Pancreatic ductal adenocarcinoma (PDAC) has emerged as the fourth leading cause of cancer related death, which account for 7% and 8% of overall cancer mortality in man and woman, respectively(Siegel et al., 2023). With the continuous improvement of early detection and treatment, the mortality of the 4 major cancers, such as lung, breast, prostate, colorectum was declined, unfortunately, the death rate of PDAC was increased in the past years(Siegel et al., 2023). Most PDAC patients were diagnosed at advanced stage for they were asymptomatic in early stage(Mizrahi et al., 2020). Only up to 20% of patients are eligible for surgery resection and even after completed operation, the 5-year survival of PDAC patients remains only 25%(Mizrahi et al., 2020, Gillen et al., 2010). Chemotherapy with gemcitabine significantly improve the disease-free survival and overall survival of patients after resection and has been the mainstay of treatment for patients with metastasis(Oettle et al., 2013; Burris et al., 1997). But the therapeutic effect is limited due to chemotherapy resistance(Takezaki et al., 2016). Therefore, further understanding of the molecular mechanism under chemotherapy resistance may benefit patients with prolonged survival.

Circular RNAs (circRNAs) are a novel class of noncoding RNAs, which is formed by single-stranded RNA molecules with covalent binding. With the development of bioinformatics and high-throughput sequencing technology, a large number of circRNAs were discovered and identified. The characteristic covalently closed loop structure enables the circRNAs to tolerate the digestion of RNA exonucleases or RNase R( J.T. Granados-Riveron, Aquino-Jarquin., 2016; Jeck et al., 2013). Consequently, circRNAs are more stable than their linear counterparts and may serve as diagnostic biomarkers and therapeutic targets(Jeck et al., 2013). Previous studies have demonstrated that circRNAs have diverse functions such as miRNA sponge, interacting with RNA-binding proteins (RBPs), regulating transcription and alternative splicing(Li et al., 2018). Due to the diversity of their functions, circRNAs play important roles in human diseases including neurological diseases, cardiovascular diseases, cancers, etc.(Han et al., 2018). Previous studies have emphasized the importance of circRNAs in the progression of multiple cancers(Kristensen et al., 2022), but their roles in PDAC progression are unclear and remain to be investigated.

MEK/ERK signaling pathway is the pivotal component of mitogen-activated protein kinase (MAPK) cascades which is involved in multiple biological processes including cell proliferation, cell cycle, gene expression, cell metabolism, cell motility and apoptosis(Roberts and CJ., 2007). Both MEK and ERK have two identified isoforms with high sequence conversation and functional similarity, and referred to as MEK1/2 and ERK 1/2(Manning et al., 2002). The highly narrow substrate specificity of MEK1/2 make it only have one sole downstream target, ERK1/2, which can be phosphorylated and activated by active MEK1/2(Roskoski., 2012). Excessive activation of MEK/ERK pathway occurs in various diseases, including cancer, inflammation, developmental disorders and neurological disorders(Guo et al., 2020). Furthermore, activation and dysfunction of MEK/ERK pathway is the most abundant oncogenic factor and major trigger of almost all cancer types(Khotskaya et al., 2017; Garcia-Gomez et al., 2018).

Tripartite motif (TRIM) proteins mediate the ubiquitylation of eukaryotic cells in post-translational modifications and this ubiquitylation eliminates the short-lived regulatory proteins contributing to cell cycle regulation, cellular signaling, DNA repair, morphogenesis, protein quality control and transcriptional regulation(Hatakeyama, 2011). Previous studies have demonstrated that some members TRIM play vital roles in carcinogenesis. TRIM29 protein is encoded by the ATDC gene which is located at chromosome 11q23. Unlike most other TRIM proteins, TRIM29 has no E3 ubiquitin ligase activity due to the lack of RING domain(Richard et al., 1993). Additionally, TRIM29 leads to p53 export from the nucleus via directly binding to p53 and overexpression of TRIM29 inhibits acetylation of p53 via TIP60 degradation(Sho et al., 2011; Yuan et al., 2010). Collectively, these findings emphasize the vital roles of TRIM29 in the pathological process of cancers.

In the present study, we find TRIM29 is upregulated in PDAC and TRIM29 overexpression is associated with gemcitabine resistance and MEK/ERK signaling pathway. Functional studies demonstrate that TRIM29 enhances GEM resistance of PDAC cells. Mechanistic investigations suggest TRIM29 is regulated by circRPS29 via sponging miR-770-5p, which is confirmed by dual luciferase reporter assays, fluorescent in situ hybridization (FISH), circRNA immunoprecipitation (circRIP) and RNA pull-down assays. Gain- and loss-of-function experiments further demonstrate that circRPS29/miR-770-5p/TRIM29 axis regulates GEM resistance, proliferation, and metastasis of PDAC cells via activating MEK/ERK signaling pathway. More importantly, rescue assays in vivo reveal that circRPS29/miR-770-5p axis promotes tumor growth via upregulating TRIM29 expression and activating ERK1/2.

Collectively, the findings of this study reveal a novel signaling cascade in PDAC progression and provide new insights to explore the potential therapeutic targets of PDAC.