Adenosine downregulates DPPIV on HT-29 colon cancer cells by stimulating protein tyrosine phosphatase(s) and reducing ERK1/2 activity via a novel pathway

Departments of 1 Pharmacology, 2 Microbiology and Immunology, and 3 Pathology, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada Submitted 17 May 2005 ; accepted in final form 29 March 2006 The multifunctional cell-surface protein dipeptidyl peptidase IV (DPPIV/CD26) is aberrantly expressed in many cancers and plays a key role in tumorigenesis and metastasis. Its diverse cellular roles include modulation of chemokine activity by cleaving dipeptides from the chemokine NH 2 -terminus, perturbation of extracellular nucleoside metabolism by binding the ecto-enzyme adenosine deaminase, and interaction with the extracellular matrix by binding proteins such as collagen and fibronectin. We have recently shown that DPPIV can be downregulated from the cell surface of HT-29 colorectal carcinoma cells by adenosine, which is a metabolite that becomes concentrated in the extracellular fluid of hypoxic solid tumors. Most of the known responses to adenosine are mediated through four different subtypes of G protein-coupled adenosine receptors: A 1 , A 2A , A 2B , and A 3 . We report here that adenosine downregulation of DPPIV from the surface of HT-29 cells occurs independently of these classic receptor subtypes, and is mediated by a novel cell-surface mechanism that induces an increase in protein tyrosine phosphatase activity. The increase in protein tyrosine phosphatase activity leads to a decrease in the tyrosine phosphorylation of ERK1/2 MAP kinase that in turn links to the decline in DPPIV mRNA and protein. The downregulation of DPPIV occurs independently of changes in the activities of protein kinases A or C, phosphatidylinositol 3-kinase, other serine/threonine phosphatases, or the p38 or JNK MAP kinases. This novel action of adenosine has implications for our ability to manipulate adenosine-dependent events within the solid tumor microenvironment. CD26; deaminase binding protein; mitogen-activated protein kinases; nucleosides; tumor microenvironment Address for reprint requests and other correspondence: J. Blay, Dept. of Pharmacology, Faculty of Medicine, Sir Charles Tupper Medical Bldg., Dalhousie Univ., 1459 Oxford St., Halifax, Nova Scotia, Canada B3H 1X5 (e-mail: jonathan.blay{at}dal.ca )