The cancer stem cell (CSC) model posits the presence of a small number of CSCs in the heterogeneous cancer cell population that are ultimately responsible for tumor initiation, as well as cancer recurrence and metastasis. CSCs have been isolated from a variety of human cancers and are able to generate a hierarchical and heterogeneous cancer cell population. CSCs are also resistant to conventional chemo- and radio-therapies. Here we report that ionizing radiation can induce stem cell-like properties in heterogeneous cancer cells. Exposure of non-stem cancer cells to ionizing radiation enhanced spherogenesis, and this was accompanied by upregulation of the pluripotency genes Sox2 and Oct3/4. Knockdown of Sox2 or Oct3/4 inhibited radiation–induced spherogenesis and increased cellular sensitivity to radiation. These data demonstrate that ionizing radiation can activate stemness pathways in heterogeneous cancer cells, resulting in the enrichment of a CSC subpopulation with higher resistance to radiotherapy.

The precursor homocysteine is metabolized either through the methionine cycle to produce methionine or through the transsulfuration pathway to synthesize cysteine. Alternatively, cysteine can be obtained through uptake of its oxidized form, cystine. Many cancer cells exhibit methionine dependency such that their proliferation is impaired in growth media in which methionine is replaced by homocysteine. Here, we showed that oncogenic PIK3CA and decreased expression of SLC7A11, a gene that encodes a cystine transporter also known as xCT, correlated with increased methionine dependency in breast cancer cells. Oncogenic PIK3CA was sufficient to confer methionine dependency to mammary epithelial cells, in part by decreasing cystine uptake through the transcriptional and posttranslational inhibition of xCT. Manipulation of xCT activity altered the proliferation of breast cancer cells in methionine-deficient, homocysteine-containing media, suggesting that it functionally contributed to methionine dependency. We propose that concurrent with decreased cystine uptake through xCT, PIK3CA mutant cells use homocysteine through the transsulfuration pathway to synthesize cysteine. Consequently, less homocysteine is available to produce methionine, contributing to methionine dependency. These results indicate that oncogenic PIK3CA alters methionine and cysteine utilization, in part by inhibiting xCT, to contribute to the methionine dependency phenotype in breast cancer cells.

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