Citrullination and the protein code: crosstalk between post-translational modifications in cancer

PADI1 aggravates endoplasmic reticulum stress in trophoblast cells by inhibiting the FAK-ERK signaling pathway

Preeclampsia (PE) is a multisystemic syndrome of pregnancy that seriously affects maternal and infant healthcare. Here, we identified PADI1 as an abnormally highly expressed gene in PE and investigated its effect on trophoblast cells. According to the analysis results from GEO datasets GSE186257 and GSE143953, PADI1 is highly expressed in the placental samples of PE patients. PADI1 knockdown promoted the growth and migration of trophoblast cells. HTR-8 and Swan-71 cells were treated with 200 nmol/L TG to simulate ERS in vitro. PADI1 knockdown inhibited TG-induced ERS and apoptosis. Mechanistically, PADI1 knockdown downregulated ERS-related factors including IRE1α, XBP1, CHOP, ATF6 and GRP78. The FAK/ERK1/2 signaling pathway has been identified as a downstream target of PADI1 in trophoblast cells. Inhibition of FAK/ERK1/2 effectively hindered the enhancement of cell activity by PADI1 knockdown in TG-treated trophoblast cells. In conclusion, PADI1 was highly expressed in the placental tissues of PE patients. PADI1 knockdown inhibited the ERS-induced apoptosis in trophoblast cells through FAK/ERK1/2 signaling pathway, suggesting the potential role of PADI1 in PE prevention and treatment.

Placental Protein Citrullination Signatures Are Modified in Early- and Late-Onset Fetal Growth Restriction

Fetal growth restriction (FGR) is an obstetric condition most frequently caused by placental dysfunction. It is a major cause of perinatal morbidity with limited treatment options, so identifying the underpinning mechanisms is important. Peptidylarginine deiminases (PADs) are calcium-activated enzymes that mediate post-translational citrullination (deimination) of proteins, through conversion of arginine to citrulline. Protein citrullination leads to irreversible changes in protein structure and function and is implicated in many pathobiological processes. Whether placental protein citrullination occurs in FGR is poorly understood. We assessed protein citrullination and PAD isozyme abundance (PAD1, 2, 3, 4 and 6) in human placental samples from pregnancies complicated by early- and late-onset FGR, compared to appropriate-for-gestational-age (AGA) controls. Proteomic mass spectrometry demonstrated that the placental citrullinome profile changed in both early- and late-onset FGR, with 112 and 345 uniquely citrullinated proteins identified in early- and late-onset samples, respectively. Forty-four proteins were citrullinated only in control AGA placentas. The proteins that were uniquely citrullinated in FGR placentas were enriched for gene ontology (GO) terms related to neurological, developmental, immune and metabolic pathways. A greater number of GO and human phenotype pathways were functionally enriched for citrullinated proteins in late- compared with early-onset FGR. Correspondingly, late-onset but not early-onset FGR was associated with significantly increased placental abundance of PAD2 and citrullinated histone H3, determined by Western blotting. PAD3 was downregulated in early-onset FGR while abundance of PAD 1, 4 and 6 was less altered in FGR. Our findings show that placental protein citrullination is altered in FGR placentas, potentially contributing to the pathobiology of placental dysfunction.

Insights into the complexities of Citrullination: From immune regulation to autoimmune disease

Citrullination, a post-translational modification that changes arginine to citrulline in proteins, is vital for immune response modulation and cell signaling. Catalyzed by peptidyl arginine deiminases (PADs), citrullination is linked to various diseases, particularly autoimmune disorders like rheumatoid arthritis (RA). Citrullinated proteins can trigger the production of anti-citrullinated protein antibodies (ACPAs), included in RA classification criteria. The immune response to citrullination involves both innate and adaptive immunity, affecting monocytes/macrophages, neutrophils, dendritic cells, natural killer cells, B cells, and T cells. Citrullination contributes to disease development in RA and other conditions such as multiple sclerosis, sepsis, and cancer. Therapeutic strategies targeting citrullination and its effects are being explored, including B cell depletion therapies, T cell-directed approaches, PAD inhibitors, and citrullinated peptide-based vaccines. Understanding the interplay between citrullination and the immune system may lead to novel diagnostic tools and targeted therapies for autoimmune diseases and beyond.

Association of PADI4 Gene Polymorphisms With Susceptibility to Rheumatoid Arthritis: Evidence From 24 Case-Control Studies

This study aims to investigate the association of rs11203366, rs11203367, rs874881, rs2240340 and rs1748033 polymorphisms of protein-arginine deiminase type 4 (PADI4) gene with the risk of rheumatoid arthritis (RA) through a meta-analysis that was followed with a bioinformatics approach. The data were collected from reputable articles and underwent quantitative analysis, followed by in silico analysis using some bioinformatics tools. The results showed that rs874881 polymorphism in Latino (G vs. C: OR = 1.35, 95% CI = 1.11-1.65, p = 0.003; GG + CG vs. CC: OR = 2.02, 95% CI = 1.41-2.89, p = 0.0001; CG vs. CC + GG: OR = 1.38, 95% CI = 1.04-1.83, p = 0.027; GG vs. CC: OR = 2.09, 95% CI = 1.35-3.23, p = 0.001; CG vs. CC: OR = 1.98, 95% CI = 1.36-2.87, p = 0.00033) and rs1748033 in Caucasian population (T vs. C: OR = 1.25, 95% CI = 1.07-1.45, p = 0.005; TT vs. CT + CC: OR = 1.34, 95% CI = 1.09-1.64, p = 0.005, TT + CT vs. CC: OR = 1.26, 95% CI = 1.09-1.44, p = 0.001; TT vs. CC: OR = 1.59, 95% CI = 1.13-2.23, p = 0.007; CT vs. CC: OR = 1.20, 95% CI: 1.04-1.39, p = 0.015) are associated with increased risk of RA. Moreover, rs11203366 (G vs. A: OR = 1.46, 95% CI = 1.19-1.78, p = 0.0002, GG vs. AG + AA: OR = 1.42, 95% CI = 1.01-2.01, p = 0.043; GG + AG vs. AA: OR = 2.03, 95% CI = 1.45-2.86, p = 0.00004; GG vs. AA: OR = 2.29, 95% CI = 1.49-3.51, p = 0.0002; AG vs. AA: OR = 1.93, 95% CI = 1.35-2.76, p = 0.0003) and rs11203367 (T vs. C: OR = 1.50, 95% CI = 1.23-1.83, p = 0.00007; TT vs. CT + CC: OR = 1.56, 95% CI = 1.12-2.18, p = 0.009; TT + CT vs. CC: OR = 2.02, 95% CI = 1.43-2.84, p = 0.00007, TT vs. CC: OR = 2.43, 95% CI = 1.59-3.71, p = 0.0004; CT vs. CC: OR = 1.86, 95% CI = 1.30-2.68, p = 0.0007) had an impact in the Latino population. Bioinformatics tools showed the effect of these polymorphisms on gene function. These findings suggest that rs11203366, rs11203367, rs874881 and rs1748033 polymorphisms may be genetic risk factors for RA. Moreover, differences between populations suggest that ethnicity may play an important role in the effect of these polymorphisms on RA risk.

Roles of post-translational modifications of UHRF1 in cancer

UHRF1 as a member of RING-finger type E3 ubiquitin ligases family, is an epigenetic regulator with five structural domains. It has been involved in the regulation of a series of biological functions, such as DNA replication, DNA methylation, and DNA damage repair. Additionally, aberrant overexpression of UHRF1 has been observed in over ten cancer types, indicating that UHRF1 is a typical oncogene. The overexpression of UHRF1 repressed the transcription of such tumor-suppressor genes as CDKN2A, BRCA1, and CDH1 through DNMT1-mediated DNA methylation. In addition to the upstream transcription factors regulating gene transcription, post-translational modifications (PTMs) also contribute to abnormal overexpression of UHRF1 in cancerous tissues. The types of PTM include phosphorylation, acetylation, methylationand ubiquitination, which regulate protein stability, histone methyltransferase activity, intracellular localization and the interaction with binding partners. Recently, several novel PTM types of UHRF1 have been reported, but the detailed mechanisms remain unclear. This comprehensive review summarized the types of UHRF1 PTMs, as well as their biological functions. A deep understanding of these crucial mechanisms of UHRF1 is pivotal for the development of novel UHRF1-targeted anti-cancer therapeutic strategies in the future.

Orthogonal Translation for Site-Specific Installation of Post-translational Modifications

Post-translational modifications (PTMs) endow proteins with new properties to respond to environmental changes or growth needs. With the development of advanced proteomics techniques, hundreds of distinct types of PTMs have been observed in a wide range of proteins from bacteria, archaea, and eukarya. To identify the roles of these PTMs, scientists have applied various approaches. However, high dynamics, low stoichiometry, and crosstalk between PTMs make it almost impossible to obtain homogeneously modified proteins for characterization of the site-specific effect of individual PTM on target proteins. To solve this problem, the genetic code expansion (GCE) strategy has been introduced into the field of PTM studies. Instead of modifying proteins after translation, GCE incorporates modified amino acids into proteins during translation, thus generating site-specifically modified proteins at target positions. In this review, we summarize the development of GCE systems for orthogonal translation for site-specific installation of PTMs.