Tyrosine phosphorylation of CARM1 promotes its enzymatic activity and alters its target specificity

An important epigenetic component of tyrosine kinase signaling is the phosphorylation of histones, and epigenetic readers, writers, and erasers. Phosphorylation of protein arginine methyltransferases (PRMTs), have been shown to enhance and impair their enzymatic activity. In this study, we show that the hyperactivation of Janus kinase 2 (JAK2) by the V617F mutation phosphorylates tyrosine residues (Y149 and Y334) in coactivator-associated arginine methyltransferase 1 (CARM1), an important target in hematologic malignancies, increasing its methyltransferase activity and altering its target specificity. While non-phosphorylatable CARM1 methylates some established substrates (e.g. BAF155 and PABP1), only phospho-CARM1 methylates the RUNX1 transcription factor, on R223 and R319. Furthermore, cells expressing non-phosphorylatable CARM1 have impaired cell-cycle progression and increased apoptosis, compared to cells expressing phosphorylatable, wild-type CARM1, with reduced expression of genes associated with G2/M cell cycle progression and anti-apoptosis. The presence of the JAK2-V617F mutant kinase renders acute myeloid leukemia (AML) cells less sensitive to CARM1 inhibition, and we show that the dual targeting of JAK2 and CARM1 is more effective than monotherapy in AML cells expressing phospho-CARM1. Thus, the phosphorylation of CARM1 by hyperactivated JAK2 regulates its methyltransferase activity, helps select its substrates, and is required for the maximal proliferation of malignant myeloid cells.

Abstract A04: Targeted protein degradation as a strategy to overcome non-covalent BTK inhibitor resistance in lymphoma

Bruton’s tyrosine kinase (BTK) is a target for multiple generations of covalent (irreversible) and non-covalent (reversible) inhibitors due to its critical role in the proliferation and survival of B-cell malignancies. Similar to drug resistance mechanisms in other cancers, resistance to covalent BTK inhibitors (BTKi) in chronic lymphocytic leukemia (CLL), such as ibrutinib, arise through on-target BTK mutations at the BTK C481 residue (the binding site of ibrutinib), which allow escape from BTK inhibition. Non-covalent BTKi represent a new avenue to overcome resistance to the clinically approved covalent BTKi; however, we have recently discovered mechanisms of resistance to non-covalent BTK inhibition in patients with CLL (Wang, Xi, Thompson, Montoya et al NEJM 2022). We used bulk and single cell genomic analyses that identified a series of acquired BTK mutations in a cohort of CLL patients that relapsed on the phase I/II clinical trial of pirtobrutinib. We discovered mutations (BTK V416L, A428D, M437R, T474I, L528W) that occur at critical residues within the catalytic kinase domain of BTK and conferred resistance to both non-covalent and covalent BTKis. Based on structural models of BTK, we observed that these mutations physically impede drug binding, and disrupt the normal kinase activity of BTK but can, upon B-cell receptor stimulation, sustain AKT, ERK, and NF𝜅B signaling as well intracellular Ca2+ release in the presence of pirtobrutinib. In addition, CITE-seq analyses of 53,722 cells from this cohort identified pre-existing leukemic and immune cell states associated with development of resistance in patients. These findings identify novel mechanisms to engender clinical resistance to non-covalent BTKis, including contribution of the immune microenvironment to response to BTK inhibition. Together our data suggest that mutations at the BTK kinase domain may alter conformation of BTK’s non-kinase protein interaction domains thus allowing BTK to be used as a scaffold for other signaling molecules to phosphorylate phospholipase C gamma 2 (PLC𝛾2), the direct downstream target of BTK. We have performed mass spectrometry phosphoproteomics to evaluate potential signaling molecules that are active and could bypass BTK in catalytically inactive BTK mutant cells. Given the above, we decided to target BTK’s non-kinase function by investigating targeted protein degraders of BTK using BTK-degrading protein targeting chimeras (PROTACS). The immediate advantages of using protein degraders are (1) Binding may occur at any site of the target protein, and (2) PROTACS can act catalytically to bind to and degrade multiple target proteins. Our studies have shown that several of the BTK mutants that engendered clinical resistance to covalent and non-covalent BTKi are sensitive to these degraders, elucidating an exciting alternative therapeutic approach for patients who become resistant to BTKi therapies. We are continuing to test the efficacy of BTK PROTAC degraders in overcoming acquired resistance mechanisms in CLL as well as other B-cell lymphomas.

Citation Format: Skye Montoya, Eric Wang, Jessie Bourcier, Sana Chaudhry, Tulasigeri Totiger, Alejandro Pardo, Gabriel Pardo, Maurizio Affer, Jacob Jahn, Anthony Mato, Omar Abdel-Wahab, Justin Taylor. Targeted protein degradation as a strategy to overcome non-covalent BTK inhibitor resistance in lymphoma [abstract]. In: Proceedings of the Third AACR International Meeting: Advances in Malignant Lymphoma: Maximizing the Basic-Translational Interface for Clinical Application; 2022 Jun 23-26; Boston, MA. Philadelphia (PA): AACR; Blood Cancer Discov 2022;3(5_Suppl):Abstract nr A04.

Tobacco Carcinogen-Induced Production of GM-CSF Activates CREB to Promote Pancreatic Cancer

Although smoking is a significant risk factor for pancreatic ductal adenocarcinoma (PDAC), the molecular mechanisms underlying PDAC development and progression in smokers are still unclear. Here, we show the role of cyclic AMP response element-binding protein (CREB) in the pathogenesis of smoking-induced PDAC. Smokers had significantly higher levels of activated CREB when compared with nonsmokers. Cell lines derived from normal pancreas and pancreatic intraepithelial neoplasm (PanIN) exhibited low baseline pCREB levels compared with PDAC cell lines. Furthermore, elevated CREB expression correlated with reduced survival in patients with PDAC. Depletion of CREB significantly reduced tumor burden after tobacco-specific nitrosamine 4-(methyl nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) treatment, suggesting a CREB-dependent contribution to PDAC growth and progression in smokers. Conversely, NNK accelerated PanIN lesion and PDAC formation via GM-CSF-mediated activation of CREB in a PDAC mouse model. CREB inhibition (CREBi) in mice more effectively reduced primary tumor burden compared with control or GM-CSF blockade alone following NNK exposure. GM-CSF played a role in the recruitment of tumor-associated macrophages (TAM) and regulatory T cell (Treg) expansion and promotion, whereas CREBi significantly reduced TAM and Treg populations in NNK-exposed mice. Overall, these results suggest that NNK exposure leads to activation of CREB through GM-CSF, promoting inflammatory and Akt pathways. Direct inhibition of CREB, but not GM-CSF, effectively abrogates these effects and inhibits tumor progression, offering a viable therapeutic strategy for patients with PDAC.Significance: These findings identify GM-CSF-induced CREB as a driver of pancreatic cancer in smokers and demonstrate the therapeutic potential of targeting CREB to reduce PDAC tumor growth.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/21/6146/F1.large.jpg Cancer Res; 78(21); 6146-58. ©2018 AACR.

Abstract 5042: CREB transgenic mice to study alcohol-associated pancreatic carcinogenesis

Background: Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related deaths in the United States. Chronic alcohol (>60 grams/day) consumption is strongly associated with the risk of PDAC development. The metabolites generated from alcohol have been shown to cause significant pancreatic injury. Although the association of alcohol with PDAC progression has been established, the details of the underlying molecular and cellular mechanisms governing this process are unknown. Our study revealed that alcohol-associated pancreatic carcinogenesis correlates with CXCL12-induced activation of cyclic AMP response element binding protein (CREB). In this study, we examined newly developed CREB knockout (CREBfl/fl) mouse in pancreas.

Methods: Human pancreatic epithelial lines (HPNE), HPNE cells with KRAS (HPNE-KRAS), pancreatic stellate cells (PSCs), pancreatic intraepithelial neoplasia (PanIN) mouse cell lines (LSL-KrasG12D/+; Pdx1Cre/+) and cancer-associated fibroblasts (CAFs) cells were exposed to chronic alcohol (50 mM) and analyzed for phospho-kinases in cell lysates and cytokines in conditioned media. Inducible Ptf1aCreERTM;KRASG12D/+ (iPK) mice and CREB knockout (CREBfl/fl) with KRAS activation [Ptf1aCreERTM;KRASG12D/+;CREBfl/fl (iPKCREBKO)] mice were used to investigate the effect of alcohol on CREB activation and the role of CREB in alcohol-associated PDAC pathogenesis. iPK and iPKCREBKO mice were exposed to Lieber-DeCarli alcohol diet for up to 14 weeks with or without caerulein injections. The number of acinar cells (amylase), ducts (cytokeratin 19), PanIN lesions (alcian blue positive), fibrosis (sirius red) and activation of CREB were measured by immunohistochemistry at 6 and 14 weeks of alcohol exposure in vivo. Serum obtained from alcohol-fed iPK and iPKCREBKO mice were analyzed for significant cytokine release upon alcohol exposure in vivo. We then determined the biologic effects of pharmacologic CREB inhibition in iPK mice exposed to alcohol diet.

Results: We found increased pCREB levels in HPNE and HPNE-KRAS cells upon treatment with alcohol, which was further associated with the up-regulation of the CXCL12 protein in the conditioned media. Serum CXCL12 and tissue pCREB levels were high in alcohol fed iPK mice when compared to their corresponding control diet-fed mice. Blocking CXCL12 with Plerixafor, a CXCL12-CXCR4 axis inhibitor, decreased alcohol-induced pCREB levels. Finally, exposure of iPK mice to an alcohol diet coupled with cerulein administration significantly increased the number of PanIN lesions (alcian blue+ cells), and decreased acinar cells when compared to alcohol-fed iPKCREBKO mice.

Conclusion: These findings implicate CREB as a critical oncogenic driver in alcohol addiction-induced pancreatic carcinogenesis. Additionally, CXCL12 represents an important mediator of CREB activation, which can be pharmacologically targeted.

Citation Format: Tulasigeri Totiger, Supriya Srinivasan, Michael VanSaun, Chandrashekhar Joshi, Chanjuan Shi, Xizi Dai, Rajinder Dawra, Alexander Gaidarski, Eric Nestler, Nipun Merchant, Nagaraj Nagathihalli. CREB transgenic mice to study alcohol-associated pancreatic carcinogenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5042.

Abstract 1249: Animal model in the prevention of alcoholic pancreatitis

Background: Alcohol abuse is a major risk factor for pancreatitis. Alcohol addiction-induced molecular pathogenesis of pancreatitis remains obscure, and no current effective treatment exists. Therefore, approaches to investigate pathogenesis, prevention and cellular mechanisms by which alcohol causes pancreatitis are necessary for establishing therapeutics. Our efforts demonstrate that alcohol induces activation of a major intracellular transcriptional factor, cyclic AMP response element-binding protein (CREB). We further investigated the functional role of CREB in alcohol-induced pathogenesis of pancreatitis using cellular and genetic mouse models of pancreas.

Materials and Methods: Human tissue microarrays were immunostained to determine the significance of pCREB expression among pancreatic tissues obtained from normal and chronic pancreatitis. Rat acinar cell line AR42J and mouse PSCs (mPSCs) were exposed to alcohol (50 mMol/L). Inducible Ptf1aCreERTM knockin mice and Ptf1aCreERTM;CREBfl/fl (iPC) mice were fed with Lieber Decarli diet alcohol or regular diet for 14 weeks with or without caerulein (50 µg/kg). Mice were then euthanized 24 hours after the last caerulein injection, and pancreas tissues were processed for morphometric analysis (necrosis, vacuolization, hemorrhage, edema and inflammation) and immunohistochemical analysis of amylase, trichrome blue, SMA, collagen 1, fibronectin and pCREB expression. To determine whether alcohol accelerated morbidity in mice, we evaluated pathogenesis of chronic pancreatitis by analyzing acinar atrophy and pancreatic fibrosis.

Results: Expression of pCREB was significantly higher (p <0.001) in chronic pancreatitis vs. normal patient tissues, confirming the role of activated CREB in pancreatitis. Activated CREB levels were very high in alcohol-fed Ptf1aCreERTM mice when compared with control diet-fed mice. Pancreatic sections from alcohol-fed mice challenged with caerulein revealed significantly higher score of acinar cell vacuolization and necrosis, inflammatory infiltrate and hemorrhage compared with minimal lesions in control diet-fed animals receiving caerulein. Pancreatic sections from alcohol-fed Ptf1aCreERTM animals showed higher score of histologic injury, extracellular matrix deposition, collagen deposition and increased pancreatic fibrosis when compared with control-fed mice. iPC mice showed decrease in the pathogenesis of chronic pancreatitis when compared to Ptf1aCreERTM mice with alcohol.

Conclusion: CREB is overexpressed in pancreatitis and alcohol activates CREB, which then drives pathogenesis of pancreatitis. Severity of pancreatitis in response to alcohol is diminished in the absence of CREB. Therefore, we conclude that targeting CREB represents a promising treatment for alcohol-induced pancreatitis.

Citation Format: Supriya Srinivasan, Tulasigeri Totiger, Michael VanSaun, Fanuel Messaggio, Chanjuan Shi, Austin Dosch, Eric Nestler, Nipun Merchant, Nagaraj Nagathihalli. Animal model in the prevention of alcoholic pancreatitis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1249.