Holder Jerry Ryan, Xiang Zhimin, Bauzo Rayna M, Haskell-Luevano Carrie
University of Florida, Department of Medicinal Chemistry, Gainesville, Florida 32610-0485, USA.
J Med Chem. 2002 Dec 19;45(26):5736-44. doi: 10.1021/jm020296e.
The melanocortin pathway is involved in the regulation of several physiological functions including skin pigmentation, steroidogenesis, obesity, energy homeostasis, and exocrine gland function. This melanocortin pathway consists of five known G-protein coupled receptors, endogenous agonists derived from the proopiomelanocortin (POMC) gene transcript, the endogenous antagonists Agouti and the Agouti-related protein (AGRP) and signals through the intracellular cAMP signal transduction pathway. The endogenous melanocortin agonists contain the putative message sequence "His-Phe-Arg-Trp," postulated to be important for melanocortin receptor molecular recognition and stimulation. Herein, we report a tetrapeptide library, based upon the template Ac-His-D-Phe-Arg-Trp-NH(2), consisting of 20 members that have been modified at the Trp(9) position (alpha-MSH numbering) and pharmacologically characterized for agonist activity at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R. Results from this study yielded compounds that ranged in pharmacological properties from equipotent to a loss of melanocortin receptor activity at up to 100 microM concentrations. Interestingly, modification of the Trp(9) in the tetrapeptide template at the MC1R resulted in only up to a 220-fold potency change, while at the MC4R and MC5R, up to a 9700-fold decrease in potency was observed, suggesting the MC1R is more tolerant of the modifications examined herein. The most notable results of this study include identification that the Trp(9) indole moiety in the tetrapeptide template is important for melanocortin-3 receptor agonist potency, and that this position can be used to design melanocortin ligands possessing receptor selectivity for the peripherally expressed MC1 and MC5 versus the centrally expressed MC3 and MC4 receptors. Specifically, the Ac-His-D-Phe-Arg-Tic-NH(2) and the Ac-His-D-Phe-Arg-Bip-NH(2) tetrapeptides possessed nanomolar MC1R and MC5R potency but micromolar MC3R and MC4R agonist potency. Additionally, these studies identified that substitution of the Trp amino acid with either Nal(2') or D-Nal(2') resulted in equipotent melanocortin receptor potency, suggesting that the chemically reactive Trp indole side chain may be replaced with the nonreactive Nal(2') moiety for the design of nonpeptide melanocortin receptor agonists.
黑皮质素途径参与多种生理功能的调节，包括皮肤色素沉着、类固醇生成、肥胖、能量稳态和外分泌腺功能。该黑皮质素途径由五个已知的G蛋白偶联受体、源自阿黑皮素原（POMC）基因转录本的内源性激动剂、内源性拮抗剂刺鼠信号蛋白和刺鼠相关蛋白（AGRP）组成，并通过细胞内cAMP信号转导途径传递信号。内源性黑皮质素激动剂包含假定的信息序列“His-Phe-Arg-Trp”，推测该序列对黑皮质素受体的分子识别和刺激很重要。在此，我们报道了一个基于模板Ac-His-D-Phe-Arg-Trp-NH₂的四肽库，该库由20个成员组成，这些成员在Trp(9)位置（α-MSH编号）进行了修饰，并对其在小鼠黑皮质素受体MC1R、MC3R、MC4R和MC5R上的激动剂活性进行了药理学表征。这项研究的结果产生了一系列化合物，其药理性质从等效到在高达100μM浓度下丧失黑皮质素受体活性不等。有趣的是，在MC1R处对四肽模板中的Trp(9)进行修饰仅导致效力变化高达220倍，而在MC4R和MC5R处，观察到效力下降高达9700倍，这表明MC1R对本文研究的修饰更具耐受性。这项研究最显著的结果包括确定四肽模板中的Trp(9)吲哚部分对黑皮质素-3受体激动剂效力很重要，并且该位置可用于设计对在外周表达的MC1和MC5与在中枢表达的MC3和MC4受体具有受体选择性的黑皮质素配体。具体而言，Ac-His-D-Phe-Arg-Tic-NH₂和Ac-His-D-Phe-Arg-Bip-NH₂四肽具有纳摩尔级的MC1R和MC5R效力，但具有微摩尔级的MC3R和MC4R激动剂效力。此外，这些研究确定用Nal(2')或D-Nal(2')替代Trp氨基酸会导致等效的黑皮质素受体效力，这表明在设计非肽类黑皮质素受体激动剂时，具有化学反应性的Trp吲哚侧链可以被无反应性的Nal(2')部分取代。
