A (PKA) signaling pathway. Desacetyl-alpha-MSH activated mouse MC1, MC3, MC4 and MC5 receptors with EC50s = 0.13, 0.96, 0.53, and 0.84 nM, and alpha-MSH activated these receptors with EC50s = 0.17, 0.88, 1.05, and 1.34 nM, respectively. Mouse agouti protein competitively antagonized alpha-MSH and desacetyl-alpha-MSH coupling to the MC1-R similarly. In contrast, mouse agouti protein antagonized desacetyl-alpha-MSH much more effectively and potently than alpha-MSH coupling the MC4-R to the PKA signaling pathway. Furthermore, mouse agouti protein (10 nM) significantly reduced (1.4-fold) the maximum response of mMC4-R to desacetyl-alpha-MSH and 100 nM mouse agouti significantly increased (4.8-fold) the EC50. Minimal antagonism of alpha-MSH coupling mMC4-R to the PKA signaling pathway was observed with 10 nM mouse agouti, whereas both 50 and 100 nM mouse agouti appeared to reduce the maximum reponse (1.1- and 1.3-fold, respectively) and increase the EC50 (2.5- and 3.4-fold respectively). Mouse agouti protein did not significantly antagonize either alpha-MSH or desacetyl-alpha-MSH coupling mouse MC3 and MC5 receptors. Understanding the similarities and differences in activation of melanocortin receptors by desacetyl-alpha-MSH and alpha-MSH will contribute to delineating the functional roles for these endogenous melanocortin peptides.

Thalidomide increases both intra-tumoural tumour necrosis factor-alpha production and anti-tumour activity in response to 5,6-dimethylxanthenone-4-acetic acid

5,6-Dimethylxanthenone-4-acetic acid (DMXAA), synthesized in this laboratory and currently in phase I clinical trial, is a low molecular weight inducer of tumour necrosis factor-alpha (TNF-alpha). Administration of DMXAA to mice with established transplantable tumours elicits rapid vascular collapse selectively in the tumour, followed by extensive haemorrhagic necrosis mediated primarily through the production of TNF-alpha. In this report we have investigated the synthesis of TNF-alpha mRNA in hepatic, splenic and tumour tissue. Co-administration of thalidomide with DMXAA increased anti-tumour activity and increased intra-tumoural TNF-alpha production approximately tenfold over that obtained with DMXAA alone. Thalidomide increased splenic TNF-alpha production slightly but significantly decreased serum and hepatic levels of TNF-alpha induced with DMXAA. Lipopolysaccharide (LPS) induced 300-fold higher serum TNF-alpha than did DMXAA at the maximum tolerated dose, but induced similar amounts of TNF-alpha in spleen, liver and tumour. Splenic TNF-alpha activity induced with LPS was slightly increased with thalidomide, but serum and liver TNF-alpha levels were suppressed. Thalidomide did not increase intra-tumoural TNF-alpha production induced with LPS, in sharp contrast to that obtained with DMXAA. While thalidomide improved the anti-tumour response to DMXAA, it had no effect on the anti-tumour action of LPS that did not induce a significant growth delay or cures against the Colon 38 tumour. The increase in the anti-tumour action by thalidomide in combination with DMXAA corresponded to an increase in intra-tumoural TNF-alpha production. Co-administration of thalidomide may represent a novel approach to improving selective intra-tumoural TNF-alpha production and anti-tumour efficacy of DMXAA.

Melanocortin-4 receptor messenger RNA expression is up-regulated in the non-damaged striatum following unilateral hypoxic-ischaemic brain injury

Melanocortin peptides (alpha-melanocyte-stimulating hormone, adrenocorticotropin and fragments thereof) have been shown to have numerous effects on the central nervous system, including recovery from nerve injury and retention of learned behaviour, but the mechanism of action of these peptides is unknown. A family of five melanocortin receptors have recently been discovered, two of which (melanocortin-3 and melanocortin-4 receptors) have been mapped in the rat brain. We have tested the hypothesis that the expression of one or more of the messenger RNAs for three melanocortin receptors (melanocortin-3, melanocortin-4 and melanocortin-5 receptors) would be altered in rat brain following unilateral transient hypoxic-ischaemic brain injury. In this study, using in situ hybridization, we show that melanocortin-4 receptor messenger RNA was up-regulated in the striatum in the non-damaged hemisphere within 24 h after severe hypoxic-ischaemic injury compared with control brains (P<0.05). In a small group of animals, this induction was not blocked by treatment with the anticonvulsant, carbamazepine. Expression of melanocortin-3 receptor messenger RNA in the brain was not altered in this hypoxic-ischaemic injury model and melanocortin-5 receptor messenger RNA was not detected in either control or hypoxic-ischaemic injured rat brains. We hypothesize that the up-regulation of melanocortin-4 receptor messenger RNA expression in the contralateral striatum may be involved in transfer of function to the uninjured hemisphere following unilateral brain injury.

Stimulation of tumors to synthesize tumor necrosis factor-alpha in situ using 5,6-dimethylxanthenone-4-acetic acid: a novel approach to cancer therapy

The selective induction of tumor vascular collapse represents an exciting approach to cancer treatment. However, clinical evaluation of tumor necrosis factor-alpha (TNF), an agent that accomplishes this goal, has been limited by systemic toxicity, and clinical approaches using bacterial components to induce TNF production have also been disappointing. Our laboratory has developed synthetic low molecular weight inducers of TNF, including 5,6-dimethylxanthenone-4-acetic acid (DMXAA), as an alternative strategy. DMXAA induces rapid vascular collapse in transplantable murine tumors and induces TNF synthesis in vitro in both murine and human systems. We show here that the extent of DMXAA-induced TNF synthesis is greater in tumors than that in the spleen, liver, or serum. As shown by in situ hybridization studies of the murine Colon 38 tumor, DMXAA induced tumor as well as host cells to express TNF mRNA. The distribution of cells containing TNF mRNA in tumor tissues after DMXAA administration contrasted significantly with that obtained after lipopolysaccharide (LPS) treatment, although splenic and hepatic tissues showed a similar distribution of TNF mRNA-positive cells. In the Colon 38 tumor, the action of LPS was limited to host cells in the periphery of the vessels. DMXAA treatment induced 7-fold higher peak TNF levels in tumor than in serum. In contrast, LPS treatment induced 9-fold higher TNF levels in serum than in tumor. DMXAA induced 35-fold higher TNF activity in the Colon 38 tissue than did LPS. One ovarian, one squamous, and three melanoma human tumor xenografts implanted in athymic nude mice expressed TNF mRNA of human and murine origin in response to DMXAA, confirming that DMXAA can activate both host and tumor cells. The use of low molecular weight agents to induce TNF synthesis in situ in the tumor represents a novel approach to TNF-mediated therapy of cancers.

Melanocortin-4 receptor mRNA expression in the developing autonomic and central nervous systems

MC4-R mRNA expression was investigated in fetal rats (E14-E20) using in situ hybridisation. The autonomic nervous system showed the highest levels of MC4-R mRNA expression. In the spinal cord, dense signal was seen over the future intermediolateral cell column and dorsal horn. In the brain, MC4-R was expressed by E14 in diencephalon neuroepithelia, telencephalon, lamina terminalis and spinal trigeminal nucleus and was expressed by E19 throughout many regions of the brain.

Melanin-concentrating hormone: a functional melanocortin antagonist in the hypothalamus

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) demonstrate opposite actions on skin coloration in teleost fish. Both peptides are present in the mammalian brain, although their specific physiological roles remain largely unknown. In this study, we examined the interactions between MCH and alpha-MSH after intracerebroventricular administration in rats. MCH increased food intake in a dose-dependent manner and lowered plasma glucocorticoid levels through a mechanism involving ACTH. In contrast, alpha-MSH decreased food intake and increased glucocorticoid levels. MCH, at a twofold molar excess, antagonized both actions of alpha-MSH. alpha-MSH, at a threefold molar excess, blocked the orexigenic properties of MCH. MCH did not block alpha-MSH binding or the ability of alpha-MSH to induce cAMP in cells expressing either the MC3 or MC4 receptor, the principal brain alpha-MSH receptor subtypes. These data suggest that MCH and alpha-MSH exert opposing and antagonistic influences on feeding behavior and the stress response and may function in a coordinate manner to regulate metabolism through a novel mechanism mediated in part by an MCH receptor.

Melanocortin receptor binding determinants in the agouti protein

The agouti protein plays an important role in the development of diabetes and obesity in rodents and has been shown to be a potent antagonist of melanocortin receptors. For this reason alanine-scanning mutagenesis was performed on the agouti protein carboxyl terminus to locate residues important for melanocortin receptor binding inhibition. When agouti residues Arg116 and Phe118 are changed to alanine, very large decreases in agouti affinity for melanocortin receptor 1, 3, and 4 result. Mutation of Phe117 to alanine causes a similar increase in agouti KI app at melanocortin receptor 4. Substitution of agouti residue Asp108 with alanine results in large increases in KI app for all three melanocortin receptors examined. All of these residues are conserved in the agouti-related transcript, ART, whose expression is up-regulated in animal models of obesity. The three-dimensional structure of the agouti carboxyl terminus was modeled, and residues which decrease receptor binding by a factor of > or = 15 when mutated to alanine localize to one side of the structure. These agouti variants with altered receptor selectivity may be useful in determining the role of melanocortin receptors in diabetes and obesity.

Obesity, diabetes and functions for proopiomelanocortin-derived peptides

Melanocortin peptides (adrenocorticotropin (ACTH), alpha-,beta-, and gamma-melanocyte stimulating hormone (MSH), and fragments thereof) derived from proopiomelanocortin (POMC) have a diverse array of biological activities, many of which have yet to be fully elucidated. The recent cloning of a family of five distinct melanocortin receptors through which these peptides act has provided the tools to further our understanding of melanocortin peptide functions. Early work on melanocortin peptides focused on their roles in pigmentation, adrenocortical function, the immune, central and peripheral nervous systems. Although melanocortin peptides have long been known to affect lipolysis, characterisation of the melanocortin receptors has opened up several lines of evidence for important roles in the development of obesity, insulin resistance and type II diabetes. We present here a review of the current evidence for melanocortin peptides playing such a role, and based on this evidence, a model for melanocortin peptides and their receptors in maintaining energy balance.

Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain

POMC, the precursor of ACTH, MSH, and beta-endorphin peptides, is expressed in the pituitary and in two sites in the brain, in the arcuate nucleus of the hypothalamus and the commissural nucleus of the solitary tract of the brain stem. Little is known regarding the functions of melanocortin (ACTH and MSH) peptides in the brain. We report here the detailed neuroanatomical distribution of the MC4-R mRNA in the adult rat brain. The melanocortin 3 receptor (MC3-R), characterized previously, was found to be expressed in arcuate nucleus neurons and in a subset of their presumptive terminal fields but in few regions of the brainstem. The highly conserved MC4-R is much more widely expressed than MC3-R and is pharmacologically distinct. MC4-R mRNA was found in multiple sites in virtually every brain region, including the cortex, thalamus, hypothalamus, brainstem, and spinal cord. Unlike the MC3-R, MC4-R mRNA is found in both parvicellular and magnocellular neurons of the paraventricular nucleus of the hypothalamus, suggesting a role in the central control of pituitary function. MC4-R is also unique in its expression in numerous cortical and brainstem nuclei. Together, MC3-R and/or MC-4R mRNA are found in every nucleus reported to bind MSH in the adult rat brain and define neuronal circuitry known to be involved in the control of diverse neuroendocrine and autonomic functions. The high degree of conservation, distinct pharmacology, and unique neuronal distribution of the MC4 receptor suggest specific and complex roles for the melanocortin peptides in neuroendocrine and autonomic control.

Mapping of the ACTH, MSH, and neural (MC3 and MC4) melanocortin receptors in the mouse and human

The melanocortin peptides regulate a wide variety of physiological processes, including pigmentation and glucocorticoid production, and also have several activities in the central and peripheral nervous systems. The melanocortin receptor family includes the melanocyte-stimulating hormone receptor (MSH-R), adrenocorticotropic hormone receptor (ACTH-R), and two neural receptors, MC3-R and MC4-R. In the human these receptors map to 16q24 (MSH-R), 18p11.2 (ACTH-R), 20q13.2 (MC3-R), and 18q22 (MC4-R). The corresponding locations in the mouse are 8, 18, and 2; a variant for mapping MC4-R has not yet been identified. The data reported here also show that the neural MC3 receptor maps close to a disease locus for benign neonatal epilepsy in human and near the E1-2 epilepsy susceptibility locus in the mouse.

The human melanocyte stimulating hormone receptor has evolved to become "super-sensitive" to melanocortin peptides

Melanocyte-stimulating hormone (MSH) stimulates pigmentation in mammals by activating specific cell surface MSH receptors (MC1-Rs) on melanocytes. MC1-Rs on normal human melanocytes have been difficult to detect and characterise. The pharmacological characterisation of a cloned human MC1-R (hMC1-R) is reported here, and directly compared with that of a cloned mouse MC1-R (mMC1-R). The human and mouse MC1-Rs are equally sensitive (EC50 = 1-2 pM) to the super potent analogue of alpha-MSH, NDP-MSH. In contrast with the mMC1-R, the hMC1-R is also very sensitive to alpha-MSH (EC50 = 2 pM), ACTH (EC50 = 8 pM), and Lys gamma 3-MSH (EC50 < 10(-10) M). This suggests that in man, in contrast with rodents, both ACTH and Lys gamma 3-MSH may have physiological roles in pigmentation.

ACTH induces up-regulation of ACTH receptor mRNA in mouse and human adrenocortical cell lines

Corticotropin (ACTH) binds to specific receptors in the adrenal cortex and thereby regulates glucocorticoid and mineralocorticoid production. The number of ACTH binding sites on adrenocortical cells is increased by exposure of cells to activators of the cAMP pathway. The mechanism responsible for the increase in ACTH binding sites is not known. We therefore studied the levels of ACTH-R mRNA in mouse Y-1 and human NCI-H295 (H295) adrenocortical carcinoma cell lines. ACTH induced an increase in mouse ACTH-R mRNA in Y-1 cells that was time and dose dependent, increasing 6-fold over basal levels following exposure to 10(-8) M ACTH for 19-24 h. The amount of human ACTH-R mRNA in H295 cells increased 2-4-fold following a 24 h exposure to 10(-8) M ACTH, 1 mM dbcAMP, or 10(-5) M Forskolin. Treatment of H295 cells with angiotensin II (A-II) was found to dramatically increase the level of ACTH-R mRNA. These data indicate that regulation of ACTH-R mRNA levels is at least one mechanism by which ACTH and A-II elevate the number of ACTH binding sites in the adrenocortical cells.

Identification of a receptor for gamma melanotropin and other proopiomelanocortin peptides in the hypothalamus and limbic system

Corticotropin (ACTH) and melanotropin (MSH) peptides (melanocortins) are produced not only in the pituitary but also in the brain, with highest concentrations in the arcuate nucleus of the hypothalamus and the commisural nucleus of the solitary tract. We have identified a receptor for MSH and ACTH peptides that is specifically expressed in regions of the hypothalamus and limbic system. This melanocortin receptor (MC3-R) is found in neurons of the arcuate nucleus known to express proopiomelanocortin (POMC) and in a subset of the nuclei to which these neurons send projections. The MC3-R is 43% identical to the MSH receptor present in melanocytes and is strongly coupled to adenylyl cyclase. Unlike the MSH or ACTH receptors, MC3-R is potently activated by gamma-MSH peptides, POMC products that were named for their amino acid homology with alpha- and beta-MSH, but lack melanotropic activity. The primary biological role of the gamma-MSH peptides is not yet understood. The location and properties of this receptor provide a pharmacological basis for the action of POMC peptides produced in the brain and possibly a specific physiological role for gamma-MSH.

Molecular genetics of the ACTH and melanocyte-stimulating hormone receptors

The related ACTH and melanocyte-stimulating hormone (MSH) receptors control adrenal steroidogenesis and pigmentation in response to an overlapping set of peptides derived from the proopiomelanocortin (POMC) molecule. The recent cloning of these receptors has already opened up a new understanding of their role in normal and pathologic functioning of the adrenal cortex, and of the process of pigmentation. The murine MSH receptor maps to a genetic locus called extension, a locus known since early in this century to control the relative amounts of the two major types of melanins: eumelanin and phaeomelanin. The highly variable pigmentation phenotypes resulting from different extension locus alleles are caused by structural mutations in the MSH receptor that alter the degree of its signal-transducing capacity. A mutation in the ACTH receptor in a patient with ACTH resistance has also recently been reported. It is likely that the etiology of this rare disease includes mutations that affect the functioning of the ACTH receptor.

Pigmentation phenotypes of variant extension locus alleles result from point mutations that alter MSH receptor function

Coat colors in the chestnut horse, the yellow Labrador retriever, the red fox, and one type of yellow mouse are due to recessive alleles at the extension locus. Similarly, dominant alleles at this locus are often responsible for dark coat colors in mammals, such as the melanic form of the leopard, Panthera pardus. We show here that the murine extension locus encodes the melanocyte-stimulating hormone (MSH) receptor. In mice, the recessive yellow allele (e) results from a frameshift that produces a prematurely terminated, nonfunctioning receptor. The sombre (Eso and Eso-3J) and tobacco darkening (Etob) alleles, which both have dominant melanizing effects, results from point mutations that produce hyperactive MSH receptors. The Eso-3J receptor is constitutively activated, while the Etob receptor remains hormone responsive and produces a greater activation of its effector, adenylyl cyclase, than does the wild-type allele.

The cloning of a family of genes that encode the melanocortin receptors

Melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) regulate pigmentation and adrenal cortical function, respectively. These peptides also have a variety of biological activities in other areas, including the brain, the pituitary, and the immune system. A complete understanding of the biological activities of these hormones requires the isolation and characterization of their corresponding receptors. The murine and human MSH receptors (MSH-Rs) and a human ACTH receptor (ACTH-R) were cloned. These receptors define a subfamily of receptors coupled to guanine nucleotide-binding proteins that may include the cannabinoid receptor.

Effect of insulin receptor down regulation on insulin-stimulated thymidine incorporation in cultured human fibroblasts and tumor cell lines

Insulin receptors in transformed tissue are relatively resistant to down regulation by insulin, and although receptor downregulation reduces rapid onset biologic responses to insulin in normal tissue, this is not observed in tumor cells. The present study compares longterm insulin responses (thymidine incorporation and cell growth) in normal human fibroblasts with responses in human tumor cell lines (MCF-7, T-47D and HCT-8) to determine whether these responses are also resistant to the effects of receptor down regulation. Thymidine incorporation into fibroblasts was more responsive to insulin than was incorporation into tumor cells, although stimulation of uptake into fibroblasts was not paralleled by changes in cell replication. In contrast, physiological insulin concentrations inhibited, and high concentrations of insulin stimulated, thymidine incorporation and cell replication in MCF-7 and T-47D cells. All insulin concentrations inhibited thymidine incorporation in HCT-8 cells without affecting cell replication. The responsiveness of fibroblasts, MCF-7 and HCT-8 cells to insulin was unaltered by down regulation of insulin receptors prior to measuring thymidine incorporation, whereas receptor down regulation paradoxically increased the responsiveness of T-47D cells to insulin. Exposure of fibroblasts to 5 x 10(-8) M dexamethasone for 24h increased their responsiveness to insulin but did not influence the response of MCF-7 or HCT-8 cells, whereas insulin-stimulated incorporation of thymidine in T-47D cells was inhibited. Thus, receptor down regulation does not influence the longterm biologic response to insulin in normal cells, and paradoxically increases responsiveness in one of three tumor cell lines. These changes may contribute to the well-described stimulatory effects of insulin on tumor cell growth and inhibition of this response with dexamethasone may be relevant to cancer treatment programs.

Vanadate regulates glucose transporter (Glut-1) expression in NIH3T3 mouse fibroblasts

Vanadate, the major oxidized form of the essential trace element vanadium, has rapid effects on glucose transport in vitro and more delayed effects on glucose transport in vivo. We addressed the question that one potential mechanism for the delayed effects of vanadate on glucose homeostasis could be altered expression of one or more of the genes encoding glucose transporters. To do this we studied vanadate regulation of Glut-1 and Glut-4 in NIH3T3 mouse fibroblasts. Vanadate (5-40 microM) induced cells to proliferate to higher cell densities, and in addition, 40 microM vanadate caused the cells to exhibit a transformed morphology. Glut-1 mRNA was maximally induced 4- to 5-fold over the control value after 6-h exposure to 30 microM vanadate. Unlike the response to serum and growth factors, the vanadate-induced increase in Glut-1 mRNA remained elevated over the control value in the presence of vanadate for 5 days. The vanadate effect was serum dependent and was fully reversible when vanadate was removed from the medium. In the absence of vanadate, the half-life of Glut-1 mRNA was 0.5-1 h, whereas after treatment for 5 h with 30 microM vanadate the half-life was increased to 1.5-2 h. Thus, mRNA stabilization accounts for at least a part of the increase in glucose transporter mRNA levels after vanadate treatment. Glut-4 mRNA was not detected in these cells in either the absence or presence of vanadate. While the importance of this increased Glut-1 gene expression for the vanadate effect on normalization of blood glucose in vivo remains to be determined, an association between vanadate-induced cell proliferation and transformed phenotype, and vanadate-induced Glut-1 mRNA in vitro has been made. Possible potential therapeutic use of vanadate for treatment of diabetes must, therefore, be viewed with caution.

Loss of insulin binding and insulin receptor mRNA in a transformed human fetal fibroblast cell line

Insulin binding and insulin receptor gene expression have been assessed in cultured fetal (WI38) and SV40 transformed fetal (WI38/VA13) human fibroblasts to determine whether transformation influences the expression of insulin receptors. The transformed cell line had virtually no insulin binding and extremely low levels of insulin receptor mRNA. No apparent gene deletion or rearrangement was detected and therefore the marked decrease in insulin receptor gene expression seen in WI38/VA13 cells is an important example of negative regulation of insulin receptor gene expression. This cell line could serve as a model for studies of the mechanism for negative regulation of insulin receptor gene expression. Overexpression of the insulin receptor gene in these cells may reveal insights into the role of the insulin receptor in tumor biology.

Overproduction of the beta 1 form of protein kinase C enhances phorbol ester induction of glucose transporter mRNA

Phorbol esters bind to and activate a family of Protein Kinase C (PKC) proteins, although the degree to which the various PKC forms mediate specific biological functions is unknown. We and others have previously shown that, after exposure to phorbol esters, cultured fibroblasts exhibit increased expression of the mRNA encoding the HepG2/brain glucose transporter (GT mRNA). We therefore studied phorbol ester regulation of GT mRNA in rat fibroblasts which do (R6-PKC3) or do not (R6-C1) stably overproduce a full-length cDNA encoding the PKC beta 1 isotype. When PKC beta 1 is overproduced in a cell that normally has undetectable levels, it is capable of increasing HepG2/brain GT mRNA in response to alpha-D-glucose phorbol 12-myristate 13-acetate (TPA). The level of GT mRNA is maximally induced 6 to 8-fold over basal levels 6 h after exposure to TPA (10 ng/ml) in R6-PKC3 cells that overproduce PKC beta 1, but only 2-fold over basal levels in the control R6-C1 cells. This TPA induced increase in the level of GT mRNA was observed as early as 1 h, peaked by 6-8 h and decreased markedly by 24 h in both cell types. The effect of PKC beta 1 on GT mRNA expression is probably mediated through enhancement of transcription, since the stability of GT mRNA was only minimally affected by TPA. Unlike the enhancement of TPA induced GT mRNA expression caused by overexpression of PKC beta 1, the responses of GT mRNA to calf serum, platelet-derived growth factor, epidermal growth factor, insulin or insulin-like growth factor-1 were the same in both cell types. After pretreatment with 1000 ng/ml TPA in 0.5% calf serum for 24 h, PKC activity was down-regulated and both R6-C1 and R6-PKC3 cells showed complete down-regulation of the GT mRNA responses to an additional treatment with 1000 ng/ml TPA. In contrast to the marked loss of responsiveness to TPA and PKC down-regulation, the responses of GT mRNA to serum, PDGF, EGF, insulin and IGF-1 were unaffected by down-regulation. Thus, our results provide direct evidence for both PKC-dependent and independent pathways regulating GT gene expression. Furthermore, it appears that the level of PKC beta 1 production, rather than down-stream signal transduction events, is the rate-limiting step in the pathway by which TPA induces an increase in GT mRNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Measurement of nuclear oestrogen receptors in human breast tumours

Recent reports suggest that oestrogen receptors detected in the cytosolic fraction of homogenized human breast tumours might be mainly nuclear receptor released into the cytoplasm during tissue processing. This study thus compares the tumour content of steroid hormone receptors in conventional cytosolic receptor assays with direct measurements of receptor in the cell nucleus. Unoccupied cytoplasmic oestrogen receptors (cER), cytoplasmic progesterone receptors (cPR) and total (occupied plus unoccupied) nuclear oestrogen receptors (nER) have been measured in parallel in human breast tumour tissue using biochemical radioreceptor assay. Of 125 tumours, 62% and 61% were positive for cER and cPR, respectively, 50% contained nER with high affinity for oestradiol (nER I) and 13% expressed nER with low affinity for oestradiol (nER II). The concentration of cER correlated significantly with age, cPR and log nER I. A significant proportion of tumours which were positive for both cytosol receptors also possessed nuclear receptors with high and low affinity for oestrogen. It is possible that this group of tumours which are positive for cER, cPR, nER I and nER II will respond well to hormone therapy.

Abnormal insulin-receptor down regulation and dissociation of down regulation from insulin biological action in cultured human tumor cells

The sensitivity of insulin receptors to down regulation by insulin has been measured in cultured human tumor cells (breast tumor cell lines MCF-7, T-47D, and colon tumor cell line HCT-8). Insulin receptors on breast tumor cells were resistant to down regulation (15-17% maximum loss of insulin binding after 4 h exposure to 170 nM insulin). HCT-8 cells were sensitive to down regulation after 4 h exposure to 3.8 nM insulin, but the extent of down regulation then lessened at higher concentrations of insulin. This paradoxical behavior was associated with increasing affinity of insulin receptors for insulin following exposure to hormone. Insulin-stimulated [3H]leucine incorporation into protein was measured in parallel with studies of receptor regulation to assess the effect of preexposure of cells to insulin on cell metabolism. Maximum down regulation of receptors in all three types of tumor cell by prior exposure to insulin did not significantly alter the responsiveness of any of the cell lines to insulin. Thus insulin receptor down regulation is abnormal in these tumor lines compared with reported studies in normal cells, and this may contribute a metabolic advantage to these malignant cells over normal tissues.

Effects of metformin and glibenclamide on insulin receptors in fibroblasts and tumor cells in vitro

The effects of the oral hypoglycemic agents metformin and glibenclamide on receptor binding of insulin and insulin-induced receptor down-regulation were studied in cultured normal human fibroblasts, human breast tumors (cell lines MCF-7 and T-47D) and a human colon tumor (cell line HCT-8) in order to identify differences in receptor regulation between normal and transformed cells. Binding of 125I-insulin was not significantly altered in any cell type following 24-h exposure of cells to concentrations of these agents equivalent to the in vivo therapeutic range, either in the presence or absence of insulin. Glibenclamide (2 microM) and metformin (1-10 microM) induced a 13-28% reduction in insulin receptor down regulation in fibroblasts exposed to 1.7 x 10(-8)M-insulin, the loss of binding on exposure to insulin decreasing from 55% to 40-48%. Both agents induced a smaller percentage reduction in insulin-induced receptor down-regulation in HCT-8 cells exposed to 1.7 x 10(-8)M insulin for 4 h (26% binding loss reducing to 14-20% in the presence of drug) and in MCF-7 and T-47D cells exposed to 1.7 x 10(-7)M insulin for 4h (16% binding loss reducing to 10%). It remains to be shown whether these small differences in sensitivity of receptors in malignant and normal cells to regulation by insulin and oral hypoglycemic agents can be exploited clinically in cancer management programs.

Insulin receptor regulation in cultured human tumor cells

Insulin binding to monolayer cell cultures of human fibroblasts, human colon carcinoma (HCT-8, HT-29), human breast carcinoma (MCF-7, T-47D), and melanoma (MM-96) was measured using 125I-insulin. Binding was time and temperature dependent in all cell lines, and only one cell line (MM-96) degraded 125I-insulin. High-affinity insulin-binding sites (Kd = 1.4 X 10(-9) M to 0.4 X 10(-10) M) were detected in all cell lines, and insulin-binding capacity ranged from 0.6 to 14 fmol/10(6) cells. Receptor down-regulation was studied by exposing cells to increasing concentrations of unlabeled insulin, dissociating surface-bound insulin and measuring residual receptors by 125I-insulin uptake. Exposure of tumor cells to greater than 10(-6) M insulin for 2 hr at 37 degrees led to a decrease in the number of insulin binding sites in MM-96 and colon cell lines only, with maximum down-regulation ranging from 58% (MM-96) to 88% (HCT-8) receptor loss. The decrease in insulin binding was due to a decreased number of receptors per cell with no change in affinity. Monolayers exposed to 1.7 X 10(-5) M unlabeled insulin for 7 hr at 37 degrees invariably showed greater than 50% receptor loss. However, monolayers exposed to 1.7 X 10(-8) M unlabeled insulin for 7 hr at 37 degrees showed less marked (0 to 39%) down-regulation. In comparison, human fibroblasts showed 57% receptor loss after exposure to 3.5 X 10(-9) M unlabeled insulin for 7 hr. Thus, markedly supraphysiological concentrations of insulin are required to down-regulate insulin receptors in tumor cell lines compared with normal cells. This suggests a tumor-associated resistance to receptor down-regulation.

Influence of thyroidectomy and prolactin suppression on the growth of N-nitrosomethylurea-induced rat mammary carcinomas

Mammary carcinomas were induced in female Sprague-Dawley rats with N-nitrosomethylurea. Thyroidectomy increased the serum prolactin and reduced serum growth hormone levels of 17 rats without affecting tumor growth. Pergolide mesylate, 80 micrograms twice daily for 7 days, suppressed the serum prolactin of another 17 animals; seven of 17 tumors continued to grow, four became static, and six (35%) underwent partial regression. Treatment with pergolide mesylate plus thyroidectomy reduced both serum prolactin and growth hormone in all of 14 rats, caused regression of ten of the 14 tumors (71%), while two became static, and two continued to grow. Five of the ten regressions were complete. Only the combined thyroidectomy-pergolide treatment group showed a significant difference in posttreatment surface area compared with the controls (p less than 0.001). Ovine growth hormone, 40 micrograms/hr delivered by s.c. osmotic minipumps for 7 days, stimulated regrowth of six of seven tumors undergoing regression in response to thyroidectomy plus pergolide; the other one became static. Thyroxine, 2 micrograms/100 g body weight, stimulated regrowth of the tumors in another six thyroidectomized rats despite continued suppression of prolactin by pergolide. Thus, regression of N-nitrosomethylurea-induced mammary tumors produced by thyroidectomy plus pergolide is due to the combined suppression of circulating growth hormone and prolactin.

Investigation and management of symptomatic hyperprolactinaemia

Fifteen patients with infertility and hyperprolactinaemia have been investigated using tests of prolactin and LH secretion, and treated by prolactin suppression. In addition, 4 patients with hyperprolactinaemia not desiring fertility were also investigated. Of the total group, 16 had galactorrhoea and 15 had amenorrhoea. Pituitary tumours were present in 6 patients and 4 had pituitary microadenomas. Prolactin levels measured by both radioimmunoassay and radioreceptor assay were elevated before treatment and fell during therapy with bromoergocryptine (7.5 mg daily). Tests of prolactin release with TRH and chlorpromazine before treatment did not distinguish patients with functional hyperprolactinaemia from those with pituitary tumours. Basal plasma gonadotrophin concentrations were not elevated despite subnormal urinary oestrogen levels. The serum LH response to LRH was normal during hyperprolactinaemia, but LH release in response to oestrogen provocation was impaired in 14 of 17 patients. During prolactin suppression, mean oestrogen excretion rose significantly and the oestrogen provocation test became normal in all except 2 patients. Pregnancy occurred in all of the 15 patients desiring fertility. Abortion has occurred in 4 patients, all of whom are currently pregnant again. Nine pregnancies have reached term, with no complications from pituitary expansion. It appears that during hyperprolactinaemia there are defects in both positive and negative feedback of oestrogen on LH secretion, and that prolactin suppression in such patients is highly effective in restoring fertility.

Progesterone and oestrogen receptors in human breast cancer

Receptors for progesterone were found in 27% of 98 human breast tumours, and for oestrogen in 57% of 191 tumours. With one exception, progesterone receptors were found only in tumours which also contained oestrogen receptors. Levels of oestrogen receptor in positive tumours rose significantly with patient age whereas progesterone receptors were unchanged. Progesterone receptor levels were lower in lymph node metastases than in primary tumours, and oestrogen receptor levels were lower in large tumours (greater than 5 cm diameter) compared to small lesions. Receptor levels were not significantly correlated with circulating concentrations of either oestrogen or prolactin. The implications of receptor measurements in assessing hormone responsiveness of breast tumours are discussed.