Suppression of tumour growth by orally administered osteopontin is accompanied by alterations in tumour blood vessels

Background: The integrin-binding protein osteopontin is strongly associated with tumour development, yet is an abundant dietary component as a constituent of human and bovine milk. Therefore, we tested the effect of orally administered osteopontin (o-OPN) on the development of subcutaneous tumours in mice. Methods: Bovine milk osteopontin was administered in drinking water to tumour-bearing immune-competent mice. Tumour growth, proliferation, necrosis, apoptosis and blood vessel size and number were measured. Expression of the α 9 integrin was determined. Results: o-OPN suppressed tumour growth, increased the extent of necrosis, and induced formation of abnormally large blood vessels. Anti-OPN reactivity detected in the plasma of OPN-null mice fed OPN suggested that tumour-blocking peptides were absorbed during digestion, but the o-OPN effect was likely distinct from that of an RGD peptide. Expression of the α 9 integrin was detected on both tumour cells and blood vessels. Potential active peptides from the α 9 binding site of OPN were identified by mass spectrometry following in vitro digestion, and injection of these peptides suppressed tumour growth. Conclusions: These results suggest that peptides derived from o-OPN are absorbed and interfere with tumour growth and normal vessel development. o-OPN-derived peptides that target the α 9 integrin are likely involved.