1. Gamma-MSH peptides in the pituitary: effects, target cells, and receptors
C Denef, E Swinnen, J Lu Ann N Y Acad Sci . 2003 Jun;994:123-32. doi: 10.1111/j.1749-6632.2003.tb03171.x.
The melanocortin (MC) gamma3-MSH is believed to signal through the MC3 receptor. We showed that it induces a sustained increase in intracellular free calcium levels ([Ca(2+)](i)) in a subpopulation of pituitary cells. Most of the cells responding to gamma3-MSH express more than one pituitary hormone mRNA. The effect of gamma3-MSH is blocked by SHU9119, a MC3R and MC4R antagonist, in only 50% of the responsive cells, suggesting that in half of these cells the mediating receptor is not the MC3R. Low picomolar doses of gamma3-MSH increase [Ca(2+)](i) in the growth hormone (GH)- and prolactin (PRL)-secreting GH3 cell line. gamma2-MSH and alpha-MSH display a similar effect. SHU9119 does not affect the gamma3-MSH-induced [Ca(2+)](i) response. MTII, a potent synthetic agonist of the MC3R, MC4R, and MC5R, also shows no or low potency in increasing [Ca(2+)](i). By means of RT-PCR, the mRNA of the MC2R, MC3R, and MC4R receptors is undetectable. Experiments testing gamma2-MSH analogues with single alanine replacements show that, unlike the classic MCRs, the His(5)-Phe(6)-Arg(7)-Trp(8) sequence in gamma2-MSH is not a core sequence for activating the gamma-MSH receptor in GH3 cells, whereas Met(3) is essential. Low nanomolar doses of gamma-MSH increase intracellular cAMP levels. Blockade of protein kinase A abolishes the [Ca(2+)](i) responses to gamma3-MSH. gamma2-MSH increases binding of [S(35)]GTPgammaS to membrane preparations of GH3 cells. The pharmacological characteristics of gamma-MSH peptides and analogues on [Ca(2+)](i) and the signal-transduction pathways present strong evidence for the expression of a hitherto uncharacterized gamma-MSH receptor in GH3 cells, belonging to the G protein-coupled receptor family.
2. 111In-CHX-A"-Rhenium-cyclized-[Cys3,4,10,D-Phe7,Arg11]α-MSH3-13
Kam Leung
Malignant melanoma is the most deadly form of skin cancer (1). Early and accurate diagnosis is necessary for surgery and successful treatment (2). The melanocortin (MC) system is a neuropeptide network of the skin, and it is involved in pigmentation regulation, cortisol production, and many other physiological processes (3). Most cutaneous cell types express MC receptors, pro-opiomelanocortin (POMC), and prohormone convertases, and they also release MCs. Melanotropin-stimulating hormones (α-, β-, and γ-MSH) are derived from POMC by the proteolytic action of prohormone convertases. There are five MC receptors (MC1R to MC5R), which belong to the G-protein-coupled receptor superfamily, and these receptors have been found to be overexpressed in melanoma cells (4, 5). α-MSH (Ac-Ser1-Tyr2-Ser3-Met4-Glu5-His6-Phe7-Arg8-Trp9-Gly10-Lys11-Pro12-Val13-NH2), produced by the brain and the pituitary gland, is a tridecapeptide (13 amino acids) and is the most potent melanotropic peptide (6) in the regulation of skin pigmentationviaMC1R.Positron emission tomography (PET) imaging with [18F]fluoro-2-deoxy-2-d-glucose ([18F]FDG) in cancer has been approved by United States Food and Drug Administration with many on-going clinical trials. Although [18F]FDG is effective in the detection of melanoma, it is not melanoma-specific and some melanoma cells do not take up [18F]FDG (7, 8). Radiolabeled α-MSH peptide analogs have been shown to specifically bind to MC1R that is overexpressed on human and mouse melanoma cells (4, 5, 7, 9, 10).N-(2-Aminoethyl)-trans-1,2-diaminocyclohexane-N,N",N"'-pentaacetic acid (CHX-A") has been successfully coupled to α-MSH peptide analogs for radiolabeling with a variety of radionuclides (7, 11, 12). A rhenium-cyclized α-MSH analog, Re-[Cys3,4,10,D-Phe7,Arg11]α-MSH3-13(ReCCMSH(Arg11)), was conjugated with CHX-A" (CHX-A"-ReCCMSH(Arg11)) and radiolabeled with111In (111In-CHX-A"-ReCCMSH(Arg11)) as a molecular imaging agent for single-photon emission computed tomography (SPECT) to target melanoma (13). Rhenium-mediated cyclization exhibits significantly reduced nonspecific renal radioactivity accumulation, high tumor uptake and retention coupled with rapid clearance kinetics, compared with its free sulfhydryl and disulfide bond-containing counterparts (10).111In is a gamma emitter with a physical half-life of 2.8 days.
3. Improved synthesis and biological evaluation of chelator-modified α-MSH analogs prepared by copper-free click chemistry
Michael K Schultz, Molly E Martin, M Sue O'Dorisio, Nicholas J Baumhover, Kyle C Kloepping, Sharavathi G Parameswarappa, F Christopher Pigge Bioorg Med Chem Lett . 2011 Oct 1;21(19):5757-61. doi: 10.1016/j.bmcl.2011.08.017.
Radionuclide chelators (DOTA, NOTA) functionalized with a monofluorocyclooctyne group were prepared. These materials reacted rapidly and in high yield with a fully deprotected azide-modified peptide via Cu-free click chemistry under mild reaction conditions (aqueous solution, room temperature). The resulting bioconjugates bind with high affinity and specificity to their cell-surface receptor targets in vitro and appear stable to degradation in mouse serum over 3h of incubation at 37°C.
