Understanding the Polyamine and mTOR Pathway Interaction in Breast Cancer Cell Growth.

The polyamines putrescine, spermidine and spermine are nutrient-like polycationic molecules involved in metabolic processes and signaling pathways linked to cell growth and cancer. One important pathway is the PI3K/Akt pathway where studies have shown that polyamines mediate downstream growth effects. Downstream of PI3K/Akt is the mTOR signaling pathway, a nutrient-sensing pathway that regulate translation initiation through 4EBP1 and p70S6K phosphorylation and, along with the PI3K/Akt, is frequently dysregulated in breast cancer. In this study, we investigated the effect of intracellular polyamine modulation on mTORC1 downstream protein and general translation state in two breast cancer cell lines, MCF-7 and MDA-MB-231. The effect of mTORC1 pathway inhibition on the growth and intracellular polyamines was also measured. Results showed that polyamine modulation alters 4EBP1 and p70S6K phosphorylation and translation initiation in the breast cancer cells. mTOR siRNA gene knockdown also inhibited cell growth and decreased putrescine and spermidine content. Co-treatment of inhibitors of polyamine biosynthesis and mTORC1 pathway induced greater cytotoxicity and translation inhibition in the breast cancer cells. Taken together, these data suggest that polyamines promote cell growth in part through interaction with mTOR pathway. Similarly intracellular polyamine content appears to be linked to mTOR pathway regulation. Finally, dual inhibition of polyamine and mTOR pathways may provide therapeutic benefits in some breast cancers.

Characterising the Response of Human Breast Cancer Cells to Polyamine Modulation.

Breast cancer is a complex heterogeneous disease with multiple underlying causes. The polyamines putrescine, spermidine, and spermine are polycationic molecules essential for cell proliferation. Their biosynthesis is upregulated in breast cancer and they contribute to disease progression. While elevated polyamines are linked to breast cancer cell proliferation, there is little evidence to suggest breast cancer cells of different hormone receptor status are equally dependent on polyamines. In this study, we characterized the responses of two breast cancer cells, ER+ (oestrogen receptor positive) MCF-7 and ER- MDA-MB-231 cell lines, to polyamine modulation and determined the requirement of each polyamine for cancer cell growth. The cells were exposed to DFMO (a polyamine pathway inhibitor) at various concentrations under different conditions, after which several growth parameters were determined. Exposure of both cell lines to DFMO induced differential growth responses, MCF-7 cells showed greater sensitivity to polyamine pathway inhibition at various DFMO concentrations than the MDA-MB-231 cells. Analysis of intracellular DFMO after withdrawal from growth medium showed residual DFMO in the cells with concomitant decreases in polyamine content, ODC protein level, and cell growth. Addition of exogenous polyamines reversed the cell growth inhibition, and this growth recovery appears to be partly dependent on the spermidine content of the cell. Similarly, DFMO exposure inhibits the global translation state of the cells, with spermidine addition reversing the inhibition of translation in the breast cancer cells. Taken together, these data suggest that breast cancer cells are differentially sensitive to the antitumour effects of polyamine depletion, thus, targeting polyamine metabolism might be therapeutically beneficial in breast cancer management based on their subtype.