Photoaffinity labelling of MSH receptors reveals a dual role of calcium in melanophore stimulation

Interaction of melanin-concentrating hormone (MCH), neuropeptide E-I (NEI), neuropeptide G-E (NGE), and alpha-MSH with melanocortin and MCH receptors on mouse B16 melanoma cells

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) are known to exhibit mostly functionally antagonistic, but in some cases agonistic activities, e.g., in pigment cells and in the brain. Neuropeptide E-I (NEI) displays functional MCH-antagonist and MSH-agonist activity in different behavioral paradigms; the role of neuropeptide G-E (NGE) is not known. This study addressed the question of possible molecular interactions between alpha-MSH, MCH and the MCH-precursor-derived peptides NEI and NGE at the level of the pigment cell MCH receptor subtype (MCH-Rpc) and the different melanocortin (MC) receptors. Radioreceptor assays using [125I]MCH, [125l]alpha-MSH and [125I]NEI as radioligands and bioassays were performed with MCI-R-positive and MC1-R-negative mouse B16 melanoma cells and with COS cells expressing the different MC receptors. The IC50s of alpha-MSH and NEI or NGE for [125I]MCH displacement from mouse MCH-Rpc were 80-fold and, respectively, >300-fold higher than that of MCH, and the IC50s for MCH and NEI or NGE for [125I]alpha-MSH displacement from mouse MC1-R were 50,000-fold and >200,000-fold higher than that of alpha-MSH. No high-affinity binding sites for NEI were detected on B16 melanoma cells and there was no significant displacement of [1251]alpha-MSH by MCH, NEI or NGE with MC3-R, MC4-R and MC5-R expressed in COS cells. At concentrations of 100 nM to 10 microM, however, MCH, NEI and NGE induced cAMP formation and melanin synthesis which could be blocked by agouti protein or inhibitors of adenylate cyclase or protein kinase A. This shows that mammalian MCH-precursor-derived peptides may mimic MSH signalling via MC1-R activation at relatively high, but physiologically still relevant concentrations, as e.g. found in autocrine/paracrine signalling mechanisms.

A microplate binding assay for the somatostatin type-2 receptor (SSTR2)

The clinical importance of somatostatin type-2 receptors (SSTR2) and the study of novel analogues of somatostatin such as OctreoScan or [Tyr3]-octreotide containing DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) as metal chelator led us to develop a methodology to monitor the expression of SSTR2 on tumours of pancreatic origin (e.g. rat AR4-2J cancer cells). Usual binding assay protocols using the commercial [125I][Tyr1]-somatostatin radioligand failed, even in the presence of a cocktail of protease inhibitors with a broad spectrum of activity, possibly due to the high susceptibility of this tracer to proteases expressed in pancreatic cells. We prepared our own radioligand [125I][Tyr2]-octreotide which was shown to be much more resistant to degradation after incubation with AR4-2J plasma membranes. As expected, the increased stability of [125I][Tyr3]-octreotide was associated with good binding to SSTR2. Addition of appropriate protease inhibitors further increased the specific binding of [125I][Tyr3]-octreotide to AR4-2J plasma membranes without affecting the stability of the tracer, suggesting that the protease inhibitors also protect the integrity of SSTR2. Optimal conditions (time, temperature, medium) were developed for a binding assay in 96-well plates using AR4-2J plasma membranes in order to make the assay suitable for high-throughput analysis. This protocol was the basis for studying the in vivo regulation of SSTR2 expression in AR4-2J cells implanted into scid mice after exposure to different compounds.

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.

Signaling mechanisms controlled by melanocortins in melanoma, lacrimal, and brain astroglial cells

Melanocortins appear to be involved as regulators in an ever growing number of physiological processes in cells and tissues of diverse functions. While such trends are apparent also in the case of other peptide hormones, it appears that melanocortin receptors can be regarded as unique among G-protein-linked receptors due to their special need for extracellular Ca2+ which may relate to some, yet undetermined selectivity of their actions. The physiological role that Ca2+ may be playing and the diverse signaling mechanisms regulated, as well as the nature of the cell-specific responses elicited in melanocortin-sensitive cells/tissues, have yet to be elucidated. Likewise, it will be of interest to establish the relationship of melanocortins to processes like growth and differentiation of cells, as well as to higher, more complex processes such as those regulated in the CNS.

Peptides containing multiple photolabels: a new tool for the analysis of ligand-receptor interactions. Reversible long-lasting stimulation and inhibition of MSH receptors by multiple photocrosslinks with alpha-MSH

Crosslinking of MSH receptors on melanophores of the lizard Anolis carolinensis with analogues of alpha-MSH containing a photoreactive group in position 1, 7, 9 or 13 leads to long-lasting receptor stimulation. Reversibility of this long-lasting stimulation is obtained by employing a disulfide-containing photoreactive group which can be cleaved from the receptor by thiol reagents [Ref. 3]. When two photoreactive groups are simultaneously present on the alpha-MSH molecule (e.g in positions 1 + 9; 1 + 13; 7 + 13, or 9 + 13), identical results were obtained and long-lasting receptor stimulation was not altered after cleavage of one single crosslink. alpha-MSH analogues with three photoreactive groups in positions 1 + 7 + 13 led to irreversible receptor stimulation whereas one compound with the photoreactive groups in positions 1 + 9 + 13 induced reversible receptor inactivation which could be changed into long-lasting stimulation by cleaving the crosslink at position 1 of alpha-MSH. 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.

Photoaffinity labelling of melanoma cell MSH receptors

UV-irradiation at 365 nm of cultured Cloudman S91 mouse melanoma cells in the presence of photoreactive alpha-MSH analogues induced longlasting receptor stimulation as revealed by the ensuring activation of tyrosinase. Receptor labelling was more efficient with 4-diazirinophenyl and 2-nitro-4-azidophenyl photolabels than with 4-azidophenyl, and was further increased when superpotent [Nle4,D-Phe7]-alpha-MSH was used as ligand. Incubation of B16 melanoma cell membranes with mono-iodinated [Nle4,D-Phe7,Trp-(Naps)9]-alpha-MSH followed by UV-irradiation at 310-550 nm labelled a single band on SDS-PAGE with a molecular mass approximately or equal to 45 kDa. The displacement curve obtained in a competitive photolabelling experiment paralleled that of the binding assay, demonstrating that the labelling was specific.

Photoaffinity labelling of MSH receptors on Anolis melanophores: effects of catecholamines, calcium and forskolin

Photoaffinity labelling of MSH receptors on Anolis melanophores was used as a tool for studying the effects of catecholamines, calcium and forskolin on hormone-receptor interaction and receptor-adenylate cyclase coupling. Covalent attachment of photoreactive alpha-MSH to its receptor was suppressed in calcium-free buffer but was hardly influenced by catecholamines or forskolin. The longlasting signal generated by the covalent MSH-receptor complex was readily and reversibly abolished by adrenaline, noradrenaline, dopamine or clonidine or by the absence of calcium. The suppression of pigment dispersion by catecholamines was blocked by the simultaneous presence of yohimbine but not prazosin, indicating that the catecholamines antagonize the alpha-MSH signal by inhibitory action on the adenylate cyclase system through an alpha-2 receptor. Forskolin, which stimulates melanophores by direct action on the catalytic unit of the adenylate cyclase and at about the same speed as alpha-MSH, produced a slower and weaker response in the presence of noradrenaline. If MSH receptors were covalently labelled and then exposed to noradrenaline, the characteristics of the forskolin-induced response were identical to those of unlabelled cells that had not been exposed to noradrenaline. This may point to a partial restoration of receptor-adenylate cyclase coupling by forskolin. The results show that the longlasting stimulation of Anolis melanophores by photoaffinity labelling proceeds via a permanently stimulated adenylate-cyclase system whose coupling to the receptor depends on calcium and is abolished by alpha-2 receptor agonists. Calcium is also essential for hormone-receptor binding.

Calmodulin activation of adenylate cyclase in the mouse B16 melanoma

Calmodulin antagonists inhibited hormone-stimulated cyclic AMP accumulation in both cultured cells and cell lysates of mouse B16 melanoma. Particulate preparations of B16 melanoma contained 34-45% of total cell calmodulin, which could not be dissociated by extensive washing irrespective of the presence of EGTA in the buffer. The adenylate cyclase activity in such preparations was unaffected by the addition of exogenous calmodulin. However, the rare-earth-metal ion La3+, which can mimic or replace Ca2+ in many systems, produced an immediate inhibition of agonist-stimulated adenylate cyclase activity and preincubation of particulate preparations was La3+ followed by washing with La3+-free buffer dissociated calmodulin (96% loss) from particulate preparations. The loss of calmodulin from particulate preparations was associated with a decrease in agonist responsiveness (74%) and a marked change in the Ca2+-sensitivity of the enzyme, low concentrations of calcium (approx. 10 nM) now failing to stimulate enzyme activity, high concentrations of calcium (greater than or equal to 100 nM) producing greater-than-normal inhibition of enzyme activity. Direct activation of adenylate cyclase by the addition of pure calmodulin was now demonstrable in such calmodulin-depleted particulate preparations. Half-maximal stimulation of agonist-responsive adenylate cyclase occurred at 80 nM-calmodulin in the presence of 10 microM free Ca2+. Maximal stimulation by calmodulin (at 300-600 nM) restored enzyme activity to 89 +/- 5% (mean +/- S.E.M., n = 7) of the activity in untreated, calmodulin-intact, preparations.

Calmodulin activation of cyclic AMP phosphodiesterase in the B16 mouse melanoma

Mouse B16 melanoma extracts of both cultured cells and tumour tissue contain cyclic AMP phosphodiesterase activity, with 95% present in the soluble fraction. Although activation of the enzyme by added calmodulin did not occur, it was found that endogenous calmodulin was present at a level sufficient to activate fully the enzyme. The ability of Ca-calmodulin to stimulate cyclic AMP phosphodiesterase in this tissue was shown by the inhibitory effect of N-(6-aminohexyl)-5-chloronaphthalenesulphonamide (W7), a known calmodulin antagonist; by the activation of the enzyme with exogenous calmodulin observed in supernatants depleted of endogenous calmodulin by passage over fluphenazine-Sepharose 6B in the presence of Ca2+; by the Ca-dependent binding of the enzyme to calmodulin-agarose and its activation by Ca-calmodulin after elution from the column with EGTA-containing buffer. It was calculated that about 50% of the total cyclic AMP phosphodiesterase activity was calmodulin-activated in this tissue.

Calcium requirement for alpha-MSH action on melanophores: studies with forskolin

alpha-MSH-induced pigment dispersion in melanophores shows an absolute requirement for extracellular Ca2+. To localize Ca2+ sites involved in the mechanism of action of alpha-MSH we studied the effects of Ca2+ deprivation on alpha-MSH and forskolin-induced melanophore responses. In an in vitro melanophore system employing ventral tailfins of Xenopus tadpoles, melanophore responses were assayed in terms of pigment dispersion and the phosphorylation state of a 53 kDa melanophore-specific protein. In the same melanophore system alpha-MSH has been shown to specifically increase the phosphorylation of this 53 kDa protein. Forskolin induces a dose-dependent pigment dispersion (EC50 7 X 10(-7) M). In contrast to the dispersion induced by alpha-MSH forskolin-induced dispersion does not require extracellular Ca2+. Moreover, in a Ca2+-free medium melanophores with permanently activated MSH-receptors aggregate, but can be redispersed by the addition of forskolin. Forskolin increases 53 kDa phosphorylation in a dose-dependent manner. Maximal stimulation with forskolin (10(-5) M) is four-fold and equals maximal 53 kDa phosphorylation obtainable with alpha-MSH. The MSH-induced increase in 53 kDa phosphorylation is inhibited by Ca2+ deprivation, whereas the forakolin-induced increase is unaffected. Our results suggest that alpha-MSH and forskolin stimulate melanophores through a common pathway and confirm that cAMP is a second messenger in alpha-MSH action in this system. We conclude that the Ca2+ sites in the mechanism of alpha-MSH action on melanophores precede adenylate cyclase activation.

Photoaffinity labelling of peptide hormone receptors

Photoreactive peptide derivatives for the labelling of hormone receptors are usually prepared by inserting a chemically stable aryl azide or nitroaryl azide into a specific site of the molecule, such as an alpha or omega amino or carboxyl group, or into the side-chain of an Arg, Cys, His, Trp or Tyr. With p-azidophenylalanine (Pap), a more or less isosteric replacement of Tyr or Phe can be achieved when other alterations would impair the biological activity of the hormone. Reversible attachment of photoreactive groups via S-S linkage to, e.g., SH-Trp makes it possible to release the covalently bound hormone from the receptor. This is an advantage when photolabelling is used for the isolation of receptors. Photoaffinity labelling of intact cell systems may reveal insights into dynamic aspects, such as receptor inactivation and turnover or the temporal involvement of second messengers, and may be particularly useful for the study of peptide action on target cells occurring only in small numbers. The Xenopus MSH-melanophore system which is stimulated irreversibly upon UV-irradiation of photoreactive alpha-MSH represents such an example.

Calcium requirement for alpha-MSH action on tail-fin melanophores of xenopus tadpoles

The role of Ca2+ in alpha-MSH action on melanophores was studied, in vitro, with a bioassay on ventral tail-fin pieces from tadpoles of Xenopus laevis. Melanosome dispersion induced by alpha-MSH required 1-2 mM extracellular Ca2+. Gradual lowering of the extracellular Ca2+ levels produced a concentration-dependent inhibition of the alpha-MSH response; complete inhibition was obtained in a Ca2+-free medium containing 10-4 M EGTA. In Mg2+-free medium, normal dispersion was observed. The Ca2+ antagonists verapamil (10-4 M), methoxy-verapamil (10-4 M) and La3+ (10-3 M) inhibited the dispersion induced by 3 X 10-9 M alpha-MSH, whereas ruthenium red (10-3 M) was without effect. The ionophore A23187 mimicked the effect of the hormone. Melanosome movement per se was evidently independent of Ca2+, because cAMP and dibutyryl-cAMP induced a full dispersion in the absence of Ca2+. These results show that extracellular Ca2+ is specifically required for alpha-MSH action on tail-fin melanophores in vitro and suggests a Ca2+ influx concomitant with the action of the hormone. Possible intra- and extra-cellular Ca2+ sites are discussed.

Calcium sites in MSH stimulation of xenopus melanophores: studies with photoreactive alpha-MSH

Photo-affinity labelling of MSH receptors on tail-fin melanophores of Xenopus tadpoles with p-azidophenylalanine 13-alpha-MSH (Pap13)-alpha-MSH) or p-azidophenylacetyl-serine1-alpha-MSH ([Apac-Ser1]-alpha-MSH) resulted in a long-lasting stimulation of the melanophores which cannot be reversed despite continuous washing. The generation of this irreversible response is inhibited when photo-affinity labelling is performed in a Ca2+-free medium or in the presence of Ca2+ antagonists. The irreversible stimulation produced in normal medium is completely reversed upon removal of Ca2+ , but is not affected by Ca2+ antagonists or melatonin. Re-addition of Ca2+ after temporary removal restores to irreversible stimulation, even in the presence of Ca2+ antagonists or melatonin. This proves that covalent alpha-MSH-receptor complexes remain fully functional despite temporary deprivation of ca2+. Racemized alpha-MSH, which binds 'tightly' to the receptor and produces a long-lasting effect, is removed from the receptor in Ca2+-free medium, but not by Ca2+ antagonists or melatonin. These results confirm earlier results showing that at least 2 Ca2+ sites are involved in alpha-MSH action, one associated with MSH-receptor binding and the other with the subsequent generation of the effect. The dual role of Ca2+ is not the result of EGTA present; it is specific (Mg2+ has no effect) and is identical for the two different photoreactive alpha-MSH derivatives.