Abstract 3045: PEPT1 as a tumor promoter and novel drug target to treat pancreatic cancer
Authors: Schniers Bradley , Bhutia Yangzom 
Published: August 2019

Abstract
Pancreatic ductal adenocarcinoma (PDAC) is the most lethal of all cancers. Gemcitabine is currently used as a first line therapy but with a very low success rate. With this projection in mind, it is imperative to discover a more effective treatment for PDAC. Our lab works on the Peptide Transporter 1 (PEPT1)/SLC15A1. PEPT1 is typically expressed in the small intestine, kidney, and bile duct and transports a wide array of di- and tri-peptides and peptide-like drugs. Literature evidence has shown PEPT1 to be upregulated in some PDAC cell lines. Our aim here was to first corroborate the literature evidence, then investigate if PEPT1 is a tumor promoter and finally understand the mechanistic aspect of its upregulation. Using Real-time PCR and Western blotting, we checked the expression of PEPT1 mRNA and protein, respectively in pancreatic cancer cell lines vs. normal pancreatic cell lines. It was interesting to note that PEPT1 was selectively and significantly upregulated only in cancer cells. We further corroborated the data using patient-derived xenografts (PDXs) and tumor samples from PDAC patients. Additionally, we performed radiolabeled glycylsarcosine (3H-Gly-Sar) uptake to check the functionality of PEPT1. The results of 3H-Gly-Sar uptake correlated with the protein expression in the cancer cells. To further investigate if PEPT1 could be a tumor promoter we performed a CRISPR/Cas9 mediated knockout of PEPT1. AsPC-1 was used as the model cell line. Following PEPT1 knockout, we injected AsPC-1/Control and AsPC-1/PEPT1-knockout cells in athymic nude mice and monitored the tumor growth. Interestingly, we found that the absence of PEPT1 significantly reduced the tumor growth suggesting its role as a tumor promoter. It is known that tumor cells generate large amounts of lactic acid by accelerating aerobic glycolysis. This metabolic switch leads to cellular acidification, but tumor cells circumvent this obstacle by releasing lactate and H+into the extracellular medium. Since PEPT1 is a proton-coupled transporter we hypothesized that lactate regulates its expression. To test this, we performed RT-PCR to check the expression of Pept1 in Gpr81/wildtype and Gpr81/knock-out intestinal samples. Since GPR81 is a lactic acid receptor we hypothesized if its activation by lactate would in turn activate PEPT1. Surprisingly, we found that lactate/GPR81 complex actually regulates PEPT1 expression. Further, we found that lactate also increases the expression of MMP-1, which will then break down the extracellular matrix protein collagen into large peptides. These peptides could be further hydrolyzed into dipeptides by DPP-IV/CD26, which could be the mechanism to generate dipeptide substrates for PEPT1 and thereby couple the process to amino acid nutrition for pancreatic cancer cells. In summary, PEPT1 promotes pancreatic cancer and therefore could be used as a drug target to treat PDAC. However, the molecular mechanisms needs further elucidation.
Citation Format: Bradley Schniers, Yangzom Bhutia. PEPT1 as a tumor promoter and novel drug target to treat pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3045.