L-精氨酸代谢在决定巨噬细胞M1/M2极化中起着至关重要的作用。M1巨噬细胞表达诱导型一氧化氮合酶(iNOS)，而M2巨噬细胞表达精氨酸酶1并将精氨酸代谢为一氧化氮和尿素。肿瘤微环境促进了M2巨噬细胞的极化，并从而将精氨酸的代谢命运从一氧化氮转向尿素的产生。重要的是，M2巨噬细胞或肿瘤相关巨噬细胞(TAMs)的浸润与多种癌症类型的不良预后相关。褪黑激素已被广泛报道具有抗肿瘤和免疫调节性质。然而，它是否以及如何影响TAMs的极化尚未阐明。考虑到精氨酸代谢在巨噬细胞极化中的关键作用，我们对TAMs中L-精氨酸的命运以及褪黑激素是否能够恢复它的命运感兴趣。我们使用Dalton淋巴瘤的小鼠模型，并建立了体外TAMs模型。对于TAMs，我们使用承载肿瘤的腹水激活巨噬细胞，在存在或不存在脂多糖的情况下。在这些组中，我们评估了L-精氨酸代谢，并评估了褪黑激素在这些组中的效果，其中包括精氨酸的代谢命运以及iNOS和精氨酸酶1的表达。此外，在肿瘤携带宿主的体内系统中评估了褪黑激素的效果。体外TAMs模型显示出Th2细胞因子谱，减少的吞噬活性和增强的伤口愈合能力。在调查精氨酸代谢时，我们观察到TAMs中尿素水平升高，精氨酸酶1的活性和表达增加。此外，我们观察到TAMs中LPS诱导的一氧化氮水平降低；然而，它们的iNOS表达与相媲美。通过褪黑激素处理，尿素水平显著降低，而一氧化氮水平的降低不及其在TAMs中的处理时观察到的显著降低。此外，褪黑激素显著降低了精氨酸酶的活性和转录水平和转译水平的表达，而iNOS表达仅在转译水平受到影响。这一效应进一步在体内系统中进行了研究，褪黑激素处理在肿瘤微环境内逆转了精氨酸的代谢命运，从尿素向一氧化氮。这一效应进一步与体内系统中的促凋亡肿瘤细胞死亡相关联。我们的结果加强了褪黑激素的免疫调节作用，并为在癌症条件下激活抗肿瘤免疫应答提供了强有力的前景。肿瘤相关巨噬细胞(TAMs)内的L-精氨酸代谢与其受褪黑激素调控的调控。© 2023. 作者，独家授权给德国施普林格出版社，隶属于施普林格自然出版集团。

L-Arginine metabolism plays a crucial role in determining the M1/M2 polarization of macrophages. The M1 macrophages express inducible nitric oxide synthase (iNOS), while the M2 macrophages express arginase 1 and metabolize arginine into nitric oxide and urea, respectively. The tumor microenvironment promotes M2 macrophage polarization and consequently switches the metabolic fate of arginine from nitric oxide towards urea production. Importantly, infiltration of M2 macrophages or tumor-associated macrophages (TAMs) has been correlated with poor prognosis of various cancer types. Melatonin is well reported to have antitumor and immunomodulatory properties. However, whether and how it impacts the polarization of TAMs has not been elucidated. Considering the crucial role of arginine metabolism in macrophage polarization, we were interested to know the fate of L-arginine in TAMs and whether it can be reinstated by melatonin or not. We used a murine model of Dalton's lymphoma and established an in vitro model of TAMs. For TAMs, we used the ascitic fluid of tumor-bearing hosts to activate the macrophages in the presence and absence of lipopolysaccharide (LPS). In these groups, L-arginine metabolism was evaluated, and then the effect of melatonin was assessed in these groups, wherein the metabolic fate of arginine as well as the expression of iNOS and arginase 1 were checked. Furthermore, in the in vivo system of the tumor-bearing host, the effect of melatonin was assessed. The in vitro model of TAMs showed a Th2 cytokine profile, reduced phagocytic activity, and increased wound healing ability. Upon investigating arginine metabolism, we observed high urea levels with increased activity and expression of arginase 1 in TAMs. Furthermore, we observed reduced levels of LPS-induced nitric oxide in TAMs; however, their iNOS expression was comparable. With melatonin treatment, urea level decreased significantly, while the reduction in nitric oxide level was not as significant as observed in its absence in TAMs. Also, melatonin significantly reduced arginase activity and expression at the transcriptional and translational levels, while iNOS expression was affected only at the translational level. This effect was further investigated in the in vivo system, wherein melatonin treatment reversed the metabolic fate of arginine, from urea towards nitric oxide, within the tumor microenvironment. This effect was further correlated with pro-apoptotic tumor cell death in the in vivo system. Our results reinforced the immunomodulatory role of melatonin and offered a strong prospect for activating the anti-tumor immune response in cancer conditions. L-arginine metabolism in tumor-associated macrophages (TAMs) within tumor microenvironment and its modulation by melatonin.© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.