Effect of MELANOTAN, [Nle(4), D-Phe(7)]-alpha-MSH, on melanin synthesis in humans with MC1R variant alleles

A gel-forming α-MSH analog promotes lasting melanogenesis

The α-MSH peptide plays a significant role in the regulation of pigmentation via the melanocortin 1 receptor (MC1R). It increases the DNA repair capacity of melanocytes and reduces the incidence of skin cancers. As such, α-MSH analogs could have the utility for protecting against UV-induced skin DNA damage in susceptible patients. Recently, α-MSH analogs have been approved for the treatment of erythropoietic protoporphyria, hypoactive sexual desire, or pediatric obesity. However, the delivery of these drugs requires inconvenient implants or frequent injections. We recently found that select palmitoylated melanocortin analogs such as afamelanotide and adrenocorticotropin peptides self-assemble to form liquid gels in situ. To explore the utility of these novel analogs, we studied their pharmacological characteristics in vitro and in vivo. Acylated afamelanotide (DDE 313) and ACTH1-24 (DDE314) analogs form liquid gels at 6-20% and have a significantly increased viscosity at >2.5% compared to original analogs. Using the DDE313 analog as a prototype, we showed gel-formation reduces the passage of DDE313 through Centricon filters, and subcutaneous injection of analog gel in rats leads to the sustained presence of the peptide in circulation for >12 days. In addition, DDE313 darkened the skin of frogs for >4 weeks, whereas those injected with an equivalent dose of afamelanotide lost the tanning response within a few days. Because self-assembled gels allow sustained activation of melanocortin receptors, further studies of these analogs may allow the development of effective and convenient tanning therapies to prophylactically protect against UV-induced malignant transformation of skin cells in susceptible patients.

Peptide-Based Strategies for Targeted Tumor Treatment and Imaging

Cancer is one of the leading causes of death worldwide. The development of cancer-specific diagnostic agents and anticancer toxins would improve patient survival. The current and standard types of medical care for cancer patients, including surgery, radiotherapy, and chemotherapy, are not able to treat all cancers. A new treatment strategy utilizing tumor targeting peptides to selectively deliver drugs or applicable active agents to solid tumors is becoming a promising approach. In this review, we discuss the different tumor-homing peptides discovered through combinatorial library screening, as well as native active peptides. The different structure–function relationship data that have been used to improve the peptide’s activity and conjugation strategies are highlighted.

Chemoprevention agents for melanoma: A path forward into phase 3 clinical trials

Recent progress in the treatment of advanced melanoma has led to unprecedented improvements in overall survival and, as these new melanoma treatments have been developed and deployed in the clinic, much has been learned about the natural history of the disease. Now is the time to apply that knowledge toward the design and clinical evaluation of new chemoprevention agents. Melanoma chemoprevention has the potential to reduce dramatically both the morbidity and the high costs associated with treating patients who have metastatic disease. In this work, scientific and clinical melanoma experts from the national Melanoma Prevention Working Group, composed of National Cancer Trials Network investigators, discuss research aimed at discovering and developing (or repurposing) drugs and natural products for the prevention of melanoma and propose an updated pipeline for translating the most promising agents into the clinic. The mechanism of action, preclinical data, epidemiological evidence, and results from available clinical trials are discussed for each class of compounds. Selected keratinocyte carcinoma chemoprevention studies also are considered, and a rationale for their inclusion is presented. These data are summarized in a table that lists the type and level of evidence available for each class of agents. Also included in the discussion is an assessment of additional research necessary and the likelihood that a given compound may be a suitable candidate for a phase 3 clinical trial within the next 5 years.

Temporal cAMP Signaling Selectivity by Natural and Synthetic MC4R Agonists

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in the brain, where it controls energy balance through pathways including α-melanocyte-stimulating hormone (α-MSH)-dependent signaling. We have reported that the MC4R can exist in an active conformation that signals constitutively by increasing cAMP levels in the absence of receptor desensitization. We asked whether synthetic MC4R agonists differ in their ability to increase intracellular cAMP over time in Neuro2A cells expressing endogenous MC4R and exogenous, epitope-tagged hemagglutinin-MC4R-green fluorescent protein. By analyzing intracellular cAMP in a temporally resolved Förster resonance energy transfer assay, we show that withdrawal of α-MSH leads to a quick reversal of cAMP induction. By contrast, the synthetic agonist melanotan II (MTII) induces a cAMP signal that persists for at least 1 hour after removal of MTII from the medium and cannot be antagonized by agouti related protein. Similarly, in mHypoE-42 immortalized hypothalamic neurons, MTII, but not α-MSH, induced persistent AMP kinase signal, which occurs downstream of increased cAMP. By using a fluorescence recovery after photobleaching assay, it appears that the receptor exposed to MTII continues to signal after being internalized. Similar to MTII, the synthetic MC4R agonists, THIQ and BIM-22511, but not LY2112688, induced prolonged cAMP signaling after agonist withdrawal. However, agonist-exposed MC4R desensitized to the same extent, regardless of the ligand used and regardless of differences in receptor intracellular retention kinetics. In conclusion, α-MSH and LY2112688, when compared with MTII, THIQ, and BIM-22511, vary in the duration of the acute cAMP response, showing distinct temporal signaling selectivity, possibly linked to specific cell compartments from which cAMP signals may originate.

ACTH: The forgotten therapy

Although anti-inflammatory drugs are among the most common class of marketed drugs, chronic inflammatory conditions such as rheumatoid arthritis, multiple sclerosis or inflammatory bowel disease still represent unmet needs. New first-in-class drugs might be discovered in the future but the repurpose and further development of old drugs also offers promise for these conditions. This is the case of the melanocortin adrenocorticotropin hormone, ACTH, used in patients since 1952 but regarded as the last therapeutic option when other medications, such as glucocorticoids, cannot be used. Better understanding on its physiological and pharmacological mechanisms of actions and new insights on melanocortin receptors biology have revived the interest on rescuing this old and effective drug. ACTH does not only induce cortisol production, as previously assumed, but it also exerts anti-inflammatory actions by targeting melanocortin receptors present on immune cells. The endogenous agonists for these receptors (ACTH, α-, β-, and γ-melanocyte stimulating hormones), are also produced locally by immune cells, indicating the existence of an endogenous anti-inflammatory tissue-protective circuit involving the melanocortin system. These findings suggested that new ACTH-like melanocortin drugs devoid of steroidogenic actions, and hence side effects, could be developed. This review summarizes the actions of ACTH and melanocortin drugs, their role as endogenous pro-resolving mediators, their current clinical use and provides an overview on how recent advances on GPCR functioning may lead to a novel class of drugs.

Cutaneous porphyrias part II: treatment strategies

The porphyrias are diverse in pathophysiology, clinical presentation, severity, and prognosis, presenting a diagnostic and therapeutic challenge. Although not easily curable, the dermatological manifestations of these diseases, photosensitivity and associated cutaneous pathology, can be effectively prevented and managed. Sun avoidance is essential, and patient education regarding the irreversibility of photocutaneous damage is a necessary corollary. Beyond preventative measures, the care of fragile, vulnerable skin, and pain management, each of the porphyrias has a limited number of unique additional therapeutic options. Many of the treatments have been published only in small case series or anecdotal reports and do not have well-understood nor proven mechanisms of action. This article presents a comprehensive review of available therapeutic options and long-term management recommendations for the cutaneous porphyrias.

The regulation of epidermal melanogenesis via cAMP and/or PKC signaling pathways: insights for the development of hypopigmenting agents

Abnormal pigmentation, particularly hyperpigmentation, is major issue of concern for people with colored skin. Several hypopigmenting agents, which exert their action by inhibiting tyrosinase activity and/or transcription, have been used for treatment. However, results have been discouraging. To manage abnormal pigmentation properly, the mechanisms of melanogenesis should be understood. Endogenous and exogenous factors affect melanogenesis via intracellular machineries. cAMP and PKC are critical factors of important transduction pathways and cross-talk between them could amplify the melanogenic effect. Here, factors involved in melanogenesis regulation via cAMP and/or PKC pathways are reviewed with their action mechanisms.

The melanocortin 1 receptor (MC1R) inhibits the inflammatory response in Raw 264.7 cells and atopic dermatitis (AD) mouse model

The alpha melanocyte stimulating hormone receptor (MC1R) is one of five G-protein coupled receptors belonging to the melanocortin subfamily, MC1R gene has been known to play a major role in regulating of fur color in mammals, and α-MSH and ACTH are endogenous nonselective agonists for MC1R. However, we found that MC1R was highly expressed in Raw 264.7 cells which were important inflammatory cells involved in the initiation of inflammatory responses. In addition, Cyclic AMP is not only a key molecule in the MC1R signal transduction pathway, but dampen innate immune-mediated responses. These intriguing biological results triggered the further conformation studies; it suggested that MC1R was very likely to be an important role in immunoregulation. In this study, we were to investigate the immunosuppressive effects of MC1R on inflammation in lipopolysaccharide (LPS) stimulated Raw 264.7 cells and LPS induced vivo 2-chloro-1,3,5-trinitrobenzene (TNCB)-induced atopic dermatitis (AD) model. The effects of the MC1R antagonist psoralen on pro-inflammatory cytokines and signaling pathways were analyzed by enzyme-linked immunosorbent assay, western blot, real-time fluorescence quantitative PCR and Histological analysis. Our results show a consistent and marked effect of high concentrations of MC1R antagonist psoralen increased the level of MC1R mRNA in Raw 264.7 cells by cumulative feedback regulation through preferential binding of MC1R. Moreover, as evidenced by inhibiting the LPS-induced TNF-α, IL-6 and enhancing the expression level of cyclic AMP protein in vitro. In vivo study it was also observed that psoralen promoted on histopathologic changes in the skin tissue of TNCB-induced AD mice. Taken together, our results suggest that MC1R decrease the inflammation in vitro and vivo, and might be a therapeutic signaling pathway to against inflammatory diseases.

Melanotropic peptides: more than just 'Barbie drugs' and 'sun-tan jabs'?

While ultraviolet radiation (UVR) is a major cause of skin ageing and carcinogenesis, public pursuit of a novel tanning strategy circumventing the need for UVR is increasingly reported in the media and scientific press. This involves the subcutaneous self-administration of unregulated products labelled as melanotan I and/or II, synthetic analogues of α-melanocyte stimulating hormone (α-MSH), as obtained via the internet, tanning salons and gyms. The Medicines and Healthcare products Regulatory Authority has recently raised awareness of the public health risk of transmission of blood-borne viruses from the needle sharing that may occur, and of the potential impurity of these products. Dermatologists should also be aware that these agents can complicate the clinical presentation of patients with pigmented lesions; their use may be suspected in unexpectedly tanned individuals with rapidly pigmenting naevi. Meanwhile, the regulated α-MSH analogue afamelanotide (Clinuvel Pharmaceuticals Ltd, Melbourne, Australia) is showing promise for its photoprotective potential, and is undergoing phase II and III clinical trials in people with photosensitivity disorders and those prone to nonmelanoma skin cancer. The photoprotective and other biological effects of α-MSH analogues await full determination.

Melanocyte-stimulating hormone directly enhances UV-Induced DNA repair in keratinocytes by a xeroderma pigmentosum group A-dependent mechanism

Melanocyte-stimulating hormone (MSH) reduces UV-induced DNA damage through the induction of pigmentation. In this study, we provide evidence that MSH also enhances DNA repair in skin keratinocytes by modulating the function of DNA repair molecules. Intracutaneous injection of MSH prevented UV-induced DNA damage in human and mouse skin independent of its effects on melanogenesis. In keratinocytes, MSH bound to the melanocyte melanocortin receptor type 1 and activated adenylate cyclase activity, which in turn activated Xeroderma pigmentosum group A (XPA)-binding protein 1 and induced nuclear translocation of XPA, a critical factor controlling nucleotide excision repair signaling pathways. Together, our findings reveal a novel pigmentation-independent mechanism that underlies MSH-mediated DNA repair following UVB irradiation.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

LIGR, a protease-activated receptor-2-derived peptide, enhances skin pigmentation without inducing inflammatory processes

The protease-activated receptor-2 (PAR-2) is a seven transmembrane G-protein-coupled receptor that could be activated by serine protease cleavage or by synthetic peptide agonists. We showed earlier that activation of PAR-2 with Ser-Leu-Ile-Gly-Arg-Leu-NH(2) (SLIGRL), a known PAR-2 activating peptide, induces keratinocyte phagocytosis and increases skin pigmentation, indicating that PAR-2 regulates pigmentation by controlling phagocytosis of melanosomes. Here, we show that Leu-Ile-Gly-Arg-NH(2) (LIGR) can also induce skin pigmentation. Both SLIGRL and LIGR increased melanin deposition in vitro and in vivo, and visibly darkened human skins grafted onto severe combined immuno-deficient (SCID) mice. Both SLIGRL and LIGR stimulated Rho-GTP activation resulting in keratinocyte phagocytosis. Interestingly, LIGR activates only a subset of the PAR-2 signaling pathways, and unlike SLIGRL, it does not induce inflammatory processes. LIGR did not affect many PAR-2 signaling pathways, including [Ca(2+)] mobilization, cAMP induction, the induction of cyclooxgenase-2 (COX-2) expression and the secretion of prostaglandin E2, interleukin-6 and -8. PAR-2 siRNA inhibited LIGR-induced phagocytosis, indicating that LIGR signals via PAR-2. Our data suggest that LIGR is a more specific regulator of PAR-2-induced pigmentation relative to SLIGRL. Therefore, enhancing skin pigmentation by topical applications of LIGR may result in a desired tanned-like skin color, without enhancing inflammatory processes, and without the need of UV exposure.

Ammonium chloride and l-tyrosine enhance melanogenesis in vitro but not in vivo even in combination with ultraviolet radiation

BACKGROUND/PURPOSE

Melanogenesis can be induced in vitro in melanoma cells and melanocytes by adding substances able to neutralize intracellular acidic organelles like melanosomes. Further addition of l-tyrosine enhances the melanogenesis by increasing the tyrosinase activity. As such, the property of tyrosine as a pigmentation enhancer is used in promoting creams containing tyrosine. The objective of this study was to investigate whether such an effect could actually be seen in a short term in vivo mouse study.

METHODS

Lightly pigmented C3.Cg/TifBomTac hairless mice capable of pigmenting had ammonium chloride (NH4Cl) and/or l-tyrosine applied topically (1/day for 3 weeks). Pigmentation of the mice was determined using the Kodak Gray Scale at days 0, 7, 14, and 21.

RESULTS

Both NH4Cl and l-tyrosine yielded no significant effect, either alone or in combination, when applied using either hydrogel or moisturizing cream. Exposing mice to simulated solar radiation (4 standard erythema doses, 3/week) yielded increased pigmentation. However, no statistically significant difference was found between treatment with simulated solar radiation alone or in combination with NH4Cl and l-tyrosine.

CONCLUSION

In spite of the commercial value of adding l-tyrosine to 'pigmentation-enhancing' creams, topically applied l-tyrosine showed no pigmentation-enhancing effect, neither alone nor in combination with ultraviolet (UV) radiation, providing a basis to contest such promotional measures.

Cell surface expression of melanocortin-1 receptor on HaCaT keratinocytes and alpha-melanocortin stimulation do not affect the formation and repair of UVB-induced DNA photoproducts

Ultraviolet (UV) exposure induces an up-regulation of melanocortin-1 receptor (MC1R) expression in human skin and the alpha-melanocyte-stimulating hormone (alpha-MSH) may reduce UVB-induced DNA damage in normal human melanocytes. Using high-performance liquid chromatography coupled to tandem mass spectrometry, we investigated the formation and repair of DNA lesions in UVB-irradiated HaCaT cells stably transfected with the wild type MC1R gene (HaCaT-MC1R). Similar levels of 8 bipyrimidine photoproducts including cyclobutane pyrimidine dimers (CPDs) (T<>T, T<>C, C<>T), (6-4) photoproducts ((6-4)PPs) (TT-(6-4)PPs, TC-(6-4)PPs) and their Dewar valence isomers together with 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) were found to be generated in both non-transfected and HaCaT-MC1R cells after UVB exposure. Time-course studies of DNA photoproduct yields indicated that the DNA repair ability depended upon radiation doses. It was shown that (6-4)PPs were removed from the DNA of UVB-irradiated cells much more efficiently than CPDs. The repair efficiency of 8-oxodGuo, CPDs and (6-4)PPs was relatively similar in both cell lines and was not modified by stimulation with alpha-MSH before UVB-exposure. In conclusion, cell surface-enforced expression of MC1Rs on HaCaT keratinocytes and alpha-MSH stimulation do not affect the formation of UVB-induced DNA photoproducts and their subsequent repair.