A Cocktail of Specific Inhibitors of HER-2, PI3K, and mTOR Radiosensitises Human Breast Cancer Cells

Abstract

Intrinsic tumour radioresistance limits the benefit of radiotherapy. Targeted treatment modalities that are singly effective for triple-negative breast cancer are lacking, partly due to paucity of relevant targets as they are devoid of the human epidermal growth factor receptor 2 (HER-2), progesterone receptor (PR), and oestrogen receptor (ER); or to resistance to single-target therapies as a consequence of cellular heterogeneity. Concomitant targeting of cell signaling entities other than HER-2, PR and ER may sensitise triple-negative tumours to radiotherapy. In this study, we investigated the effect of an HER-2 inhibitor (TAK-165) and a dual inhibitor of phosphoinositide 3-kinase (PI3K) and mammalian target for rapamycin (mTOR) (NVP-BEZ235) in three human breast cancer cell lines. The potential of simultaneous inhibition of HER-2, PI3K and mTOR with a cocktail of the specific inhibitors TAK-165 and NVP-BEZ235, to radiosensitise human breast cancer cells in vitro was examined using the colony forming assay. Combined inhibition of HER-2, PI3K, and mTOR resulted in significant radiosensitisation in all cell lines, independent of HER-2, ER, or PR status. Radiosensitisation was more prominent in ER-and PR-negative cells expressing higher levels of epidermal growth factor receptor (EGFR). These data suggest that a cocktail of TAK-165 and NVP-BEZ235 could potentially be effective in the treatment of triple-negative breast cancer.

Combined Inhibition of PI3K, mTOR and Bcl-2 Significantly Radiosensitises Progesterone and Oestrogen Receptor Negative Breast Cancer Cells

“…However, in the ER and PR positive MCF-7 cell line [10,11], inhibition of PI3K, mTOR and Bcl-2 results in less than 2-fold radiosensitisation. These findings are consistent with the radiosensitisation reported elsewhere for pre-treatment of cell cultures with a cocktail of HER2, PI3K and mTOR inhibitors [6]. The marginal radiosensitisation seen in the MCF-7 cells can be attributed to the fact that they are PI3K mutated, in contrast to their PI3K wild-type counterparts, MDA-MB-231 and MCF-12A [12,13].…”

“…It is not clear why no radiomodulatory effect was seen in the MCF-7 cell line, given that these cells express as high as 4.5-fold Bcl-2 in comparison with the MDA-MB-231 cells [16,17]. The modest radiation modifying factors seen in all cell lines after pretreatment with the PI3K and mTOR inhibitor (Table 1) are consistent with those reported previously [6], and do not seem to depend on PI3K status, whereas the MCF-7 cells are PI3K mutant [12,13].…”

“…Inadequate expression of these antigens that could potentially be therapeutic targets may be responsible for the apparent resistance of triple-negative breast cancer to existing treatment regimens like radiotherapy. Inhibition of residual HER2, phosphoinositide 3-kinase (PI3K), and mammalian target of rapamycin (mTOR) radiosensitises tumour cells [3][4][5][6][7]. Therefore, it was hypothesised that concomitant inhibition of phosphoinositide 3-kinase (PI3K), mammalian target of rapamycin (mTOR), and the pro-survival gene (Bcl-2) may sensitise TNBC to radiotherapy to a larger extent.…”

Inhibition of PI3K and mTOR Sensitises Oestrogen Receptor Positive Human Breast Cancer Cells to a Large Fraction of Radiation Dose

“…There is evidence to show that inhibition of proteins involved in the PI3K/Akt/mTOR pathway can result in significant radiosensitisation of glioma and breast cancer cells over radiation absorbed doses of up to 8 Gy [15,25,26]. In recent studies, a similar radiosensitisation was demonstrated at 2 Gy in breast and prostate cell lines when pre-treated with inhibitors of HER-2, PI3K and mTOR [21,22]. In this investigation, HER-2 inhibitor (TAK-165) and dual inhibitor of PI3K and mTOR (NVP-BEZ235) were evaluated for their radiomodulatory effects at 6 Gy in 3 breast cell lines.…”

“…Therefore, targeting residual HER-2 and downstream signalling components of the EGFR family members may further potentiate the therapeutic benefit of stereotactic radiotherapy for triple-negative breast cancer. It was recently demonstrated that inhibition of phosphoinositide-3-kinase (PI3K) and mammalian target of rapamycin (mTOR) sensitised breast and prostate cancer cells to 2 Gy of radiation, whilst protecting normal prostate cells [21,22].…”