Abstract P2-06-14: The role of SLC7A5 (LAT1) in endocrine therapy-resistant breast cancer

Endocrine therapies are commonly used to treat estrogen receptor-positive (ER+) breast cancers, which comprise 70% of all new breast cancer cases. Unfortunately, emergence of resistance to these therapies presents a major clinical challenge. Cancer cells can adapt to the dysregulation of cellular metabolism induced by endocrine therapy in order to evade cell death.Central to this adaptation is the scavenging of free-formed amino acids from the tumor microenvironment. For example, we found 109 solute carrier (SLC) mRNAs to be differentially expressed between endocrine-sensitive and resistant cells. We began our mechanistic studies of these genes with SLC family 7 member 5 (SLC7A5 or LAT1). SLC7A5 is a key component of a transmembrane transporter, which can complex with CD98 and increase the uptake of large, neutral amino acids (such as leucine or tyrosine).

We used a panel of endocrine therapy-resistant (LCC9) and sensitive (MCF7; LCC1) breast cancer cells. SLC7A5 expression was upregulated by estrogen in MCF7 and LCC1 cells; this induction was blocked by fulvestrant treatment. Basal expression of the SLC7A5 protein in the absence of estrogen was 2.75-fold higher in LCC9 cells compared with MCF7 cells; SLC7A5 mRNA expression was 71-fold higher. Fulvestrant treatment did not significantly alter SLC7A5 mRNA or protein expression in LCC9 cells. Inhibiting SLC7A5 function using either a pharmacological inhibitor (JPH203), or depleting expression using siRNA, led to significant suppression of LCC9 cell growth. Cell cycle analysis revealed that SLC7A5 depletion caused cells to accumulate in the G1-phase, with a concurrent reduction of cells in S-phase. In four publicly available datasets of ER+, tamoxifen treated breast cancer patients, high expression of SLC7A5 was significantly associated with poor relapse-free survival.

This study uncovers a novel adaptive mechanism in endocrine therapy-resistant breast cancer cells that is facilitated by increased expression of SLC7A5, which enables them to supplement their increased metabolic needs and promoting cell growth. Blocking the functions of SLC7A5, perhaps in conjunction with inhibition of autophagy, may therefore offer a new avenue of potential therapeutic intervention against endocrine therapy-resistant breast cancers.

Citation Format: Sevigny CM, Sengupta S, Luo Z, Jin L, Pearce D, Clarke R. The role of SLC7A5 (LAT1) in endocrine therapy-resistant breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-06-14.

Abstract P4-10-01: Estrogen induced apoptosis can be mimicked by targeting unfolded protein response

Estrogen signaling is considered to promote growth in estrogen receptor positive (ER+) breast cancers and its actions can be blocked by antagonists like tamoxifen or by inhibiting the synthesis of estrogen by aromatase inhibitors. Both of these classes of drug are used to treat ER+ breast cancers in the clinic. Paradoxically, before the discovery of tamoxifen and aromatase inhibitors high dose estrogen (HDE) was the choice of endocrine therapy to treat post-menopausal breast cancers. Recent clinical trials have observed 30% clinical benefit rate with high as well as low doses of estrogen-therapy in aromatase-inhibitor resistant breast cancers. Despite its clinical success, the precise underlying mechanism of estrogen-therapy by which it triggers the tumor regression remains unknown. Studies in the laboratory have indicated that unfolded protein response (UPR) and apoptotic pathways may play important role in estrogen-induced apoptosis. Using MCF7:5C cells, which can proliferate independent of estrogen and are hyper-sensitive to estrogen as evident by induction of apoptosis, we demonstrate that increased global protein translational load as the trigger for estrogen-induced apoptosis. This subsequently leads to endoplasmic reticulum (EnR) stress and activates the protein kinase-RNA-like endoplasmic reticulum kinase (PERK) pathway of UPR. Our results also suggest that sustained phosphorylation of eukaryotic initiation factor 2-alpha (eIF2-α), a downstream target of PERK activation, may be crucial in estrogen-induced apoptosis. Phosphorylation of eIF2-α attenuated global translation but preferentially allowed high expression of transcription factors, including, activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP). ATF4 and CHOP are known to activate apoptosis. Notably, we were able to recapitulate this phenotype by pharmacologically inhibiting the regulatory subunits of the protein phosphatase 1, GADD34 (growth arrest and DNA damage inducible protein) and CReP (constitutive repressor of eIF2α phosphorylation), that are responsible for de-phosphorylation of eIF2-α. This was evident in MCF7:5C cells as well as another estrogen-independent breast cancer cell line LCC9 that is resistant to both tamoxifen and fulvestrant but does not undergo estrogen mediated apoptosis. We further observed that the combination of 4-hydroxy-tamoxifen (4OHT) and pharmacological inhibitors of GADD34 and CReP potentiated its apoptotic action in both LCC9 and MCF7:5C cells. These results not only enhance our understanding of the apoptotic mechanism of estrogen but also provides crucial evidence that estrogen-induced apoptosis can be mimicked by manipulating the unfolded protein response even in breast cancer cells that are not susceptible to estrogen mediated apoptosis.

Citation Format: Sengupta S, Sevigny C, Clarke R. Estrogen induced apoptosis can be mimicked by targeting unfolded protein response [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-10-01.