Abstract C44: MBD3 stabilizes MYC, leading to metastatic outgrowth of pancreatic cancer in the liver
Authors: Ranjan Alok , Sinha Surajit , Ayabe Reed , Wach Michael , Ruff Samantha , Gupta Shreya , McDonald James , Remmert Kirsten , Alexander Imani , Hernandez Jonathan 
Published: March 2020

Introduction: Pancreatic cancer remains a challenging disease, with an overall cumulative 5-year survival rate less than 5%. Metastases are responsible for over 90% of cancer-related deaths. Despite significant progress made in cancer drug development, our understanding of the mechanisms driving metastatic outgrowth remains limited. Systematic dissection of the molecular mechanisms underlying metastatic progression is necessary for the development of new therapeutic strategies.

Method: To identify the important genes driving metastatic outgrowth, we performed a novel in vivo cDNA screening using a high metastatic-capacity line (M-4964Liv) and a low metastatic-capacity line (D-4964Liv), which we derived from in vivo passaging of a cell line from a KPC mouse. MBD3 regulated MYC stability was examined by cycloheximide chase assay and further confirmed by polymerase chain reaction and Western blotting. Phenotypic effect of MBD3 was evaluated by sphere assay in 3D and intrasplenic model of liver metastasis in vivo.

Results: Methyl-CpG binding domain protein 3 (MBD3) was identified in our screen. We initially evaluated MBD3 with pancreatosphere formation assays. In doing so, we showed that MBD3 overexpression leads to significant augmentation of sphere formation, while knockdown abrogates sphere-forming capacity. We subsequently validated MBD3 in vivo with liver metastases assays, through both overexpression and knockdown, as being a facilitator of metastatic outgrowth. In addition, we observed high expression of MBD3 across a panel of pancreatic cancer cell lines that readily form liver metastases upon splenic injection. To understand how MBD3 may lead to metastases, we used a cycloheximide chase assay and determined that MBD3 overexpression increases MYC stability. With knockout of MBD3 using CRISPR/Cas9, we demonstrated that MBD3 mediates MYC stabilization by increasing the expression of phospho-MYC at Ser-62 (stable form of MYC). In the absence of MBD3, we observed an increase in phospho-MYC at Thr-58 expression (unstable form of MYC). Consequently, we observed a decrease in expression of CDK7, a downstream target of MYC, after MBD3 knockdown. CDK7 is a known master regulator of cyclin-dependent kinases, and loss of CDK7 drives cells to quiescence. Intriguingly, downregulation of MYC expression, through destabilization with MBD3 knockdown, led to activation of negative feedback pathway as observed with an increase in phospho-GSK3β at Ser-9, which inactivates GSK3β and thereby decreases MYC ubiquitination.

Conclusion: MBD3 leads to stabilization of MYC in pancreatic cancer cell lines, which results in an increase in the ability to form liver metastases. Further dissection of this previously undescribed MBD-mediated mechanism may lead to new therapeutic targeting of MYC for patients with pancreatic cancer.

Citation Format: Alok Ranjan, Surajit Sinha, Reed Ayabe, Michael Wach, Samantha Ruff, Shreya Gupta, James McDonald, Kirsten Remmert, Imani Alexander, Jonathan Hernandez. MBD3 stabilizes MYC, leading to metastatic outgrowth of pancreatic cancer in the liver [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C44.