Human
breast cancer cells (MDA-MB-435s) secrete a
nucleoside diphosphate kinase (NDPK-B) as a
phosphoprotein capable of converting
diphosphate nucleosides to
triphosphate nucleotides for one round in the absence of a phosphoryl donor. Incubation of the partially purified NDPK-B (Nm23-H2 by Western blot) from [gamma32P]Pi-labeled cells with non-radioactive
ADP results in the formation of [gamma32P]
ATP (Proc. West. Pharmacol. Soc. 44: 61-63, 2001). The presence of a secreted
protein that can maintain
ATP levels in the vicinity of capillary and lymph vessels may support
cancer metastasis in several ways based on the known actions of
ATP at P2Y receptors: facilitate intravasation of
breast cancer cells that migrate from a solid
tumor, support their extravasation at a distal site, and stimulate angiogenesis. The putative role of
angiostatin (AS) as an
ATP-synthase inhibitor led us to test the notion that AS blocks NDPK-B activity. Addition of commercial AS (kringles 1-4) did not alter
enzyme activity. However, AS produced by us and never lyophilized, blocked NDPK activity in a dose-dependent fashion consistent with the notion that extracellular
ATP generation by
tumor cells may be important to the development of
metastases. The ability of 0.5 mg/ml
angiostatin to block NDPK-B activity to approximately 75% of control activity compared poorly with the
polyphenol inhibitors of. The
catechin gallates, theaflavins and
ellagic acid inhibited NDPK-B completely with the rank order of potency: EA > theaflavins > EGCG > ECG > PAPS. Our results suggest that the
biological activity of
angiostatin as a putative
metastasis inhibitor may be in part the result of nm23 inhibition and that the production, lyophilization, packaging or storage of commercial
angiostatin leads to the alteration of its
biological activity against NDPK-B.
Ellagic acid is a potent (IC50 = 10.5 microM) NDPK-B inhibitor that may prove useful in elucidating the role of
cancer-cell secreted NDPK-B in
tumor development.