To our knowledge, the term erectile dysfunction (ED) refers to the inability to achieve and sustain a sufficient penile erection for satisfactory sexual intercourse (Bacon et al., 2003; Bohm et al., 2007). The condition is prevalent among males in the middle-aged and elderly populations (Shiri et al., 2006), with an estimated prevalence rate ranging from 25% to 35% (Gleason et al., 2011). The projected global incidence of ED is expected to reach 322 million cases by 2025, as per forecasts (Ayta et al., 1999). Therefore, the prevention and early intervention for the risk factors of ED are of significant importance and require a sense of urgency.

The pathophysiology of ED involves the interplay of various factors, encompassing vasculogenic, neurogenic, hormonal, iatrogenic, anatomical, and psychogenic influences (Corona et al., 2006). Several studies show that the presence and seriousness of ED are related to endothelial dysfunction and markers and mediators of subclinical inflammation (Vlachopoulos et al., 2007). Previous studies show that patients with ED exhibit elevated levels of inflammatory markers, encompassing adhesion molecules, inflammatory cytokines, and endothelial-prothrombotic markers (Vlachopoulos et al., 2006; Chiurlia et al., 2005; Arana Rosainz Mde et al., 2011; Bocchio et al., 2004; Meldrum et al., 2012; Esposito et al., 2005). The severity of ED has been consistently associated with the regulation of inflammation, as demonstrated by numerous clinical and preclinical studies. Circulating inflammatory proteins play a significant role in the pathogenesis of ED, exerting both beneficial and deleterious effects. Suvanlli et al. reported that plasma levels of high-sensitivity CRP and fibrinogen exhibited an elevation in patients with ED when compared to individuals without ED (Sullivan et al., 2001). Another study by Zuo et al. revealed that tumor necrosis factor (TNF-α) inhibits the expression of endothelial NO synthase in endothelial cells (Anderson et al., 2004; Neumann et al., 2004; Yoshizumi et al., 1993). The injection of senescent cells, which produced proinflammatory-cytokines TNF-α and interleukin (IL)-1β, resulted in impaired erectile response in mouse models by inducing endothelial dysfunction and nerve injury (Nishimatsu et al., 2015). Previous evidence suggests that there is an inverse correlation between the International Index of Erectile Function 5(IIEF-5) score, which measures sexual performance, and circulating inflammatory parameters, such as IL-6 and IL-1 (Vlachopoulos et al., 2006). The serum levels of inflammatory factors such as IL-1, IL-2, and IL-10 in type 2 diabetic mellitus rats with ED were significantly elevated compared to those in control rats during preclinical animal experiments (Li et al., 2017). These aforementioned studies emphasize the pathophysiological involvement of inflammatory proteins and endothelial dysfunction in the etiology of ED. Nevertheless, whether inflammation factors are the cause of ED or due to drug usage after disease progression remains controversial. Further studies are essential to elucidate the causal associations between systemic inflammatory proteins and ED.

Concerning the susceptibility of observational studies to lack control bias due to underlying confounders and reverse causality (Zhao G. et al., 2023), Mendelian randomization (MR) was employed to evaluate the causality between exposure and outcome. This analysis approach, based on genome-wide association studies (GWAS), is a robust method for constructing instrumental variables (IVs) and inferring causal effects between exposure and outcome in situations where establishing causality is challenging due to the retrospective and observational nature of the study (Ren et al., 2023). The evidence of causality provided by the MR research was reported to lie at the interface between conventional epidemiological studies and randomized controlled trials (RCTs) (Chen et al., 2023). For instance, a bidirectional MR study found that targeted interventions of specific inflammatory proteins could mitigate the risk of meningiomas (Zhang et al., 2023). In addition, another MR study suggested that genetically increased specific inflammatory regulator levels demonstrated a protective effect against the risk of ischemic stroke (Li et al., 2021). In view of the correlations between inflammatory proteins and ED and the clinical efficacy of traditional Chinese medicine (TCM) on ED, we speculate whether certain TCM monomeric compounds can achieve efficacy in the treatment of ED by interfering with these specific inflammatory proteins. Therefore, in this study, we utilized multi-omics approach and MR study to explore potential drug targets by integrating GWAS and protein quantity trait loci (pQTL) data. Additionally, through the utilization of molecular docking technology, we screen the eligible TCM monomeric compounds for regulating inflammation in the treatment and prevention of ED.

In this study, we firstly conducted multi-omics association study integrating GWAS and pQTL data to explore whether genetic evidence supported a causality of systemic inflammatory proteins with the risk of ED. Understanding the impact of inflammatory regulators in ED will provide us with further insights into the role inflammation plays in both the initiation and progression of ED. We observed a positive association between genetically predicted MIP-1α levels and the risk of ED, thereby providing molecular and epidemiological evidence supporting the potential role of targeted therapeutic drugs that regulate underlying inflammation in the treatment and prevention of ED. Additionally, we did not find any evidence indicating a causal association between ED and specific inflammatory protein through conducting reverse MR analysis. Meanwhile, for the candidate key protein MIP-1α, we utilized molecular docking technology to identify four anti-inflammatory or tonifying small molecular compounds, encompassing echinacea, pinoresinol diglucoside, hypericin, and icariin, which may be promising therapeutic targets for ED in the future.

To the best of our understanding, ED is an evolving health issue that causes sexual dysfunction in males (Zhu et al., 2023). The etiologies of ED encompass a range of factors, including endothelial dysfunction, atherosclerosis, and chronic inflammation (Li et al., 2018). Inflammation plays a significant pathophysiological role in the initiation and development of ED (Vlachopoulos et al., 2015). The presence of chronic low-grade inflammation plays a pivotal role in the pathogenesis of ED and is likely to be recognized as an intermediary stage for endothelial dysfunction (Kaya-Sezginer and Gur, 2020). Through augmented production of circulating proteins and upregulation of cellular adhesion molecules, the dysfunctional endothelium facilitates inflammation within the vascular wall, thereby establishing a conducive environment for the initiation and progression of atherosclerotic lesions in both penile vasculature and peripheral blood vessels (Vlachopoulos et al., 2007). Inflammation has been regarded as a shared factor in the pathogenesis of both ED and metabolic disorders. The inflammatory process of ED involves a complex modulation of cytokines, chemokines, and adhesion molecules (Kaya-Sezginer and Gur, 2020).

Macrophage inflammatory protein-1 alpha (MIP-1α), also referred to as CCL3, is a chemotactic cytokine possessing proinflammatory properties (Wu et al., 2019). MIP-1α has been reported to be vital for mediating inflammation responses (Noh et al., 2014; Zhang et al., 2016). MIP-1α could enhance inflammatory responses and augment the secretion of proinflammatory cytokines, such as IL-1β, TNF-α, and IL-6, which were synthesized by M1 macrophages (Chen et al., 2017). There are many studies on the correlations between MIP-1α and many relevant diseases of the genitourinary system. A previous animal experimental study conducted by Najjar et al. (2017) demonstrated that the accumulation of a specific subset of myeloid-derived suppressor cells in the renal cell carcinoma parenchyma was positively correlated with the intratumoral expression of MIP-1α. Another study conducted by Lopes and Callera (2012) suggested that the levels of IL-5, IL-4, IL-2, TNF-α, and MIP-1α were not significantly affected by irradiation in three-dimensional conformal radiotherapy of prostate cancer, while the levels of IL-6 seemed to reach their highest point after a duration of 15 days of radiotherapy. This evidence indicated that MIP-1α might be involved in the initiation and progression of urologic neoplasms. What’s more, the data from current research also revealed a positive association between serum levels of MIP-1α and IP-10, and this finding indicated that chemokines were involved in urinary tract inflammation of children (Gorczyca et al., 2014). However, the epidemiological evidence for the association between MIP-1α and ED was few, limited by small sample sizes, and restricted by using a case-control study design. In the present study, we observed that a higher genetically circulating level of MIP-1α was found to be significantly associated with an increased risk of ED. The evidence from experimental studies supported our findings. For instance, previous research conducted by P. Lu et al. reported that MIP-1α induced macrophages to infiltrate and produce VEGF by binding to CCR5 and eventually promoted angiogenesis (Lu et al., 2008). Although these studies demonstrated an association between circulating levels of MIP-1α and the risk of ED, the findings from these observational studies remained inconclusive and ambiguous. This situation was primarily attributed to the limited sample size and underlying confounding variables. Notably, our reverse MR analysis showed no associations between the risk of ED and serum MIP-1α levels. Thus, these results indicated that MIP-1α might be regarded as a potential therapeutic target for ED development. Nevertheless, given the indeterminacy of the findings, more studies are needed to validate the precise underlying biological mechanism of MIP-1α in the initiation and development of ED.

In our study, the associations between multiple inflammatory proteins and ED were assessed utilizing MR analysis. So far, several studies have explored the associations between partial inflammatory proteins and the risk of ED. Previously, J.K.Akintunde et al. reported that the erectile dysfunction caused by inflammation was associated with the alteration of NO-cGMP-dependent PKG signaling cascade through the upregulation of key cytokines and chemokine (MCP-1). Monocyte Chemoattractant Protein (MCP-1) could be considered for the management of inflammatory-mediated erectile dysfunction (IMED) (Akintunde et al., 2023). A previous study conducted by Unsworth et al. (2020) demonstrated that MCP-1 activation led to the expression of NF-kB, whereas inhibition of NF-kB has been shown to prevent inflammatory responses of TNF-α and MCP-1 in humans. These studies indicate that MCP-1 is a protective factor against ED, which goes in the same direction as our findings, although insignificant in statistics. Additionally, Vlachopoulos et al. (2006) conducted the research suggested that ED was related to elevated levels of circulating inflammatory proteins and endothelial-prothrombotic regulators. In his study, he found the presence of ED to be effectively ruled out by the detection of low levels of IL-6 and fibrinogen, as these simple biochemical substances demonstrated a satisfactory negative predictive value for ED. However, the findings of our study did not reveal any statistically significant impact of IL-6 on ED. This inconsistency in the results may be attributed to variations in sample size and diverse prevalence rates of ED across different populations. The role of TNF-α in inflammation-related ED is crucial, as it leads to an increase in arterial reactive oxygen species (ROS) and a reduction in NO levels (Carneiro et al., 2010). Hayward et al. reported that mice exhibiting human TNF-α overexpression manifested a reduction in erectile responses (Hayward et al., 2007). Moreover, the clinical research indicates an inverse association between TNF-α and sexual performance, with ED patients exhibiting higher serum levels of TNF-α independent of comorbid conditions (Karadag et al., 2007). These studies indicate that TNF-α exerts deleterious effects on erectile function. Intriguingly, for the association of TNF-α levels on the risk of ED, our findings suggest that there is no significant association between genetically predicted TNF-α and ED. These variations could be attributed to the diverse choice of IVs and GWAS summary data.

As previous studies described, the utilization of MR study has been introduced as a strategic approach to establish robust evidence for causal association between a biomarker and a disease (Mokry et al., 2015). Our study suggested that MIP-1α might be regarded as a potential therapeutic target or a biomarker for ED. The genetic factors that influence the biomarker will impact disease progression when the biomarker is a causal factor in disease progression (Khasawneh et al., 2022). Therefore, the identification of causal relationships between biomarkers and diseases by MR analysis renders them viable candidates for drug design (Khasawneh et al., 2022). However, examples are scarce in the literature regarding prospective studies that have utilized MR for drug development. In our study, we first employed molecular docking techniques to validate the interactions between the hub proteins MIP-1α and anti-inflammatory or tonifying compounds, as well as to screen potential therapeutic targets for ED. Through molecular docking, we found that the important active ingredients of echinacea, pinoresinol diglucoside, hypericin, and icariin combined well with the hub targets MIP-1α.

Inevitably, our study was subject to several limitations. First, the selection of IVs was conducted using a relaxed significance threshold of P < 5e-06, and this might lead to biased findings and false-positive variants. Nonetheless, the F-statistics of IVs all exceeded 10, indicating minimal presence of weak instrument bias. Similarly, several previous studies have also utilized the identical threshold (P < 5e-06) when assessing the associations between 41 inflammatory regulators and hypothyroidism (Lai et al., 2023). Second, the participants of GWAS included in the present study were limited to European descent, and it remains highly questionable whether the same results can be extrapolated to other races. Third, although we cannot eliminate the likelihood of pleiotropy, we have taken measures to exclude SNPs associated with potential confounding factors and performed multiple sensitivity analyses with different assumptions, such as MR-Egger regression. Fourth, after applying the Bonferroni correction, no cytokines demonstrated statistically significant correlations with ED, and only one of them (MIP-1α) showed suggestive associations. Fifth, when we explored the associations of IL-1rα and IL-8 with ED, the MR-Egger or weight-median estimates were significant, but the higher statistical power IVW approach was not statistically significant, this inconsistency could be attributed to the different choice of IVs and GWAS summary data. Lastly, the activities of inflammatory proteins are intricate, particularly those that exhibit high pleiotropy and possess the ability to exert their effects on diverse cell types. Additionally, circulating inflammatory proteins rarely demonstrate their effects alone but rather perform in regulatory networks (Ollier, 2004). Thus, these underlying associations need to be validated in larger cohorts and the potential involvement of cytokines in the regulation of ED development should be further investigated in future studies.

To sum up, our study provided support for the underlying causal association between circulating inflammatory protein MIP-1α and ED. Additionally, we utilized the molecular docking technology to identify four tonifying small molecular compounds, encompassing echinacea, pinoresinol diglucoside, hypericin, and icariin. Nevertheless, in the future, further investigations are warranted to validate these findings, elucidate potential biological mechanisms, and assess their utility as biomarkers and potential therapeutic targets for ED development.