Secondary resistance to imatinib (IM) represents a major challenge for therapy of gastrointestinal stromal tumors (GISTs). Aberrations in oncogenic pathways, including autophagy, correlate with IM resistance. Regulation of autophagy-related protein 5 (ATG5) by the ubiquitin-proteasome system is critical for autophagic activity, although the molecular mechanisms that underpin reversible deubiquitination of ATG5 have not been deciphered fully. Here, we identified USP13 as an essential deubiquitinase that stabilizes ATG5 in a process that depends on the PAK1 serine/threonine-protein kinase and which enhances autophagy and promotes IM resistance in GIST cells. USP13 preferentially is induced in GIST cells by IM and interacts with ATG5, which leads to stabilization of ATG5 through deubiquitination. Activation of PAK1 promoted phosphorylation of ATG5 thereby enhancing the interaction of ATG5 with USP13. Furthermore, N⁶-methyladenosine methyltransferase-like 3 (METTL3) mediated stabilization of USP13 mRNA that required the m⁶A reader IGF2BP2. Moreover, an inhibitor of USP13 caused ATG5 decay and co-administration of this inhibitor with 3-methyladenine boosted treatment efficacy of IM in murine xenograft models derived from GIST cells. Our findings highlight USP13 as an essential regulator of autophagy and IM resistance in GIST cells and reveal USP13 as a novel potential therapeutic target for GIST treatment.

Gastric cancer (GC) ranks third in mortality among all cancers worldwide. Circular RNAs (circRNAs) play an important role in the occurrence and development of gastric cancer. Forkhead box P2 (FOXP2), as a transcription factor, is closely associated with the development of many types of tumours. However, the regulatory network between FOXP2 and circRNAs remains to be explored. In our study, circST3GAL6 was significantly downregulated in GC and was associated with poor prognosis in GC patients. Overexpression of circST3GAL6 inhibited the malignant behaviours of GC cells, which was mediated by inducing apoptosis and autophagy. In addition, we demonstrated that circST3GAL6 regulated FOXP2 through the mir‐300 sponge. We further found that FOXP2 inhibited MET Proto‐Oncogene (MET), which was the initiating factor that regulated the classic AKT/mTOR pathway of autophagy. In conclusion, our results suggested that circST3GAL6 played a tumour suppressive role in gastric cancer through miR‐300/FOXP2 axis and regulated apoptosis and autophagy through FOXP2‐mediated transcriptional inhibition of the MET axis, which may become a potential target for GC therapy.

N⁶-methyladenosine (m⁶A) is a frequently occurring mRNA modification, which regulates mRNA stability, splicing, and translation. However, its role in drug resistance of gastrointestinal stromal tumor (GIST) is not known. Here, we report that m⁶A modification levels are elevated in imatinib-resistant GIST cells and tissues, and that methyltransferase METTL3 is one of the main protein responsible for this aberrant modification. Increased METTL3 levels contributed to imatinib resistance and worse progression-free survival of GIST patients. Mechanistic studies revealed that METTL3-mediated m⁶A modification of the 5'UTR of the multidrug transporter MRP1 mRNA promoted drug resistance of GIST by stimulating MRP1 mRNA translation, via binding with YTHDF1 and eEF-1. Further, METTL3 transcription in imatinib resistant GIST cells was activated by ETV1, leading to the increased m⁶A methylation of MRP1 mRNA. This is the first report showing a novel regulatory mechanism whereby ETV1, METTL3, and the YTHDF1/eEF-1 complex mediate the translation of MRP1 mRNA in an m⁶A-dependent manner to regulate the intracellular concentration of imatinib and drug resistance of GIST. These findings highlight MRP1 as a new potential therapeutic target for imatinib resistance of GIST.

The pentose phosphate pathway (PPP) plays a critical role in maintaining cellular redox homeostasis in tumor cells and macromolecule biosynthesis. Upregulation of the PPP has been shown in several types of tumor. However, how the PPP is regulated to confer selective growth advantages on drug resistant tumor cells is not well understood. Here we show a metabolic shift from tricarboxylic acid cycle (TCA) to PPP after a long period induction of Imatinib (IM). One of the rate-limiting enzymes of the PPP-phosphogluconate dehydrogenase (PGD), is dramatically upregulated in gastrointestinal stromal tumors (GISTs) and GIST cell lines resistant to Imatinib (IM) compared with sensitive controls. Functional studies revealed that the overexpression of PGD in resistant GIST cell lines promoted cell proliferation and suppressed cell apoptosis. Mechanistic analyses suggested that the protein level of hypoxia inducible factor-1α (HIF-1α) increased during long time stimulation of reactive oxygen species (ROS) produced by IM. Importantly, we further demonstrated that HIF-1α also had positive correlation with PGD, resulting in the change of metabolic pathway, and ultimately causing drug resistance in GIST. Our findings show that long term use of IM alters the metabolic phenotype of GIST through ROS and HIF-1α, and this may contribute to IM resistance. Our work offers preclinical proof of metabolic target as an effective strategy for the treatment of drug resistance in GIST.

The role of methyltransferase-like 3 (METTL3), which participates in catalyzing N-methyladenosine (m6A) RNA modification, in gastric cancer (GC) is unclear. Here, we found that METTL3 was overexpressed in human GC. Functionally, we verified that METTL3 promoted tumor cell proliferation and angiogenesis through a series of phenotypic experiments. Subsequently, ADAMTS9 was identified as the downstream effector of METTL3 in GC, which could be degraded by the YTHDF2-dependent pathway. Finally, the data suggested that METTL3 might facilitate GC progression through the ADAMTS9-mediated PI3K/AKT pathway. Our study unveiled the fundamental mechanisms of METTL3 in GC progression. The clinical value of METTL3 in GC deserves further exploration.

In this noninferiority study, personalized supine-prone MRI with zoomed diffusion-weighted imaging and free-breathing dynamic contrast-enhanced imaging had higher accuracy than dual-energy CT for local-regional staging of gastric cancer.