Influence of tyrosine and phenylalanine limitation of cytotoxicity of chimeric TGF-alpha toxins on B16BL6 murine melanoma in vitro.

Previous research in animals supports the use of tyrosine and phenylalanine (Tyr-Phe) restriction as an adjuvant to the treatment of cancer. In this regard, dietary restriction of Tyr-Phe specifically inhibits the growth of B16BL6 melanoma tumors, dramatically suppresses spontaneous hematogenous metastasis, and modulates the sensitivity of these tumor cells to growth factors. Two chimeric toxins, HB-TGF alpha-(PEKDEL)-K-4E and TGF alpha-(PEKDEL)-K-4E, were examined for their toxicity against the B16BL6 melanoma cell line, and the ability of Tyr-Phe limitation to modulate the potential of these toxins was examined. Tyr-Phe limitation significantly enhanced the cytotoxic effects of HB-TGF alpha-(PEKDEL)-K-4E approximately 10-fold toward B16BL6 melanoma, and free heparin diminished the cytotoxicity of HB-TGF alpha-(PEKDEL)-K-4E. Although TGF alpha-(PEKDEL)-K-4E is cytotoxic to this cell line, Tyr-Phe limitation did not affect the cytotoxicity of this toxin. Tyr-Phe limitation inhibited the synthesis and secretion of heparin-binding proteins but did not alter the expression of surface heparan sulfate proteoglycans. These data suggest that cell surface heparan sulfate proteoglycan is a target for binding and execution of the cytotoxicity of HB-TGF alpha-(PEKDEL)-K-4E and that augmentation of cytotoxicity by Tyr-Phe limitation is due to the inhibition of heparin-binding protein production.