Dipeptidyl peptidase 9 (DPP9) in human skin cells.

Dipeptidyl peptidase 9 (DPP9) is a relatively new member of the DPPIV family of prolyl dipeptidases which is ubiquitously expressed. Its role in regulation of immune responses and proliferation of epithelial carcinoma cells was reported. There is no data on possible role of DPP9 expressed in skin epithelial cells (keratinocytes) and in dermal fibroblasts. Transcriptional and protein expression of DPP9 and DPPIV was examined in fibroblasts and keratinocytes isolated from normal human skin. Localization of DPP9 and its sub-localization in Golgi were determined by immunocytochemistry staining. DPPIV-like enzyme activity was determined in cell lysates and in isolated cell fractions containing membranes (M), cytosol (C) and content of organelles/endosomes/vesicles (V). Relative contribution of DPPIV and DPP8/9 enzyme activity in these fractions was determined by using selective inhibitors: sitagliptin (selective for DPPIV) and 1G244 (selective for DPP9 and a highly homologous DPP8). Possible roles of DPP8/9 via its enzyme activity were analysed by assessment of survival and proliferative capacity of fibroblasts and HaCaT cells of keratinocyte origin in the presence of the inhibitors. Possible role of DPP9 in cell migration and/or adhesion was analysed in fibroblasts and HaCaT cells after DPP9 gene silencing. Fibroblasts and keratinocytes exerted comparable level of DPP9 both at transcriptional and protein level. Fibroblasts strongly expressed DPPIV, whereas in keratinocytes DPPIV expression was low. DPP9 expression was found in cytosol and in perinuclear area of some fibroblasts, or in scattered pattern of keratinocytes, as well as in nuclei of some cells. Only low level of DPP9 sub-localization within Golgi was observed in fibroblasts and keratinocytes. DPPIV-like enzyme activity was about 5 times higher in lysates of fibroblasts than of HaCaT cells. In fibroblasts DPPIV-like enzyme activity was mainly (65%) found in the fraction containing cell membranes (M) and was predominantly (86.9%) due to DPPIV. In contrast, in HaCaT cells the DPPIV-like enzyme activity was mainly (84.2%) found in cytosol (C) and was predominantly (95.6%) due to DPP8/9. Survival and the proliferative capacity were significantly diminished in the presence of 10μM 1G244, both in fibroblasts and in HaCaT cells, suggesting possible role of DPP8/9 enzyme activity in regulation of survival and proliferation of these cells. DPP9 gene silencing resulted in decreased adhesion of fibroblasts, as well as in decreased migration of fibroblasts and HaCaT cells. Accumulation of DPP9 on the edges of plasma membranes of fibroblasts and keratinocytes adhering to surface supports the idea of possible role of DPP9 in cell adhesion. This is the first study showing protein expression, sub-localization and possible biological roles of DPP9 expressed in isolated human skin cells. The data may be relevant for development of new drugs against skin diseases by targeting DPP9 expressed in the skin cells.