Abstract A89: Targeting the MYC activation and degradation pathway for the treatment of pancreatic cancer
Authors: Farrell Amy S. , Allen-Petersen Brittany , Daniel Colin J. , Wang Xiaoyan , Wang Zhiping , Christensen Dale J. , Sheppard Brett , Sears Rosalie C. 
Published: October 2015

Pancreatic cancer is a disease with a high rate of mortality, as it is generally diagnosed in the advanced stages when few effective treatments are available. Given the clinical aggressivity of this disease and lack of good treatment options, the development of new therapeutic agents is of the upmost importance. The c-MYC (MYC) oncoprotein is a critical transcription factor that regulates many diverse cellular functions by either activating or suppressing downstream target genes. High levels of MYC expression occur in a wide variety of human tumors, and animal models demonstrate MYC-induced tumorigenesis in many tissues. It has been shown that the MYC gene is frequently amplified in human pancreatic cancer and that MYC mRNA and/or protein are often overexpressed. However, what pathways contribute to this overexpression or whether MYC can be therapeutically targeted in pancreatic cancer is yet not known.

Work in our laboratory has elucidated a complex signaling pathway that regulates MYC protein stability and transcriptional activity. RAS activation in early G1 leads to ERK-dependent phosphorylation of MYC at Serine62 (pS62-MYC), which stabilizes MYC. pS62-MYC is then recognized by the phosphorylation-directed Pin1 prolyl isomerase, which isomerizes Proline63 (P63) to cis, enhancing MYC’s recruitment to, and transactivation of, pro-growth target genes. Later in G1, when RAS activation decreases, GSK3β phosphorylates MYC at T58 (pT58-MYC). Following a second isomerization by Pin1, the trans-specific phosphatase PP2A-B56α removes the stabilizing S62 phosphate. pT58-MYC is then recognized by the E3 ubiquitin ligase SCFFbw7 and degraded by the proteasome. We have found that this pathway is frequently deregulated in human cancer leading to elevated pS62, decreased pT58, and increased c-Myc stability and activity.

Here, we show that MYC is overexpressed and stabilized in pancreatic cancer cells. Additionally, we find aberrant MYC phosphorylation in primary human pancreatic cancer tissue, suggesting that the pathway that controls MYC phosphorylation is deregulated in human pancreatic cancer. Therefore, we hypothesized that the pathways regulating MYC phosphorylation and activation might represent new therapeutic targets. We are pursuing two different approaches to target MYC in pancreatic cancer: 1) inhibiting Pin1, the prolyl isomerase that co-activates pS62-MYC, and 2) activation of PP2A, the phosphatase that destabilizes MYC protein. We have observed decreased PP2A activity in pancreatic cancer cells, and have found that two endogenous inhibitors of PP2A, SET and CIP2A, are frequently overexpressed in pancreatic cancer cells and primary patient samples. This overexpression correlates with aberrant phosphorylation and stabilization of MYC. Furthermore, knockdown of SET or CIP2A, or inhibition of SET using a peptide mimetic (OP449), increases PP2A activity and reduces the tumorigenic potential of pancreatic cancer cells both in vitro and in vivo. In addition to increased SET and CIP2A expression in pancreatic cancer, we also observe frequent overexpression of Pin1 in human pancreatic cancer cell lines and primary patient tumor tissue and find that Pin1 knockdown or inhibition reduces the oncogenic potential of pancreatic cancer cells both in vitro and in vivo. Together, these findings suggest that MYC may play an important role in pancreatic cancer, and that targeting the pathway that regulates MYC activity and protein stability may be a viable therapeutic approach for human pancreatic cancer.

Citation Format: Amy S. Farrell, Brittany Allen-Petersen, Colin J. Daniel, Xiaoyan Wang, Zhiping Wang, Dale J. Christensen, Brett Sheppard, Rosalie C. Sears. Targeting the MYC activation and degradation pathway for the treatment of pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr A89.