Interactions of alpha-melanotropin and agouti on B16 melanoma cells: evidence for inverse agonism of agouti

alpha-Melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) and agouti control the switch between eumelanin and pheomelanin synthesis in mammalian melanocytes. Here we investigated interactions between alpha-MSH, agouti protein, cAMP elevating agents and phorbol ester on mouse B16 melanoma cells. Agouti (Kd 3.7 nmol/l) and alpha-MSH (Kd 2.3 nmol/l) had similar affinities to the MC1 melanocortin receptor. Both alpha-MSH and agouti induced MC1 receptor down-regulation. Agouti antagonized melanogenesis induced by alpha-MSH, forskolin, cholera toxin (CT), and pertussis toxin (PT). It also reduced the constitutive melanin formation of long-term cultures. Cell proliferation was inhibited by agouti (43% at 100 nM). This effect was reversed by alpha-MSH, forskolin, or CT. B16-G4F cells, a cell variant that lacks the MC1 receptor, did not respond to agouti. From these results we conclude that agouti shows the characteristics of an inverse agonist acting through the MC1 receptor.

Induction of constitutive melanogenesis in amelanotic mouse melanoma cells by transfection of the human melanocortin-1 receptor gene

The human melanocortin-1 (MC1) receptor was stably expressed in the amelanotic mouse melanoma cell clone B16-G4F which does not express its own (mouse) MC1 receptor and hence is unresponsive to alpha melanocyte stimulating hormone (alpha MSH). From several stable transfectant cell lines expressing the human MC1 receptor in relatively high numbers, three melanin producing clones (G4F-12, 14, and 15) and one amelanotic clone (G4F-7) were further analyzed in competition binding experiments and in cAMP and melanin assays. The dissociation constants (KD) for [Nle4, D-Phe7]-alpha MSH in all four clones ranged from 0.187 to 0.705 nmol/l, thus corresponding to the KD observed with the different human melanoma cell lines so far studied. Intracellular cAMP content was 3- to 5-fold higher than that of control cells, and alpha MSH induced an additional 1.5- to 1.7-fold increase. G4F-15 cells secreted melanin into the medium whereas the other clones did not secrete melanin. The extent of melanin secretion was similar to that of fully alpha MSH-stimulated B16-F1 mouse melanoma cells but the onset of secretion was delayed. alpha MSH induced an additional dose-related increase (up to 1.3-fold) in melanin production which could be suppressed by the addition of specific alpha MSH antibodies without altering the constitutive part of melanogenesis. Human and mouse agouti proteins, which inhibit basal and alpha MSH-induced melanogenesis in B16-F1 cells, both reduced alpha MSH-induced melanin production in G4F-15 cells but did not affect the constitutive melanogenesis. These results indicate that human MC1 receptor expressed in mouse B16-G4F cells induces constitutive activation of the signalling pathway controlling melanogenesis, most likely by tightly coupling to Gs alpha, in a similar manner to that reported for constitutively active receptor mutants in other systems.

Characterization of melanocortin receptor subtype expression in murine adipose tissues and in the 3T3-L1 cell line

It has been known for many years that adipocytes express high affinity ACTH and alpha-melanocyte stimulating hormone (MSH) binding sites, and that ACTH, alpha-MSH, and beta-lipotropin are potent lipolytic hormones. We show here that the adipocyte response to the melanocortin peptides results from the expression of both the MC2 (ACTH) receptor as well as the newly discovered MC5 receptor. Using RT-PCR and Northern blot hybridization, high levels of MC2 receptor messenger RNA (mRNA) were found in all adipose tissues examined in the mouse, whereas MC5 receptor mRNA was found in a subset of these. Both receptors mRNAs were also found in the 3T3-L1 cell line but only after the cells had been induced to differentiate into adipocytes. This cell line was then used to characterize the pharmacological properties of the MC2 and MC5 receptor sites in situ. The MC2 receptor exhibits properties similar to the ACTH receptor characterized in adrenocortical cells, coupling to activation of adenylyl cyclase with an EC50 of approximately 1 nM. An MSH binding site characterized in these cells is presumably the MC5 receptor, based on the observation that this is the only other melanocortin receptor mRNA detected in these cells. The MC5 receptor in the 3T3-L1 adipocyte activated adenylyl cyclase in response to alpha-MSH stimulation. Interestingly, Nle4, D-Phe7-alpha-MSH (NDP-MSH), a commonly used synthetic alpha-MSH agonist, was a potent antagonist of the MC5 receptor expressed in the 3T3-L1 cell line. Although the agouti signaling peptide is a potent antagonist of NDP-MSH binding to the MC1 and MC4 melanocortin receptors, agouti was unable to block NDP-MSH binding in the 3T3-L1 adipocyte.

Transformation of an irreversible MSH antagonist into an irreversible MSH agonist by differential receptor crosslinking using the photo-affinity technique

Photocrosslinking of receptors for alpha-melanocyte-stimulating hormone (alpha-MSH) on melanophores of frogs and lizards has been shown to induce long-lasting receptor stimulation whereby the photoreactive alpha-MSH may contain one or two photolabels in positions 1, 7, 9, or 13. The chemical synthesis and biological testing of an alpha-MSH analogue is now described which contains three photoreactive groups in positions 1, 9 and 13, one of which with a cleavable S-S disulphide bridge: [ApSSpr-Ser1, Trp(Naps)9 Pap13]-alpha-MSH. Photocrosslinking of MSH receptors on melanophores of Anolis carolinensis with this analogue led to almost complete receptor blockade which could be transformed into long-lasting receptor stimulation by exposure to a thiol reagent. By contrast, the analogue containing only two photoreactive groups in positions 9 and 13, [Trp(Naps)9, Pap13]-alpha-MSH, produced long-lasting receptor stimulation which was not altered by the thiol reagent. These results demonstrate that one and the same peptide ligand may contain structural information for both receptor activation and inhibition and that the receptor may become arrested in an activated or inhibited state by multiple photocrosslinking, depending on the relative positions of these crosslinks.

Adrenaline induces hyperpolarization in frog pituitary melanotrophs through activation of potassium channels

A patch-clamp study was conducted on cultured frog pituitary melanotrophs, in order to investigate the effects of adrenaline on the electrical activity of these cells. In the whole-cell configuration, adrenaline (1 microM) caused hyperpolarization that was accompanied by a fall in membrane input resistance and a blockage of spontaneous action potentials. Under voltage clamp, adrenaline elicited a net-outward current. The hyperpolarization became undetectable at a command voltage of -100 mV which corresponded to the equilibrium potential of potassium ions. The effect of adrenaline on membrane potential and spontaneous activity was blocked by the alpha 2-adrenergic receptor antagonist yohimbine (1-10 microM) but could not be mimicked by the alpha 2-adrenergic agonist clonidine (1-10 microM). In the cell-attached configuration, exposure of the extra-patch membrane to adrenaline increased the occurrence of single-channel currents with a slope conductance of 100 pS. The deduced reversal potential of these currents corresponded to the equilibrium potential of potassium ions. These results suggest that frog melanotrophs display an alpha 2-adrenergic receptor subtype coupled to potassium channels involved in hyperpolarization.

The protein-phosphatase inhibitor okadaic acid mimics MSH-induced and melatonin-reversible melanosome dispersion in Xenopus laevis melanophores

The present study describes the ability of 315 nM okadaic acid to induce melanosome dispersion in cultured Xenopus laevis melanophores. This effect of okadaic acid is similar to that of a-melanocyte stimulating hormone (MSH) and can be reversed by melatonin treatment; it indicates that a member of the protein-phosphatase 1 or 2A families must be active for maintenance of the aggregated state. Higher concentrations of okadaic acid (1 microM) attenuate the response of Xenopus melanophores to melatonin leading to the hypothesis that melatonin action is mediated by the calcium/calmodulin activated phosphatase 2B. This hypothesis seems unlikely, however, since the calcium/calmodulin inhibitors TFP and W7 do not prevent melatonin-induced pigment aggregation, but instead induce aggregation on their own.

Alpha-melanocyte-stimulating hormone exhibits target cell selectivity in its capacity to affect interleukin 1-inducible responses in vivo and in vitro

The ability of i.v.-administered recombinant human interleukin 1 (IL 1 beta) to increase core body temperature, stimulate an increased production of serum amyloid P substance, and augment blood levels of circulating neutrophils in mice was inhibited in a dosage-dependent manner by administration of the neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH). alpha-MSH administration was also capable of inhibiting the capacity of i.v.-administered IL 1 beta to enhance plasma levels of corticosterone and to depress the generation and/or elicitation of contact hypersensitivity responses to skin-reactive chemicals. An analog of alpha-MSH (Nle4, D-Phe7 alpha-MSH), known to be more potent than native alpha-MSH in a number of melanotropin-sensitive systems, was determined to be more active than alpha-MSH in the modification of these same in vivo responses. Neither alpha-MSH nor its analog were capable of altering the capacity of IL 1 to stimulate increased plasma levels in prostaglandin E2 (PGE2). In vitro, neither alpha-MSH nor its analog were capable of reducing the capacity of IL 1 to stimulate fibroblast production of PGE2 or to augment the proliferation of murine thymocytes exposed to phytohemagglutinin. The apparent selectivity associated with the regulatory influences of alpha-MSH on IL 1-induced responses in vivo suggests that this neuropeptide may function as an endogenous inhibitor of certain immunomodulatory and inflammatory activities of the cytokine IL 1.

Action of pro-opiomelanocortin products on the rat vas deferens

Behavioral study of pro-opiomelanocortin products indicates that beta-endorphin and corticotrophin-like peptides have antagonistic effects. However, these peptides have similar actions on the rat vas deferens. beta-endorphin, alpha-MSH and ACTH each inhibit electrically evoked contraction of the duct, but the corticotrophin derived peptides are tenfold more potent on a molar basis (ED50 = 9 nM). Pharmacological analysis shows that the action of corticotrophin-derived peptides does not involve an opiate receptor mechanism. The results are discussed in terms of the central action of the peptides.

GABAA and GABAB receptors on porcine pars intermedia cells in primary culture: functional role in modulating peptide release

A primary culture of porcine pars intermedia cells with particularly high yields has been developed. The cells, grown in monolayers, secrete the pro-opiomelanocortin-derived peptide alpha-melanocyte-stimulating hormone over several weeks. The patch-clamp technique has been used to demonstrate the presence of gamma-aminobutyrateA (GABAA) receptors on the cells. GABA or the selective GABAA receptor agonist isoguvacine produced a depolarizing increase in chloride conductance that desensitized rapidly. The response was antagonized by bicuculline and by the aminopyridazine derivative of GABA (SR 95103), a novel GABAA receptor antagonist. The effects of specific agonists for each receptor were tested on peptide release from cells maintained in a perfusion system. Isoguvacine (10 microM) potentiated Ba2+-evoked release of alpha-melanocyte-stimulating hormone, whereas (-)-baclofen (50 microM) decreased both basal and stimulated hormone release. This negative effect on peptide secretion was reproduced when GABA (50 microM) was perfused in the presence of bicuculline (10 microM) to block GABAA receptor activation. The possible mechanisms underlying these GABAA and GABAB effects on stimulus-secretion coupling in this neuroendocrine model are discussed.

Absence of inhibitory dopaminergic control of the rabbit pituitary gland intermediate lobe

No immunoreactive axons were detected with an antiserum against tyrosine hydroxylase in the rabbit intermediate lobe (IL), which thus appears to be devoid of dopaminergic (DA) innervation. Dopamine and its agonists, which classically inhibit alpha-MSH release have no inhibitory effects on rabbit IL superfused in vitro but, paradoxically, stimulate alpha-MSH release. D2 type DA receptors, known to mediate inhibitory control of dopamine on melanotropic cells, and detectable by their affinity for (3H)-spiroperidol, were as previously reported absent from the rabbit IL. The absence of (3H)-spiroperidol binding sites in the IL was further confirmed on rabbit pituitary sections by radioautography. The mechanism of DA stimulation is still not clear, but might be tentatively explained by interference with other receptors involved in the stimulation of the gland. The lack of DA inhibitory control over the rabbit IL is an exception among the species so far studied.

Naloxone prevents the analgesic action of alpha-MSH in mice

alpha-MSH (0.1, 1, 10 micrograms) was administered intracerebroventricularly and its action on pain sensitivity was investigated by the hot-plate method in mice. alpha-MSH produced dose-dependent analgesia and this analgesic effect was prevented by naloxone (1 mg/kg, s.c.). It is possible that alpha-MSH may play a role in the mechanism of pain through endogeneous opioid systems.

Enzymological studies of melanotropins

The relative stability of natural melanotropins and related synthetic analogues to serum and purified proteolytic enzymes was studied. Both alpha- and beta-MSH were rapidly inactivated by frog serum, but much more slowly by rat serum. beta-MSH was more stable than alpha-MSH to serum inactivation. Both alpha- and beta-MSH were rapidly inactivated by alpha-chymotrypsin and trypsin. The synthetic analogues, [Nle4, D-Phe7]-alpha-MSH and [Cys4, Cys10]-alpha-MSH, were totally resistant to inactivation by frog and rat serum enzymes. [Nle4, D-Phe7]-alpha-MSH was resistant to inactivation by alpha-chymotrypsin and trypsin, whereas [Cys4, Cys10]-alpha-MSH was partially resistant to these enzymes under similar conditions. Melanotropin analogues resistant to inactivation by serum enzymes may prove useful in a variety of physiological studies wherein natural melanotropins would be rapidly inactivated.

Characterization of melanin-concentrating hormone in chum salmon pituitaries

Many lower vertebrates exhibit colour change in response to the background. A dual hormonal control of colour change by two antagonistic pituitary melanophorotropic hormones was first postulated in amphibia by Hogben and Slome. It is well established that the melanotropins alpha- and beta-MSH are responsible for pigment dispersion in the integumentary melanophore of lower vertebrates and that these molecules are derived from a common precursor protein, proopiocortin, by specific processing within the intermediate lobe. No evidence has been found for an antagonistic hormone in amphibia, although the existence of such a molecule in the pituitary gland of teleost fishes has long been recognized and was termed the melanophore-concentrating hormone by Enami. Early attempts to separate the two hormones proved unsuccessful. Recently, Baker and Ball re-invoked the dual hormone concept, and it has been suggested that a melanin-concentrating hormone (MCH) is synthesized in the hypothalamus of teleosts and stored and released by the neurohyphophysis. We have now isolated a novel peptide from the pituitary of the salmon (Oncorhynchus keta) possessing an antagonistic function to MSH, and we describe here its chemical and biological characteristics.

Effects of actinomycin D and cycloheximide on the differentiation of epidermal melanocytes of newborn mice

The increase in the number of epidermal melanocytes positive to the dopa reaction after birth was suppressed in C57BL/10J strain mice when injected with actinomycin D or cycloheximide. On the other hand, no change was observed in the melanoblast-melanocyte population positive to the combined dopa-premelanin reaction. The initiation of melanogenesis in the skin of newborn mice, therefore, seems to require de novo transcription and translation. The increase in the number of melanocytes after treatment with alpha-MSH or DBc-AMP was also completely suppressed when mice were injected with actinomycin D or cycloheximide. The results suggest that normal and hormone-induced differentiation of epidermal melanocytes requires de novo transcription and translation.

[Functional state of the sympathetic-adrenal system in melanocyte-stimulating hormone deficiency under the effect of the melanocyte-inhibiting factor of the hypothalamus]

Fluorometry was applied to the study of the content of a number of amines (DOPA), adrenalin, noradrenaline) in the adrenal glands and various structures of the rat brain in endogenous deficiency of melanocyte-stimulating hormone of the hypophysis caused by parenteral administration of synthetic melanostatin (MIF) of the hypothalamus. The amines content in the adrenal glands medulla, and also in the medulla oblongata; the cerebellum, and the large hemispheres proved to change 30 to 60 min after MIF administration. The results obtained indicated that various links of the sympathico-adrenal system could take part in the mechanism of MIF action on the hypophysis.

Effect of phorbol ester tumor promoters on the expression of melanogenesis in B-16 melanoma cells

Cells of the C3 clone of B-16 melanoma synthesize melanin only at confluence after which they senesce and can no longer be passaged. Addition to the cultures of 10(-8)--10(-7) M 12-O-tetradecanoylphorbol-13-acetate (TPA) shortly after plating delayed by about 2 days the onset of melanogenesis. TPA did not, however, affect the growth of the cells or the time at which they reached confluence. The ability of a series of phorbol esters to delay melanogenesis correlated with their tumor-promoting activity on mouse skin. The optimum time for addition of TPA was within the first 24 hr after plating; the inhibitory effect decreased when TPA was added at later points. alpha-melanocyte-stimulating hormone (5 x 10(-7) M) added to B-16 cultures 24 hr after plating slowed the growth of the cells and caused them to differentiate when still subconfluent. TPA also inhibited this alpha-melanocyte-stimulating hormone-induced melanogenesis. These results suggest that TPA inhibits a very early stage in a stepwise process that leads to the differentiation of these cultures. For reasons that are not apparent, the cells eventually escape from this inhibition. The B-16 melanoma cell culture system may be useful for studying the mechanism by which TPA and related tumor promoters affect cellular differentiation.

The effect of polyphloretin phosphate on the aqueous flare response to alpha-melanocyte stimulating hormone

The breakdown of the blood aqueous barrier caused by topical prostaglandin E1 (PGE1), prostaglandin E2 (PGE2) or subcutaneous alpha-melanocyte stimulating hormone (alpha-MSH) was quantified by measurements of the aqueous flare seen in the anterior chamber. Polyphloretin phosphate (PPP) administration subcutaneously was found to effectively block the protein leakage caused by all three traumatic stimuli. The same dose of PPP given intravenously inhibited effectively the flare response to PGE1 and alpha-MSH, whereas the effect of PGE2 was only slightly decreased. Significant inhibition by subconjunctival PPP was not achieved for any of the three stimuli. Assuming that PPP is a specific PG-antagonist the present results support the eariler suggestion that PGs take part in the barrier damaging action of alpha-MSH. However, it cannot be excluded that PPP acts on a step subsequent to PG. This step might be common to PGs- and alpha-MSH-effects on the barrier, explaining why PPP inhibits both types of trauma.

Unusual alpha adrenergic receptor potency of methyldopa metabolites on melanocyte function

Catecholamines possessing alpha adrenergic receptor agonist properties induce lightening or reverse melanocyte stimulating hormone darkening of frog skin in vitro. The capacity to activate this alpha receptor by the methyldopa metabolites methyldopamine and methylnorepinephrine was compared with the capacity of the naturally occurring dopa metabolites, dopamine and norepinephrine. Melanocyte stimulating hormone-induced darkening or dispersion of the granules was reversed by each of these metabolites. Methylnorepinephrine was 10 times as potent as norepinephrine, and methyldopamine was 30- to 100-fold more potent than the naturally occurring dopamine. These inhibitory effects on melanocyte stimulating hormone could be blocked or partially impaired using the alpha adrenergic blocker, phentolamine. They were not affected by pretreatment of frogs with the monoamine oxidase inhibitor pheniprazine (Catron) nor by the application of pheniprazine, angiotensin or serotonin in vitro. This neuroendocrine model has alpha adrenergic receptor relationships analogous to those described in the central nervous system for methyldopa metabolites.

Some questions related to melanocyte-stimulating hormone

A number of questions remain unsettled about the release of melanocyte-stimulating hormone (MSH) and about its function. Even though relatively few investigators are studying this area, some generalities have emerged during the last 10 years. It now seems that release of MSH from the pituitary is inhibited by a substance present in the hypothalamus. The structure of this physiologic inhibitor of MSH release may still not be considered an established entity but there is evidence for additional mechanisms capable of exerting a fine control on the release of MSH. Contrary to some opinions, the release of MSH does not always occur together with the release of ACTH, and the release of the two hormones can be dissociated in several laboratory and clinical situations. In addition, many studies have shown that the pituitary peptide, MSH, exerts behavioral and electroencephalographic effects in both the rat and man. The hypothalamic peptide Pro-Leu-Gly-NH2 (MIF-I) also has direct effects on the central nervous system that may include alleviation of the symptoms of Parkinson's disease.

Effects of hypothalamic extracts, neurotransmitters and synthetic hypothalamic releasing hormones on adenylyl cyclase activity in the lobes of the pituitary of the dogfish (Scyliorhinus canicula L.)

Neutralized acid extracts of the median eminence of the dogfish hypothalamus were found to cause a dose-related activation of adenylyl cyclase in all lobes of the dogfish pituitary. Equal concentrations of extracts of extrahypothalamic areas of the dogfish brain did not activate the enzyme. The putative neurotransmitters melatonin, serotonin, adrenaline, noradrenaline, dopamine and acetylcholine were without effect, as were the prostaglandins E1 and E2. The effects of synthetic mammalian hypothalamic hormones were also studied. Both thyrotrophin releasing hormone and gonadotrophin releasing hormone activated the ventral lobe enzyme, but had no effect on the adenylyl cyclase of the other three lobes. The tripeptide, Pro-Leu-Gly-NH2, a possible melanocyte-stimulating hormone release-inhibiting factor had no effect on the enzyme of the neurointermediate lobe. It is suggested that all four lobes of the dogfish pituitary may be under hypothalamic control and that this control is likely to be mediated by peptide hormones, as in mammals.