Aberrant dipeptidyl peptidase IV (DPP IV/CD26) expression in human hepatocellular carcinoma

Effect of CD26/dipeptidyl peptidase IV on Jurkat sensitivity to G2/M arrest induced by topoisomerase II inhibitors

CD26/dipeptidyl peptidase IV (DPPIV) is a surface antigen with multiple functions, including a role in T-cell activation and the development of certain human cancers. We previously demonstrated that CD26/DPPIV enhanced sensitivity of Jurkat cells to doxorubicin. We now show that expression of CD26/DPPIV enhanced sensitivity of CD26 Jurkat transfectants to G(2)-M arrest mediated by the antineoplastic agent etoposide. The increased sensitivity to etoposide-induced G(2)-M arrest was associated with disruption of cell cycle-related events, including hyperphosphorylation of p34(cdc2) kinase, change in cdc25C expression and phosphorylation, and alteration in cyclin B1 expression. CD26/DPPIV-associated enhancement of doxorubicin and etoposide-induced G(2)-M arrest was also observed in serum-free media, suggesting an effect of CD26 on cell-derived processes rather than serum-derived factors. Importantly, our work elucidated a potential mechanism for the enhanced susceptibility of CD26-expressing Jurkat cells to the topoisomerase II inhibitors by demonstrating that CD26/DPPIV surface expression was associated with increased topoisomerase II alpha levels and enhanced enzyme activity. Besides being the first to show a functional association between the multifaceted molecule CD26 and the key cellular protein topoisomerase II alpha, our studies provide additional evidence of a potential role for CD26 in the treatment of selected malignancies.

CD26/DPPIV signal transduction function, but not proteolytic activity, is directly related to its expression level on human Th1 and Th2 cell lines as detected with living cell cytochemistry

CD26/DPPIV is a cell surface glycoprotein that functions both in signal transduction and as a proteolytic enzyme, dipeptidyl peptidase IV (DPPIV). To investigate how two separate functions of one molecule are regulated, we analyzed CD26 protein expression and DPPIV enzyme activity on living human T-helper 1 (Th1) and Th2 cells that express different levels of CD26/DPPIV. DPPIV activity was specifically determined with the synthetic fluorogenic substrate ala-pro-cresyl violet and CD26 protein expression was demonstrated with an FITC-conjugated CD26-specific antibody. Fluorescence of liberated cresyl violet (red) and FITC (green) was detected simultaneously on living T-cells using flow cytometry and spectrofluorometry. Th1 cells expressed three- to sixfold more CD26 protein than Th2 cells. The signal transduction function of the CD26/DPPIV complex, tested by measuring its co-stimulatory potential for proliferation, was directly related to the amount of CD26 protein at the cell surface. However, DPPIV activity was similar in both cell populations at physiological substrate concentrations because of differences in K(m) and V(max) values of DPPIV on Th1 and Th2 cells. Western blotting and zymography of Th1 and Th2 whole-cell lysates demonstrated similar patterns. This study shows that two functions of one molecule can be controlled differentially.

A growth-constrained environment drives tumor progression invivo

We recently have shown that selective growth of transplanted normal hepatocytes can be achieved in a setting of cell cycle block of endogenous parenchymal cells. Thus, massive proliferation of donor-derived normal hepatocytes was observed in the liver of rats previously given retrorsine (RS), a naturally occurring alkaloid that blocks proliferation of resident liver cells. In the present study, the fate of nodular hepatocytes transplanted into RS-treated or normal syngeneic recipients was followed. The dipeptidyl peptidase type IV-deficient (DPPIV(-)) rat model for hepatocyte transplantation was used to distinguish donor-derived cells from recipient cells. Hepatocyte nodules were chemically induced in Fischer 344, DPPIV(+) rats; livers were then perfused and larger (>5 mm) nodules were separated from surrounding tissue. Cells isolated from either tissue were then injected into normal or RS-treated DPPIV(-) recipients. One month after transplantation, grossly visible nodules (2--3 mm) were seen in RS-treated recipients transplanted with nodular cells. They grew rapidly, occupying 80--90% of the host liver at 2 months, and progressed to hepatocellular carcinoma within 4 months. By contrast, no liver nodules developed within 6 months when nodular hepatocytes were injected into the liver of recipients not exposed to RS, although small clusters of donor-derived cells were present in these animals. Taken together, these results directly point to a fundamental role played by the host environment in modulating the growth and the progression rate of altered cells during carcinogenesis. In particular, they indicate that conditions associated with growth constraint of the host tissue can drive tumor progression in vivo.

Elevated serum dipeptidyl peptidase IV (CD26, EC 3.4.14.5) activity in experimental liver cirrhosis

BACKGROUND

Dipeptidyl peptidase IV (DPP IV) is a cell surface ectoenzyme widely distributed in the rat body, present on the epithelial cells of the brush border membranes (e.g. bile canaliculi) and on the surface of reactive lymphocytes and fibroblasts. DPP IV has been implicated in hepatocyte-extracellular matrix interactions, fibroblast activation and proliferation and in T-cell activation. Aberrant DPP IV expression was found in human liver cirrhosis, and elevated serum DPP IV activity was reported in patients with primary biliary cirrhosis and chronic hepatitis C virus infection. The aim of the study was to examine serum DPP IV activity in experimental liver cirrhosis.

METHODS

Liver cirrhosis was induced by administering diethyl-nitrosamine, phenobarbital and CCl4 in Fischer-344 male rats (n = 22). Phenobarbital-treated (n = 9) and nontreated (n = 9) male rats were used as controls. Serum DPP IV activity was measured using a microplate-based continuous-monitoring assay. Recombinant rat DPP IV was used as standard and Gly-Pro-PNA was used as substrate. Enzyme activity was given in nmol mL-1 min-1 (U L-1).

RESULTS

Significantly higher DPP IV activity was found in the sera of rats with experimental liver cirrhosis (39.2 +/- 3.7; mean +/- SD) compared to phenobarbital-treated (11 +/- 4, P < 0.000002) and nontreated (10.9 +/- 0.9, P < 0.000002) rats. There was a positive correlation between DPP IV activity and concentrations of aspartate aminotransferase (r = 0.73, P = 0.0001) and alanine aminotransferase (r = 0.69, P = 0.0004).

CONCLUSIONS

The significantly higher serum DPP IV activity found in experimental liver cirrhosis is in concordance with human observations. The elevation was probably not due to the enzyme induction effect of phenobarbital. In this experimental model, serum DPP IV seems to be an indicator for chronic liver injury.

Regulation of CD26/DPPIV gene expression by interferons and retinoic acid in tumor B cells

Interferons (IFNs alpha, beta and gamma) and all trans retinoic acid (RA) have the ability to activate genes with GAS sites. We have found that the promoter of CD26/dipeptidylpeptidase IV (DPPIV) contains a consensus GAS site TTCnnnGAA located at bp-35 to -27, and computer analysis confirmed this sequence to be a putative Stat binding site. Consistent with this finding, we show that IFNs and RA rapidly enhanced CD26 gene and protein expression in chronic B lymphocytic leukemia (B-CLL) cells. Immunoblot analyses revealed that unstimulated B-CLL cells expressed detectable levels of serine/tyrosine-phosphorylated Stat1alpha, and RA and IFN-gamma treatment led to increased levels of tyrosine phosphorylation of Stat1alpha and its nuclear accumulation. As shown by electrophoretic mobility shift assay, RA and IFN-gamma increased the binding of a nuclear protein to the GAS-CD26 element. Shift-Western blotting identified Stat1alpha as the GAS-CD26 binding factor. Augmented levels of CD26 protein in malignant B cells cultured with IFNs or RA coincided with the enhancement of DPPIV activity. Taken together, our results are in favor of the IFN-/RA-mediated upregulation of CD26/DPPIV in B-CLL through the signaling pathway involving Stat1alpha and the GAS response element of CD26 promoter.