Influence of tyrosine and phenylalanine limitation on cytotoxicity of chimeric TGF‐α toxins on B16Bl6 murine melanoma in vitro

Abstract

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‐TGFα‐PE^4EKDEL and TGFα‐PE^4EKDEL, 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‐TGFα‐PE^4EKDEL approximately 10‐fold toward B16BL6 melanoma, and free heparin diminished the cytotoxicity of HB‐TGFα‐PE^4EKDEL Although TGFa‐PE^4EKDEL 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 proteo‐glycans. These data suggest that cell surface heparan sulfate proteoglycan is a target for binding and execution of the cytotoxicity of HB‐TGFα‐PE^4EKDEL and that augmentation of cytotoxicity by Tyr‐Phe limitation is due to the inhibition of heparin‐binding protein production.