Alternative splicing regulators have emerged as new players in cancer development, modulating the activities of many tumor suppressors and oncogenes and regulating the signaling pathways. However, little is known about the mechanisms by which these oncogenic splicing factors lead to cellular transformation. We have shown previously that the splicing factor serine and arginine splicing factor 1 (SRSF1; SF2/ASF) is a proto-oncogene, which is amplified in breast cancer and transforms immortal cells when overexpressed. In this study, we performed a structure-function analysis of SRSF1 and found that the RNA recognition motif 1 (RRM1) domain is required for its oncogenic activity. Deletion of RRM1 eliminated the splicing activity of SRSF1 on some of its endogenous targets. Moreover, we found that SRSF1 elevates the expression of B-Raf and activates the mitogen-activated protein kinase kinase (MEK) extracellular signal-regulated kinase (ERK) pathway and that RRM1 is required for this activation as well. B-Raf-MEK-ERK activation by SRSF1 contributes to transformation as pharmacological inhibition of MEK1 inhibits SRSF1-mediated transformation. In conclusion, RRM1 of SRSF1 is both required (and when tethered to the RS domain) also sufficient to activate the Raf-MEK-ERK pathway and to promote cellular transformation.

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Purpose: Pancreatic ductal adenocarcinoma (PDAC) is remarkably resistant to standard modalities, including radiotherapy (RT). The mechanisms of radiation resistance in general, and pancreatic cancer in particular, are poorly understood. We hypothesized that metabolic reprogramming may underlie this radioresistance, and moreover, that it would be possible to exploit these changes in metabolism for therapeutic intent. Experimental Design: We established multiple isogenic models of radioresistant PDAC cells. Metabolic profile was investigated using Nanostring technology, labeled-glucose tracing by liquid chromatography-mass spectrometry, Seahorse analysis and exposure to metabolic inhibitors. Patient-derived xenografts (PDXs) were established from patients treated with radiation and RNA sequencing performed. The PDXs were grouped according to clinical RECIST response to radiation (responsive/stable disease vs disease progression) and differential gene expression analysis was performed. Results: The radioresistant cells overexpressed pyruvate dehydrogenase kinase (PDK) and were radiosensitized by the PDK inhibitor dichloroacetate. In keeping with PDK overexpression, radioresistant cells displayed increased glycolysis and downregulated both the tricarboxylic acid cycle and oxidative phosphorylation. Metabolic flux through the pentose-phosphate pathway (PPP) was increased, as were levels of reduced glutathione; PPP inhibition dramatically potentiated radiation-induced cell death. Critically, the PPP was upregulated in PDXs derived from patients who demonstrated clinical resistance to radiotherapy. High transcription levels of 6PGD, the rate-limiting enzyme of the PPP, were associated with a poor radiological response to radiation therapy (p=0.0004) and a lower overall survival (p=0.004). Conclusions: We demonstrate that radioresistant PDAC cells divert the glycolytic flux from the tricarboxylic acid cycle and oxidative phosphorylation to the PPP, thereby increasing their antioxidant capacity and promoting nucleotide synthesis for DNA repair. Furthermore, we show that PDAC cells can be radiosensitized via PPP inhibition. Exploitation of metabolic vulnerabilities to radiosensitize tumors constitutes a novel approach to pancreatic cancer with a real potential to improve clinical outcomes. Citation Format: Ariel Shimoni-Sebag, Ifat Abramovich, Bella Agranovich, Yaarit Sirovsky, Chani Stossel, Dikla Atias, Maria Raitses-Gurevich, Yulia Glick-Gorman, Ofer Margalit, David Regev, Rotem Tal, Itay Tirosh, Talia Golan, Keren Yizhak, Eyal Gottlieb, Yaacov R. Lawrence. The pentose-phosphate pathway induces pancreatic cancer radioresistance, a preclinical study with clinical validation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2411.

High levels of stromal tumoreinfiltrating lymphocytes (sTILs) are significantly associated with chemosensitivity and improved overall survival for patients with triple negative breast cancer (TNBC). Our aim is to determine if we can significantly increase sTILs from biopsy to lumpectomy in a presurgical window of opportunity trial with pembrolizumab (pembro) and intraoperative radiation (IORT) in treatment-naïve early stage TNBC compared to matched IORT only controls. The hypothesis is that preoperative pembro treatment in TNBC patients will increase the density and change the spatial distribution of TILs from biopsy to lumpectomy. Materials/Methods: Seven women with node negative TNBC <2.5 cm were enrolled in either our presurgical trial with pembro and IORT as an upfront boost (NCT02977468) or a parallel study with IORT alone (NCT03165487). The treatment consisted of two doses of pre-operative pembro (200mg IV) given alone with the second dose-10 to-18 days from surgery; 20Gy IORT was delivered during surgery to the lumpectomy bed. All patients completed standard of care adjuvant chemotherapy and whole breast irradiation. Assessments of sTILs and intraepithelial TILs (iTILs) was performed by H&E and qmIF from the initial biopsy and lumpectomy. Deep learning algorithms for tumor annotation and lymphocyte detection were used also to measure of iTILs. Spatial clustering analysis to measure the mean dispersion with iTILs was evaluated by calculating Ball-Hall index. Transcriptomic analysis of tumor-associated and immune genes was performed using the slide-mounted tissue. Results: Seven patients were enrolled between Nov 2017-Apr 2019; 3 patients received pembro+IORT; and 4 patients received IORT alone. Initial H&E pathology assessment of sTILs demonstrated no significant changes between biopsy and lumpectomy in control patients. No consistent patterns of iTILs dispersion were observed in the control patients. Out of the 3 pembro-treated patients, one patient demonstrated a marked increase in sTILs with a shift from a neutrophil-rich to lymphoplasmacytic stromal infiltrate, where the mean dispersion of iTILs doubled. The 2 other pembro patients, one remained stable and one showed a slight increase in sTILs. The iTILs dispersion increased significantly in these patients. QmIF analysis showed overall increase in stromal immune cell infiltrate in all cases after Pembro treatment, with an increase in stromal CD68+ macrophage noted in all cases. Transcriptomic analysis showed increased expression of CD68 and CD163 after pembro treatment. Conclusion: In this preliminary analysis, we observe preoperative pembroinduced increases in sTILs with increased mean dispersion of iTILs in treatment-naïve TNBC. Further investigation is warranted to elucidate the impact of preoperative pembro in early stage TNBC and other potential therapeutic implications.

Introduction: Chemokine (C-X3-C Motif) Receptor 1 (CX3CR1) is present on a subset of the immune cells in the tumor microenvironment (TME) and plays an essential and diverse role in cancer progression. However, its potential function in the irradiated TME remains unknown.Materials and Methods: Mouse lung cancer model was performed by subcutaneously inoculating Lewis Lung Carcinoma (LLC) cells expressing luciferase (Luc-2) and mCherry cells in CX3CR1GFP/GFP , CX3CR1DTR/+ , and wild–type (WT) mice. Bioluminescence imaging, clonogenic assay, and flow cytometry were used to assess tumor progression, proliferation, and cell composition after radiation.Results: Radiation provoked a significant influx of CX3CR1-expressing immune cells, notably monocytes, into the TME. Co-culturing irradiated LLC cells with CX3CR1-deficient monocytes and macrophages resulted in reduced clonogenic survival and increased apoptosis of the cancer cells. Interestingly, depletion of CX3CR1 in macrophages led to a redistribution of the irradiated LLC cells in the S-phase, parallel to increased expression of cyclin E1, required for cell cycle G1/S transition. In addition, deletion of CX3CR1 expression in macrophages altered the cytokine secretion with a decrease of interleukin-6, a crucial mediator of cancer cell survival and proliferation. Next, LLC cells were injected subcutaneously into CX3CR1DTR/+ mice, sensitive to diphtheria toxin (DT), and WT mice. After injection, tumors were irradiated with 8Gy, and mice were treated with DT, leading to conditional ablation of CX3CR1-expressing cells. After three weeks, CX3CR1 depleted mice displayed reduced tumor progression. Furthermore, combining the S-phase specific chemotherapy gemcitabine with CX3CR1 cell ablation resulted in additional attenuation of tumor progression.Conclusion: CX3CR1-expressing mononuclear cells invade the TME after radiation therapy in a mouse lung cancer model. CX3CR1 cell depletion attenuates tumor progression following radiation and sensitizes the tumor to S–phase-specific chemotherapy. Thus, we propose a novel strategy to improve radiation sensitivity by targeting the CX3CR1-expressing immune cells.