Dipeptidyl peptidase 2 is an essential survival factor in the regulation of cell quiescence.

Most cells in the body are in a resting state and undergo cell cycle progression only upon growth factor stimulation or activation. while much research on proliferation and activation has been performed, very little about signals that maintain quiescent cells in G(0) is known, preventing cell cycle entry or apoptosis. In this study, the pathways of apoptosis induction in quiescent peripheral blood cells and fibroblasts mediated by inhibition or downregulation of Dipeptidyl Peptidase 2 (DPP2) have been explored. A decrease in DPP2 activity was found to cause resting cells to exit from G(0), accompanied by a decrease in p130, p27(Kip1) and p21(Cip1) protein levels. In addition, DPP2-inhibited or downregulated cells exhibit an increase in early G(1)/S progressors, with increases in the levels of retinoblastoma (pRb), p107 and cyclin D proteins. Furthermore, decrease of DPP2 activity leads to an increase in c-Myc and a decrease in Bcl-2, two events that have been associated with apoptosis induction. This apoptosis by DPP2 downregulation is prevented in p53(-/-) cells or by ectopic expression of proteins that suppress p53 or c-Myc activity. Thus, DPP2 is essential for maintaining lymphocytes and fibroblasts in G(0), and its inhibition results in apoptosis mediated by induction of c-Myc and p53.

Lymphocyte quiescence factor Dpp2 is transcriptionally activated by KLF2 and TOB1.

We have shown previously that dipeptidyl peptidase 2 (DPP2) activity is essential for the survival of quiescent, but not activated, lymphocytes. The specific requirement of DPP2 activity for non-dividing cells is indicative of cell cycle specific regulation of this gene product. In the present study, we tested this hypothesis by looking at contact and serum dependence of Dpp2 transcription. We found that transfected promoter-reporter activity, as well as endogenous Dpp2 transcripts, were enhanced in NIH-3T3 cells upon contact-inhibition or serum starvation. Since lung Kruppel-like factor (KLF2), a transcription factor, and TOB1, a transcriptional co-activator, have been shown to be important in maintaining T-lymphocyte quiescence and are both downregulated upon cellular activation, we also looked at the contributions of these factors to Dpp2 transcription. Using a Dpp2 promoter-reporter system, we demonstrate that KLF2 and TOB1 activate the mouse Dpp2 promoter. Finally, we show that in human PBMC, there is a decrease in levels of endogenous DPP2 transcripts upon T cell receptor activation when compared to resting cells. These results demonstrate that Dpp2 transcription is serum and contact-dependent and link two quiescence-specific transcriptional elements to the quiescence-specific requirement of DPP2 enzymatic activity.

Differential control of G0 programme in chronic lymphocytic leukaemia: a novel prognostic factor.

Chronic lymphocytic leukaemia (CLL) is a unique malignancy where quiescent B cells accumulate in the peripheral blood. Since clinical outcomes in CLL are very heterogeneous, it is of utmost importance to correctly assess the disease prognosis in each individual case. Recently, it has been shown that high ZAP-70 [Zeta-chain (T-cell receptor) associated protein kinase (70 kDa)] expression level strongly correlates with lack of IgV(H) mutations and poor prognosis in B-CLL. As CLL malignant cells are arrested in G(0), we investigated whether Dipeptidyl Peptidase 2 (DPP2), a serine protease that plays a key role in keeping cells in the quiescent state, is involved in cell-cycle control in CLL. We have previously shown that specific inhibition of DPP2 results in apoptosis of normal lymphocytes. In this study, cell apoptosis experiments were conducted in 38 patients with B-CLL. Two distinct subsets of B-CLL were identified, susceptible and resistant to DPP2-inhibition-induced apoptosis. If resistant to apoptosis (42.1%), the CLL cells have higher expression of ZAP-70 and exhibit a worse prognosis, such as shorter treatment-free time period. Thus, resistance vs. susceptibility to DPP2-inhibiton induced apoptosis can be employed as a novel prognostic factor in CLL.