Extraenzymatic functions of the dipeptidyl peptidase IV-related proteins DP8 and DP9 in cell adhesion, migration and apoptosis

The dipeptidyl peptidase IV gene family contains the four peptidases dipeptidyl peptidase IV, fibroblast activation protein, dipeptidyl peptidase 8 and dipeptidyl peptidase 9. Dipeptidyl peptidase IV and fibroblast activation protein are involved in cell-extracellular matrix interactions and tissue remodeling. Fibroblast activation protein is upregulated and dipeptidyl peptidase IV is dysregulated in chronic liver disease. The effects of dipeptidyl peptidase 8 and dipeptidyl peptidase 9 on cell adhesion, cell migration, wound healing and apoptosis were measured by using green fluorescent protein fusion proteins to identify transfected cells. Dipeptidyl peptidase 9-overexpressing cells exhibited impaired cell adhesion, migration in transwells and monolayer wound healing on collagen 1, fibronectin and Matrigel. Dipeptidyl peptidase 8-overexpressing cells exhibited impaired cell migration on collagen I and impaired wound healing on collagen I and fibronectin in comparison to the green fluorescent protein-transfected controls. Dipeptidyl peptidase 8 and dipeptidyl peptidase 9 enhanced induced apoptosis, and dipeptidyl peptidase 9 overexpression increased spontaneous apoptosis. Mechanistic investigations showed that neither the catalytic serine of dipeptidyl peptidase 8 or dipeptidyl peptidase 9 nor the Arg-Gly-Asp integrin-binding motif in dipeptidyl peptidase 9 were required for the impairment of cell survival, cell adhesion or wound healing. We have previously shown that the in vitro roles of dipeptidyl peptidase IV and fibroblast activation protein in cell-extracellular matrix interactions and apoptosis are similarly independent of catalytic activity. Dipeptidyl peptidase 9 overexpression reduced beta-catenin, tissue inhibitor of matrix metalloproteinases 2 and discoidin domain receptor 1 expression. This is the first demonstration that dipeptidyl peptidase 8 and dipeptidyl peptidase 9 influence cell-extracellular matrix interactions, and thus may regulate tissue remodeling.