Background and Aim: Dipeptidyl peptidase 9 (DPP9) is a ubiquitous intracellular peptidase that supresses NLRP1 inflammasome activation and BRCA2 activity. These actions of DPP9 suggest potential for this protease to influence tumourigenesis. Global DPP9 gene inactivation is lethal in mice and humans. Therefore, to study liver cancer generation, we produced and studied mice in which DPP9 expression is knocked down only in hepatocytes.
Methods: This hepatocyte-specific DPP9 knockout mouse (Alb-DPP9-KO) was achieved by floxing exons 5 to 7 of full-length DPP9 and crossing this DPP9 fl/fl mouse strain with a mouse strain that expresses Cre recombinase under an Albumin promoter. Mice were treated with diethylnitrosamine (DEN; once), then after weaning with thioacetamide (TAA) and an atherogenic high fat diet (HFD) until 28 weeks of age. Measurements included tumour burden, and qPCR of immunological and autophagy markers.
Results: Alb-DPP9-KO mice were healthy at all ages. Compared to littermate controls, Alb-DPP9-KO mice had reduced liver mass and subcutaneous adipose tissue mass and had lower fasting plasma glucose. Total numbers of macroscopic liver nodules and tumours, inflammation score and steatosis score did not differ between the two genotypes. However, the Alb-DPP9-KO mice had fewer small macroscopic liver nodules (<3 mm diameter) compared to littermate controls. Alb-DPP9-KO livers had increased levels of active caspase-1 protein, indicative of increased inflammasome activity, and increased levels of Nfkbib, Cxcl10 and Ccl5. However, the number of tumour infiltrating CD8+ T cells was not greater in Alb-DPP9-KO mice. The Alb-DPP9-KO mice showed increased protein levels of autophagy marker Beclin1.
Conclusion: Lifelong DPP9 depletion in hepatocytes led to reduced tumour sizes at 28 weeks of age in this experimental model. Evidence for increased caspase-1 activation is aligned with a recent similar finding in corneal epithelium. A possible role of DPP9 in energy metabolism aligns with a previous publication. Thus, DPP9 may both influence energy metabolism and suppress NLRP1 inflammasome activation in epithelial cells.