In this study, we integrated data from two large-scale GWASs to investigate the causal effects of 91 genetically proxied inflammatory cytokines on hypertrophic scar (HS) using a Mendelian randomization (MR) design. We observed a significant positive correlation between OPG and LIF levels and HS. Specifically, OPG(Arnold et al., 2022) and type I and III collagen, which are specialized forms of HS, are overproduced in keloids(Lim et al., 2009). Interestingly, OPG was present in normal keratinocyte-fibroblast cocultures but absent in keloid keratinocyte-fibroblast cocultures. Additionally, the expression of OPG may be linked to the development of degenerative changes in articular cartilage(Kaneyama et al., 2003). Furthermore, proinflammatory cytokines such as TNF-α and IL-6 may influence OPG changes(El Amrousy & El-Afify, 2020).
LIF, a member of the IL-6 cytokine family, is widely expressed across various tissue types in the body. Studies have shown that Buyang Huanwu Decoction can reduce ICH-induced glial scarring by decreasing LIF expression in rats(Kang et al., 2019). In cases of cesarean scar pregnancy, the intensity of LIF expression is significantly lower than that in the uterine cavity in the cardiac side population group and in the case of a cesarean scar in normal pregnancy(Qian et al., 2017). Moreover, increased blood levels of LIF following LIF cDNA transfection lead to cell cycle reentry of the cardiac side population, expansion of cardiac stem cells, and an increase in new cardiomyocytes, along with a reduction in the size of myocardial infarction scars(Qian et al., 2017). Additionally, LIF(Kanda et al., 2016) has been found to inhibit the infiltration of inflammatory cells and decrease the expression of the proinflammatory cytokine IL-12 while simultaneously increasing the expression of the anti-inflammatory cytokine IL-10 in injured areas of the uterine horns(Xue et al., 2019).
CDCP1, GDNF, and PD-L1 levels are significantly correlated with hypertrophic scar (HS). CDCP1, a substrate of Src family kinases, plays a role in regulating resistance to cell death when detached from the extracellular matrix (anoikis), cell migration, and breakdown of the extracellular matrix during the invasion and spread of tumors, specifically through tyrosine phosphorylation(Uekita & Sakai, 2011). Elevated levels of signal peptide-CUB-EGF-like domain-containing protein 1 (SCUBE1) were observed in adults diagnosed with acute appendicitis, indicating its potential as a novel and promising biomarker for predicting inflammation(Altuntas et al., 2023). In another study(Tokime et al., 2008), neurotrophic factors such as nerve growth factor and GDNF were suggested to be involved in the mechanism underlying the overgrowth and hypersensitivity of sensory nerves in atopic dermatitis(Huang et al., 2020). The study indicated that, along with nerve growth factor, the increased production and release of GDNF in the skin, together with an excess of IL-18, may contribute significantly to itching in atopic dermatitis patients(Noskovicova et al., 2018). Furthermore, CDCP1 levels are significantly greater in human cervical cancer tissues than in normal cervical tissues, affecting cell adhesion and fibroblast activation(Li et al., 2021). This activation, which is crucial for excessive collagen formation in abnormal scars, also influences keratinocyte migration, a process relevant to HS formation(McGovern et al., 2013).
GDNF can play a pivotal role in promoting hair follicle neogenesis and facilitating skin regeneration after wounding by influencing the fate of dermal fibroblasts. This finding implies that stimulating the GDNF signaling pathway in fibroblasts, particularly in less-regenerative animals such as humans, has the potential to promote skin regeneration, morphogenesis, and scarless wound healing(Zhang et al., 2021). The elevated production and secretion of GDNF in the skin associated with the overproduction of IL-18 may also be a potent causative factor of itching in atopic dermatitis patients, suggesting that GDNF could be a potential factor of HS-induced itching.
PD-1/PD-L1 inhibitors are promising medications for cancer therapy due to their potent effects on the adaptive immune response. BMS-202, a PD-1/PD-L1 inhibitor, effectively suppressed the proliferation, migration, and extracellular matrix deposition of HS tissues, potentially through the regulation of the ERK and transforming growth factor beta 1 (TGFβ1)/Smad signaling pathways(Cai et al., 2022). Moreover, investigations have suggested that HS, a fibroproliferative disorder of the skin with tumor-like properties, is closely associated with dysregulated inflammation. PD-L1 in fibroblast-like cells plays a positive role in the regulation of wound healing by initiating the resolution of inflammation(Wang et al., 2022).
This study represents the first Mendelian randomization study aimed at evaluating the causal relationship between hypertrophic scar (HS) and 91 inflammatory cytokines. However, several limitations need to be considered. First, due to the inherent limitations of Mendelian randomization analysis, the second and third assumptions could not be thoroughly examined, potentially introducing bias into the results. Second, our investigation relied on data from two large-scale GWASs, and subgroup analyses were not feasible due to a lack of specific demographic information and clinical records for the study patients. Third, there may be ethnic bias because the study participants were of European descent, and caution should be exercised in applying the conclusions to other racial groups. Further research is warranted to validate our findings and explore their potential application in clinical diagnosis and therapeutic interventions.