Improved host defense against L1210 leukemia by deprivation of dietary phenylalanine

A mixture of amino acids and other small molecules present in the serum suppresses the growth of murine and human tumors in vivo

Previously we have hypothesized that the small molecules which are selectively accumulated in cancer cells might participate in a non-immunological antitumor surveillance mechanism. We demonstrated earlier that a mixture of experimentally selected substances (“active mixture”, AM: l-arginine, l-histidine, l-methionine, l-phenylalanine, l-tyrosine, l-tryptophan, l-ascorbate, d-biotin, pyridoxine, riboflavin, adenine, l(-)malate) possesses a selective toxic effect in vitro on a variety of tumor cell lines, and we have shown that the AM selectively induces apoptosis of cancer cells in vitro. To explore the in vivo significance of our earlier findings we examined the antitumor effect of AM in Colon 26 murine colorectal adenocarcinoma, B16 murine melanoma, MXT murine mammary carcinoma, S180 murine sarcoma, P388 murine lymphoid leukemia, HL-60 human promyeloid leukemia, PC-3 human prostate carcinoma, and HT-29 human colon carcinoma tumor models. Treatment of tumor bearing mice with AM inhibited the growth of the tumors investigated, with an inhibitory effect ranging from 40 to 69%. The AM had a comparable antitumor effect with 5-fluorouracil and cisplatin in the Colon-26 tumor model, and combined treatment with AM and 5-fluorouracil or cisplatin resulted in an enhanced tumor growth inhibitory effect. The AM induced apoptosis through the mitochondrial pathway and induced G1 arrest in PC-3 cells and increased the number of apoptotic cells in PC-3 xenografts. These findings suggest that the AM might offer an interesting perspective in the treatment of cancer and in combination with other treatments may offer hope for a more effective cancer therapy.

Activation of caspases and cleavage of Bid are required for tyrosine and phenylalanine deficiency-induced apoptosis of human A375 melanoma cells

Deprivation of tyrosine (Tyr) and phenylalanine (Phe) inhibits growth and induces programmed cell death (apoptosis) of human A375 melanoma cells. Herein, we found that activation of caspases and release of mitochondrial cytochrome c are required for this process. Culturing A375 cells in Tyr/Phe-free medium, containing 10% dialyzed fetal bovine serum, results in activation of caspase-3-like activity. This is accompanied by decreased cell viability and increased apoptosis. Tyr/Phe deprivation also stimulates proteolytic cleavage of the DNA repair enzyme, poly(ADP-ribose) polymerase (PARP). Western blot analysis showed that caspases 3, 7, 8, and 9 are activated by deprivation of Tyr/Phe. Tyr/Phe deprivation decreases mitochondrial membrane potential, induces cleavage of Bid, increases translocation of Bax from the cytosol to mitochondria, and results in release of cytochrome c from the mitochondria to the cytosol. Apoptosis due to Tyr/Phe deprivation is almost completely inhibited by the broad-spectrum cell-permeable caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z.VAD.fmk). This inhibitor suppresses the cleavage of Bid, the release of cytochrome c from the mitochondria to the cytosol, and the cleavage of PARP. Decylubiquinone, a mitochondrial permeability transition pore inhibitor, does not suppress the activation of caspase 8 but suppresses release of cytochrome c, activation of caspase 9, and induction of apoptosis. These results indicate that activation of caspases, cleavage of Bid, and mitochondrial release of cytochrome c are required for apoptosis induced by Tyr/Phe deprivation.

Specific amino acid deficiency alters the expression of genes in human melanoma and other tumor cell lines

This study determined the effect of tyrosine (Tyr) and phenylalanine (Phe) deprivation on protein expression and phosphorylation of mitogen-activated protein kinase kinase 4 (MKK4)/stress-activated protein/Erk kinase (SEK1), a metastasis suppressor gene. Differential display and suppressive subtractive hybridization techniques identified genes modulated by Tyr and Phe deprivation. Expression of MKK4/SEK1 protein varied widely among human A375, A375SM and SB2 melanoma, PC-3 and DU145 prostate cancer, and MDA-MB-231 breast cancer cell lines and within the different lines. Phosphorylation of the MKK4/SEK1 protein similarly varied. No differences in MKK4/SEK1 gene expression or in the 41 other metastasis and tumor suppressor genes were found in A375 melanoma cells cultured in Tyr- and Phe-deprived media. A number of up-regulated and down-regulated genes in A375 melanoma cells were identified by differential display and suppressive subtractive hybridization that were pertinent to regulation of cytoskeletal organization, cell movement, gene transcription and metastasis. Two tumor marker genes, the gene for enolase and FUS/CHOP, were down-regulated by Tyr and Phe deprivation. This study shows that tumor cells display heterogeneity in their response to deprivation of Tyr and Phe and that these amino acids may be signaling molecules that regulate gene expression and function in tumor cells.

Evidence for nutrient modulation of tumor phenotype: impact of tyrosine and phenylalanine restriction

We have shown that Tyr and Phe restriction suppresses the malignant phenotype of the highly invasive and metastatic BL6 variant of B16 murine melanoma. Lung-colonizing abilities of Tyr- and Phe-modulated in vivo and in vitro variants of BL6 are inhibited following intravenous inoculation into mice fed normal diet. Although this antimetastatic effect of Tyr and Phe restriction is most likely not due to differences in attachment to endothelium, our data indicate that major impacts of Tyr and Phe restriction are at the level of the tumor, itself. Modulation of host immune responses, which in turn suppresses metastasis, does not appear to contribute significantly to the altered phenotype. Although numbers and function of T cells, mast cells, and NK cells are affected by Tyr and Phe restriction, they are not involved in the Tyr- and Phe-mediated suppression of tumor growth, metastasis, or angiogenesis. Our data do not rule out the importance of other host factors involved in the Tyr and Phe modulation of tumor phenotype. The outcome of this modulation results most likely from complex Tyr/Phe-tumor-host interactions.

Specificity of the suppression of metastatic phenotype by tyrosine and phenylalanine restriction

Amino acid restriction modulates tumor growth, although effects on metastasis are poorly documented. We demonstrate that low levels of tyrosine (Tyr) and phenylalanine (Phe) suppress metastasis of B16-BL6 melanoma and that these effects are specific to these two amino acids. Weight loss and sustained low body weight in mice fed low Tyr and Phe diet do not contribute to the antimetastatic effects. Furthermore, methionine (Met) restriction, which decreased survival of mice inoculated i.p. with B16 melanoma, only slightly inhibited spontaneous metastasis compared to the dramatic inhibition during Tyr and Phe restriction. Tyr and Phe restriction inhibited spontaneous metastasis by impairing the ability of tumor cells to establish metastatic foci and not via differential tumor cell removal from the blood. Spontaneous metastasis is blocked by Tyr and Phe intervention even in mice with established lymph node tumors. Tumors isolated from mice fed low Tyr and Phe diet reinoculated into mice fed normal diet exhibited lower experimental metastatic potential, reflected by decreased formation of lung tumor colonies and increased survival of inoculated mice. This decrease in metastatic potential is not associated with tumor chemosensitivity. These findings indicate that Tyr and Phe restriction could become an important adjuvant to effective melanoma treatment.