Breast cancer (BC) is the most common noncutaneous cancer among US women, and ~20% of these cancers overproduce the growth-promoting tyrosine kinase receptor, HER2. Although there are targeted treatments available for HER2-overexpressing BC, there remains an urgent need to identify new treatment strategies since this disease is associated with poor prognosis, resistance to therapy, and high risk of recurrence. We have established that calorie restriction (CR; 30% reduction in total energy relative to ad libitum-fed controls) has significant tumor suppressive effects across all breast cancer subtypes and has potential to be utilized as an adjunctive therapy. This study investigates the mechanisms by which CR decreases HER2-overexpressing BC progression. To mimic CR in vitro, murine MMTV-neu cells that overexpress neu, the rodent homolog of HER2, were treated with media containing reduced serum (1%), reduced glucose (1mM), or reduced serum and reduced glucose (1%/1mM) compared to control media (10% serum, 25mM glucose). MTT assays demonstrated that both serum restricted medias (1% and 1%/1mM) significantly decreased cellular viability (p<0.05) compared to control. To investigate the mechanism of this alteration, western blotting analysis of proteins associated with HER2 BC progression was conducted. In MMTV-neu cells treated with serum restricted medias (1% and 1%/1mM), there was a significant decrease in activity of tumor suppressor protein p21. Both p21 and phospho-21 expression were decreased with serum restriction, as well as the ratio of phospho-p21 relative to p21 (p<0.05). Phosphorylated p21 is localized to the cytoplasm and allows BC cells to evade apoptosis and proliferate uncontrollably. These results suggest that short-term CR, achieved specifically through serum restriction alone or in combination with glucose restriction, is associated with decreased phosphorylation and cytoplasmic localization of p21, which may be responsible for modulating anti-proliferative activities. This finding is significant because clinical studies have found that increased cytoplasmic p21 in HER2-overexpressing BC predicted reduced survival in patients at 5 years. Utilizing CR or pharmacologic regimens that mimic CR in conjunction with existing therapies may prevent or reverse cytoplasmic p21 localization in this cancer subtype, highlighting the importance of this investigation. Studies are ongoing in MMTV-neu cells transfected with plasmids encoding mutated p21 proteins that are constitutively expressed in either the cytoplasm or the nucleus. This will confirm the extent to which localization of p21 is responsible for CR-induced alterations in HER2-overexpressing BC cell proliferation.
Citation Format: Magdalena A. Rainey, Laura A. Smith, Ciara H. O'Flanagan, Stephen D. Hursting. Short-term calorie restriction alters expression of tumor suppressor p21 in HER2-overexpressing breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2396.