Occupancy of dipeptidyl peptidase IV activates an associated tyrosine kinase and triggers an apoptotic signal in human hepatocarcinoma cells

✍ Scribed by Lucia Gaetaniello; Michele Fiore; Sergio de Filippo; Nicola Pozzi; Sonia Tamasi; Claudio Pignata

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 221 KB
• Volume: 27
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.510270407

✦ Synopsis

Dipeptidyl peptidase IV (CD26/DPP-IV) is an ectoenzyme expressed on different cell types. Signaling properties and functional consequences of the CD26 triggering have been elucidated mostly on T cells, where the molecule delivers a costimulatory signal that potentiates T-cell activation through the T-cell receptor. We conducted studies in the human hepatocarcinoma-derived PLC/PRF/5 cell line to examine the signal transduction through CD26 and its functional properties in the absence of other T-cell-specific membrane molecules. Engagement of CD26 in PLC/PRF/5 cells through a specific antibody induces tyrosine phosphorylation of several proteins with maximal intensity 15 minutes after the stimulation. This effect was under the negative regulatory control of CD45 tyrosine phosphatase, in that the addition of orthovanadate clearly enhanced the phosphorylation events. Using in vitro kinase assays with CD26 immunoprecipitates, we observed that a protein or proteins with kinase activity are coprecipitated with the CD26 molecule. In addition, unlike Jurkat T cells, in which CD26 expression exerts a protective effect against apoptosis, in PLC/PRF/5 cells CD26 occupancy delivers a potent apoptotic signal. This effect was also observed in HepG2 cells, thus indicating that it represents a more general phenomenon occurring in different liver neoplastic cell lines. (HEPATOLOGY 1998;27:934-942.) Dipeptidyl peptidase IV (DPP-IV, CD26) is a 110-kd glycoprotein expressed on a great variety of different cell types, including lymphocytes, kidney, liver, and epithelial cells. 1 Peptidase activity resides in its extracellular domain. [2][3][4][5] This enzyme cleaves dipeptides from polypeptides that have L-proline or L-alanine at the penultimate amino-terminal position. 6,7 However, the physiological substrate is not known. Recent evidence indicates that this glycoprotein in T lymphocytes participates in a macromolecular complex by physically and functionally interacting in the extracellular domain with CD45 tyrosine phosphatase and adenosine deaminase. 8,9 Most of the functional properties of CD26 have been elucidated in T lymphocytes, in which the molecule is upregulated during cell activation. 5 In T lymphocytes, CD26 delivers a potent costimulatory signal that potentiates T-cell activation through the T-cell receptor (TCR). 10 Previously activated CD4 ϩ and CD8 ϩ lymphocytes respond to CD26 cross-linking with increased lymphokine secretion, proliferation, and cytotoxic activity. 11 However, this effect seems to be strictly dependent on the expression of the TCR/CD3 complex, in that CD26 perturbation in TCR -variant Jurkat cells fails to induce Ca 2ϩ mobilization and interleukin (IL)-2 production. 12 Because DPP-IV has a very short intracytoplasmic domain, consisting of six amino acids with no intrinsic kinase activity, CD26 triggering can deliver signals through association with intracellular signaling molecules. 13,14 Biochemical studies, mostly performed on CD3 ϩ /TCR ϩ T lymphocytes, revealed that cross-linking of CD26 can induce tyrosine phosphorylation of the z chain of the TCR/CD3 complex. 8,15 Furthermore, CD26 stimulation increases the autophosphorylation activity of the CD4 coprecipitated p56 lck , which belongs to the src family of entirely intracytoplasmic tyrosine kinases, and, of note, is selectively expressed on hematopoietic cells. 16 These effects have been considered related to the interaction with CD45, 8 which participates in the cellular activation process by dephosphorylating p56 lck at the negative regulatory site Tyr 505. 17 However, the overspread distribution of this ectoenzyme in nonlymphoid tissues suggests that this molecule may exert additional and not yet defined functions. There is evidence indicating that DPP-IV interacts with extracellular matrix proteins, which play an important role in the maintenance of liver architecture. In cirrhotic liver, an altered distribution of the CD26 molecule has been documented, suggesting a role of this molecule in the tissue destruction/regeneration process. 18 Therefore, information on the functions of CD26, and on the biochemical mechanisms involved in signal transduction through this molecule in cells from different tissues, may help unravel the physiological role of this ectoenzyme. In addition, increased knowledge about the biochemical machinery of CD26 may also help us understand the potential role of this macromolecular complex in the pathogenesis of liver disease.

For this purpose, we investigated early tyrosine phosphorylation events that follow CD26 triggering and the biochemical interactions of the molecule in the human hepatocarcinoma cell line PLC/PRF/5. This cell line was derived from a human hepatocellular carcinoma and widely expresses CD26 in the absence of TCR, CD3, and CD4 molecules expressed