Aminopeptidase-N (CD13; gp 150): contrasting patterns of enzymatic activity in blood from patients with myeloid or lymphoid leukemia

The alkylating prodrug J1 can be activated by aminopeptidase N, leading to a possible target directed release of melphalan

The alkylating prodrug of melphalan, J1 (melphalanyl-L-p-fluorophenylalanyl ethyl ester) is currently in early clinical trials. Preclinical studies have shown that J1-mediated cytotoxicity is dependent on hydrolytic activity of tumor cells. In this report we have analyzed potential peptidases and esterases of importance for release of free melphalan from J1. Exposure of tumor cell lines to J1 resulted in a significant increased level of free intracellular melphalan, at least tenfold at C(max), compared to exposure to melphalan at the same molar concentration. This efficient intracellular delivery could be inhibited in both magnitude and in time by bestatin, a broad spectrum inhibitor of the aminopeptidases, including the metalloproteinase aminopeptidase N (APN, EC 3.4.11.2.), and ebelactone A, an esterase inhibitor. These effects resulted, as expected, in decreased cytotoxic effects of J1. A specific role of APN in hydrolyzing J1 releasing free melphalan was demonstrated in vitro with pure APN enzyme. By using plasmid-based overexpression of APN or down regulation of endogenous APN with siRNA in different tumor cell lines we here confirm the involvement of APN in J1-mediated cytotoxic and apoptotic signaling. In conclusion, this study demonstrates a role of APN in the activation of the melphalan prodrug J1 and subsequently, its cytotoxicity. Given that APN is shown to be overexpressed in several solid tumors our data suggest that J1 may be activated in a tumor selective manner.

Incadronate inhibits aminopeptidase N expression in prostatic PC-3 cells

Bone metastasis is an important cause of morbidity in advanced prostate cancer. Bisphosphonates are widely used for the treatment and prevention of osteoporosis, but recently have been observed to be effective in controlling prostate cancer metastasis. Since aminopeptidase N (AP-N) is known to be involved in the metastasis of prostate cancer, we investigated the effect of bisphosphonate on AP-N expression. Incadronate induced inhibition of AP-N mRNA and protein expression in PC-3 cells. The inhibitory effect of AP-N mRNA expression was also observed in the cells treated with pravastatin and other nitrogen-containing bisphosphonates, which inhibit the key enzyme in the isoprenoid biosynthesis pathway. The decrease of AP-N mRNA expression induced by incadronate was inhibited by co-incubation with geranylgeranyl diphosphate (GGPP). Moreover, GGTI-286 treatment also resulted in reduced AP-N mRNA expression. The translocation of small G protein Rap1 from the cytosol to the membrane was inhibited by incadronate and pravastatin, respectively. These above results indicate that the decrease in AP-N expression elicited by bisphosphonate is related to the inhibition of the mevalonate pathway.

The main neutral aminopeptidase activity of human lymphoid tumour cell lines does not originate from the aminopeptidase N-(APN; CD13) gene

Lymphocytes and related cell lines are predominantly CD13-negative, however, there are reports describing neutral aminopeptidase activity in or on these cells. The aim of this study was to answer the question, whether this activity originates from APN-gene expression. The total cellular activities (Ala-pNA hydrolysis) of lymphoid cell lines are up to 15 times higher than that of normal lymphocytes. Despite weak or lacking CD13 surface expression all lymphoid cell lines tested contain APNmRNA as quantified by competitive RT-PCR as well as low enzymatic activity in their particulate fractions. By isoelectric focusing two enzyme species with isoelectric points of 5.4 or between 3.5 to 4.8, respectively, were detected. To investigate whether these activities result from APN-gene we established transfectants lacking cellular APN expression of the CD13-positive histiocytic cell line U937 and the CD13-negative T-cell line H9. Studies on these transfectants proved (I) that the main neutral aminopeptidase activity expressed in lymphoid cells is definitively not related to APN and (II) that APN is also expressed in lymphoid cells, although on a low level only.