Antiestrogen Resistant Cell Lines Expressing Estrogen Receptor α Mutations Upregulate the Unfolded Protein Response...

Area of Science:

• Molecular biology
• Cancer research
• Endocrinology

Background:

• Metastatic outgrowth of estrogen receptor alpha (ERα) mutations Y537S and D538G is common following endocrine therapy for ERα-positive breast cancer.
• The functional impact of these ERα mutations replacing wild-type ERα in breast cancer cells remained unclear.

Purpose of the Study:

• To characterize the biological effects of ERα mutations Y537S and D538G in breast cancer cells.
• To investigate the efficacy of a novel ERα biomodulator, BHPI, in overcoming endocrine resistance driven by these mutations.

Main Methods:

• Utilized CRISPR-Cas9 genome editing and homology directed repair to generate 14 T47D cell lines with specific ERα mutations (Y537S or D538G).
• Assessed cell proliferation using 2D and 3D cell culture, including anchorage-independent growth.
• Investigated the effects of progestins, 4-hydroxytamoxifen, fulvestrant/ICI 182,780 (ICI), and the ERα biomodulator BHPI on cell growth and gene expression.
• Analyzed the activation of the unfolded protein response (UPR) and ERα target gene expression.

Main Results:

• ERαY537S and ERαD538G mutant cells exhibited estrogen-independent growth and increased proliferation when treated with progestins.
• These mutant cells displayed constitutive activation of estrogen-ERα target genes and partial resistance to antiestrogens like ICI.
• The ERα biomodulator BHPI completely inhibited proliferation in these resistant cell lines by hyperactivating the UPR.
• Constitutive UPR activation was observed in ERαY537S cells, further induced by progesterone, and UPR-regulated genes associated with tamoxifen resistance were upregulated.

Conclusions:

• ERα mutations Y537S and D538G confer estrogen-independent growth and antiestrogen resistance in breast cancer cells.
• Progestins, UPR activation, and potentially reduced ICI-induced ERα degradation contribute to this resistance.
• The ERα biomodulator BHPI represents a promising therapeutic strategy by targeting the UPR to overcome endocrine resistance driven by ERα mutations.