A novel 40kDa N-terminal truncated carboxypeptidase E splice variant: cloning, cDNA sequence analysis and role in regulation of metastatic genes in human cancers.

Carboxypeptidase E (CPE), a prohormone processing enzyme, is a 476- amino acid protein with a signal peptide in its N-terminus and is expressed in the nervous and the endocrine systems. Recent evidence indicate CPE plays various non-enzymatic roles in the endocrine and nervous systems and in various cancers. Besides wild type (WT) CPE, a 40-kDa CPE protein that localizes in the nucleus and cytoplasm has been described in embryonic mouse brain. In this study we have cloned this CPE variant encoding the 40kDa CPE-ΔN protein from human cancer cells. RACE assay and sequence analysis confirmed existence of this CPE variant mRNA, which has 198 nucleotides removed within the first exon and 589 nucleotides from the 3'-UTR, respectively, compared to WT-CPE mRNA. Bioinformatic analysis revealed that this CPE variant mRNA has a shortened open reading frame, which starts coding from the 3rd ATG relative to WT-CPE mRNA and encodes a 40kDa N-terminus truncated CPE protein. RT-PCR and Western blot analysis showed that 40kDa CPE-ΔN is expressed in multiple cancer cell lines and tumor tissues. Overexpression of this 40kDa CPE-ΔN variant up-regulated expression of multiple metastatic genes encompassing different signaling pathways, suggesting potentially an important role of CPE-ΔN in tumor metastasis.