Investigating the roles of dipeptidyl peptidase 9 (DPP9) in inflammation

Abstract

The DPP4 family members (DPP4, DPP8, DPP9 and FAP) are atypical enzymes that can cleave the post proline bond two residues from the N-terminus of a substrate. Inhibitors of the four proteases of the DPP4 family, which includes DPP9, lower liver tumor occurrence.

Subcutaneous injection of the C57/BL6 syngeneic hepatocellular carcinoma (HCC) cell line Hep55.1c reliably produced tumors beneath the skin of mice, establishing a subcutaneous HCC model with an intact immune system. Histologically, tumors treated with a pan-DPP inhibitor, BBS-5870, had more T cell and B cell infiltration as well as more cancer cell death compared to their contralateral saline-treated control.

A hepatocyte-specific Dpp9 knockout mouse (DPP9-KO) was developed and used to explore hepatocyte Dpp9 in experimental HCC. DPP9-KO mice had reduced body, liver and subcutaneous adipose tissue mass, lower fasting plasma glucose and fewer small macroscopic liver nodules compared to DPP9-WT mice. Activated caspase-1 protein and the inflammation markers Nfkbib, Cxcl10 and Ccl5 were elevated in the DPP9-KO liver.

To investigate outcomes of reduced global abundance of Dpp9 in adult mice, we generated a conditional Dpp9 knockout mouse model. The degree of Dpp9 knockdown in these mice varied across organs, with kidneys and lungs relatively resistant to Dpp9 knockdown. Complete Dpp9 depletion produced severe diarrhoea and lethality, which were accompanied by renal and intestinal epithelial cell death.

Complete global Dpp9 knockdown can be as lethal in adults as in neonatal mice, but considerable Dpp9 loss can be tolerated. These data provide insights into how partial Dpp9 loss affects mammals, which is instructive for the toxicology of chemical inhibitors of DPP9.