Human dipeptidyl peptidase III mRNA variant I and II are expressed concurrently in multiple tumor derived cell lines and translated at comparable efficiency in vitro

Dipeptidyl peptidase III (DPP III) is an emerging biomarker of human cancers. Expression, specificity, and function of human DPP III (hDPP III) mRNA variant I (V-I), II (V-II), and III (V-III) are poorly understood. Here, we investigated expression of these variants in multiple human tumor derived cell lines. DNA sequencing revealed concurrent expression of hDPP III V-I and V-II in U87MG (glioblastoma), SCC4 (squamous cell carcinoma), SiHa (carcinoma of uterus) cells. In SKOV1 cells, a cell line derived from ovarian carcinoma where a positive correlation between histological aggressiveness of the malignancy and hDPP III expression has previously been established, only V-II could be detected. Human DPP III V-III, which lacks an in-frame coding sequence, could not be detected in any of these cell lines. 5′ untranslated region (UTR) of hDPP III V-II contains nucleotides GCA (−12 to −10 bp) upstream to the translation initiator codon (AUG). These nucleotides are absent from V-I and V-III, however, both V-I and V-II encode for the same hDPP III protein isoform-I. In vitro transcription coupled translation assay using hDPP III V-I and V-II expression vectors which contained full length V-I and V-II cDNA including the variable 5′ UTR cloned under T7 promoter, respectively revealed a comparable translational efficiency for both the variants, abrogating involvement of nucleotides GCA (−12 to −10 bp) in translation of the variants. Our results, for the first time, demonstrate concurrent expression in multiple tumor derived cell lines and a comparable in vitro translational efficiency for hDPP III V-I and II.