Dipeptidase-1: an old protein with new roles in shaping the tumor microenvironment and impacting progression in colorectal cancer

Abstract

Dipeptidase-1 (DPEP1) has been mainly studied as an apically localized, kidney-specific protein that cleaves dipeptides, but there is an emerging interest for its role in colorectal cancer (CRC). DPEP1 is one of the most upregulated genes in CRC in comparison to normal colonic epithelium and has been reported to impact proliferation, invasion, and metastasis. With the recent discovery of DPEP1’s nonenzymatic function of binding neutrophils, we investigated this novel function of DPEP1 in CRC. The Coffey lab previously found that DPEP1 is preferentially upregulated in microsatellite stable (MSS) CRC that does not respond to immunotherapy due to a paucity of CD8+ T cells in the tumor proper; in marked contrast, microsatellite instability (MSI-H) CRCs have plentiful intratumoral CD8+ T cells and have a dramatic response to immunotherapy. I found that neutrophils bind CRC cells in a DPEP1-dependent manner and that DPEP1 null mice subjected to a colon cancer-inducing regimen leads to a paucity of neutrophils in the tumor, coinciding with an influx of CD8+ T cells not seen in wild-type. Staining for mismatch repair proteins were markedly reduced, suggesting that these seemingly more aggressive tumors had acquired an MSI-H phenotype, indicative of immunotherapy responsiveness. In a separate work, the Coffey lab found that diffuse cytosolic staining for DPEP1 in CRC correlated with a worse progression-free survival of CRC patients. I explored how DPEP1, a GPI-linked cell surface protein, could have a diffuse staining pattern, discovering a previously unrecognized alternatively spliced isoform, called DPEP1 Isoform B, which is not membrane bound, but instead has an intracellular, cytosolic staining pattern. Expression of this novel isoform transforms cells to be tumorigenic and results in large invasive tumors in nude mice. Comparison of CRC cells that express the canonical isoform, DPEP1 Isoform A, and DPEP1 Isoform B demonstrates that Isoform B impacts expression of genes related to cell proliferation and epithelial-to-mesenchymal transition. My work has provided insights into multiple functions for one protein, DPEP1, in CRC and opened many avenues for future investigation.