Cyclo(his-pro) (Cyclo(histidyl-proline))

Cyclo(his-pro) (Cyclo(histidyl-proline)) is an endogenous, blood–brain-barrier-permeable cyclic dipeptide derived from thyrotropin-releasing hormone (TRH) catabolism. Cyclo(his-pro) protects cells from oxidative damage by selectively activating the transcription factor Nrf2 signalling pathway. Cyclo(his-pro) also exhibits anti-inflammtory effects and can be applied as neuroprotective agents^[1][2].

In vitro, PC12 cells were grown on glass coverslips in the presence or in the absence of 50µM Cyclo(his-pro) for 24h and then treated with 100µM H₂O₂. Cyclo(his-pro) protects PC12 cells against H₂O₂-induced apoptotic death, oxidative/nitrosative stress and calcium accumulation by inducing Nrf2 activation and upregualting oxidative/nitrosative stress-related gene expression including GR, GSTA-2 and Gpx^[3]. hSOD1(G93A) microglial cells were pretreated for 24h with 50μM Cyclo(his-pro) and then treated with 1μg/ml LPS for 3h or 24h. Cyclo(his-pro) inhibited NLRP3 inflammasome activation by reducing protein nitration via reduction in NO and ROS levels, indicative of lower peroxynitrite generation by LPS. Cyclo(his-pro)-mediated ROS attenuation, not linked to Nrf2 activation in this cellular model, is ascribed to increased soluble SOD1 activity due to the up-regulation of Hsp70 and Hsp27 expression^[4].

In vivo, non-alcoholic fatty liver disease (NAFLD) was induced in 24-week-old male C57BL/6J mice by housing animals at thermoneutral housing (30℃–32℃) and feeding with a western diet. Treatment with Cyclo(his-pro) was started at Week 7, 20mg/kg doses were administered 3 times a week per os (gavage) up till Week 24. Cyclo(his-pro) was able to counteract the harmful effects of western diet, preventing the onset of metabolic syndrome such as weight gain, insulin resistance and hyperglycaemia. Cyclo(his-pro) prevented NAFLD progression, reducing hepatic steatosis, fibrosis, and inflammation^[5]. Overnight fasted Sprague-Dawley rats were administered with either saline, 0.005mg/kg, 0.0275mg/kg or 0.05 mg/kg Cyclo(his-pro) via the carotid line immediately preceding 3g/kg glucose by gavage. exogenous Cyclo(his-pro) does not alter insulin or C-peptide levels in the fasting rat, but does dose-dependently increased the insulin response to oral glucose in rats^[6]. C57BL/6 mice received i.p. injections of Cyclo(his-pro) (2.5mg/kg) 3h prior to LPS treatment (5mg/kg) and 30min afterwards. Cyclo(his-pro) markedly reduced hepatic and cerebral TNF-α mRNA, suppressed hippocampal IL-1β, and attenuated cortical GFAP up-regulation, effectively blocking LPS-induced reactive gliosis. Cyclo(his-pro) also delivered anti-inflammatory and neuroprotective effects by diminishing microglial and astrocytic activation in cortex and hippocampus^[7].

References:
[1] Minelli A, Grottelli S, Mierla A, et al. Cyclo(His-Pro) exerts anti-inflammatory effects by modulating NF-κB and Nrf2 signalling. Int J Biochem Cell Biol. 2012 Mar;44(3):525-35.
[2] Grottelli S, Ferrari I, Pietrini G, et al. The Role of Cyclo(His-Pro) in Neurodegeneration. Int J Mol Sci. 2016 Aug 12;17(8). pii: E1332.
[3] Minelli A, Conte C, Grottelli S, et al. Cyclo(His-Pro) promotes cytoprotection by activating Nrf2-mediated up-regulation of antioxidant defence. J Cell Mol Med. 2009 Jun;13(6):1149-61.
[4] Grottelli S, Mezzasoma L, Scarpelli P, et al. Cyclo(His-Pro) inhibits NLRP3 inflammasome cascade in ALS microglial cells. Mol Cell Neurosci. 2019 Jan:94:23-31.
[5] Masi A D, Li X X, Lee D H, et al. Cyclo(His-Pro): A further step in the management of steatohepatitis.JHEP Rep. 2023 Jun 10;5(9):100815.
[6] Mizumo H, Svec F, Prasad C, Hilton C. Cyclo(His-Pro) augments the insulin response to oral glucose in rats. Life Sci. 1997;60(6):369-74.
[7] Bellezza I, Grottelli S, Mierla A L, et al. Neuroinflammation and endoplasmic reticulum stress are coregulated by cyclo(His-Pro) to prevent LPS neurotoxicity. Int J Biochem Cell Biol. 2014 Jun:51:159-69.

Cyclo(his-pro) (Cyclo(histidyl-proline))是一种内源性、可透过血脑屏障的环状二肽，来源于促甲状腺激素释放激素（TRH）的分解代谢。Cyclo(his-pro)通过选择性激活转录因子Nrf2信号通路保护细胞免受氧化损伤，并具有抗炎作用，可作为神经保护剂^[1][2]。

体外实验中，PC12细胞在玻璃盖玻片上培养，在50µM Cyclo(his-pro)存在或不存在的情况下孵育24h，随后用100µM H2O2处理。Cyclo(his-pro)通过诱导Nrf2激活并上调氧化/硝化应激相关基因（包括GR、GSTA-2和Gpx）的表达，保护PC12细胞免受H2O2诱导的凋亡性死亡、氧化/硝化应激以及钙累积损伤^[3]。hSOD1(G93A)小胶质细胞先用50μM Cyclo(his-pro)预处理24h，再用1μg/ml LPS处理3h或24h。Cyclo(his-pro)通过降低NO和ROS水平减少蛋白硝化，从而抑制NLRP3炎症小体激活，表明LPS诱导的过氧亚硝酸盐生成减少。在该细胞模型中，Cyclo(his-pro)介导的ROS减弱并非通过Nrf2激活实现，而是归因于Hsp70和Hsp27表达上调导致可溶性SOD1活性增加^[4]。

体内实验中，24周龄雄性C57BL/6J小鼠通过置于热中性环境（30℃–32℃）并喂食西方饮食诱导非酒精性脂肪性肝病（NAFLD）。从第7周开始给予Cyclo(his-pro)治疗，剂量为20mg/kg，每周经口灌胃3次，直至第24周。Cyclo(his-pro)能够抵消西方饮食的有害影响，预防代谢综合征的发生，如体重增加、胰岛素抵抗和高血糖。Cyclo(his-pro)阻止NAFLD进展，减轻肝脏脂肪变性、纤维化和炎症^[5]。禁食过夜的Sprague-Dawley大鼠经颈动脉导管给予生理盐水、0.005mg/kg、0.0275mg/kg或0.05mg/kg Cyclo(his-pro)，随后立即经口给予葡萄糖3g/kg。外源性Cyclo(his-pro)不改变禁食大鼠的胰岛素或C肽水平，但可剂量依赖性增强大鼠对口服葡萄糖的胰岛素反应^[6]。C57BL/6小鼠在LPS处理（5mg/kg）前3h及处理后30min腹腔注射Cyclo(his-pro)（2.5mg/kg）。Cyclo(his-pro)显著降低肝脏和脑组织TNF-α mRNA水平，抑制海马IL-1β表达，并减弱皮层GFAP上调，有效阻断LPS诱导的反应性胶质增生。Cyclo(his-pro)还通过减少皮层和海马的小胶质细胞和星形胶质细胞活化，发挥抗炎和神经保护作用^[7]。