Re-expression of Dipeptidylpeptidase 4 (DPP4) in melanoma cells – insights into molecular consequences

CD 26 a type-II membrane protein – also known as dipeptidyl peptidase IV – is a multifunctional enzyme covering functions as costimulatory molecule during T cell activation as well as receptor for extracellular matrix molecules like fibronectin and collagen. It functions as a cell surface peptidase. Among the substrates for the intrinsic serine protease activity there are chemokines like RANTES, MCP 1–3 and SDF1a. CD26 is expressed on epithelial cells of the intestine, on cells of the proximal tubuli of the kidney and on activated T-, B-cells and activated NK cells. It can be detected as a secreted form in the serum. Van den Oord described that CD26 expression on melanocytic cells is inversely related to the tumor stage of melanocytic lesions (1997). Later, Umadevi et al. demonstrated, that CD26 re-expression suppresses the malignant phenotype in melanocytic cells (1999). To these results Pethiyagoda added further data, that CD26 inhibits the invasive capacity of melanoma cells (2001). But the molecular signalling basics for these features of the molecule were not analysed so far.

To address the question of how CD26 regulates the phenotype of a melanocytic cell, we re-expressed CD26 in a melanoma cell line which does not express CD26 protein neither intracellularily nor on the cell surface. CD26 transfected clones were analysed by FACS analyses for CD26 surface expression. The CD26 expression status of the cell lines was also confirmed by Westernblot and during gene expression profiling. The gene expression profiles were determined by Affimetrix GeneChip U133A. The data were confirmed for selected genes by RT-PCR. By this approach we could detect the downregulation of the expression of genes such as MMP2, IL8, CXCL1, and IL1β. This gene regulation coincides with a more benign phenotype of the cells. In addition, the CD26 expressing cells exhibited a higher level of attachment to fibronectin in Boyden chambers. The analysis of the protein tyrosine phosphorylation pattern demonstrated reproducibly a differential phosphorylation of phosphotyrosine proteins of a size of 75 kDa and 42 kDa. The remaining phosphotyrosine pattern was not changed. The identification of these proteins might give rise to the identification of the signaling pathway of CD26 leading to the characteristic malignancy suppressive phenotype.