Solute Carriers and Heat Shock Proteins Modulate Endocrine Therapy Resistance in Hormone Positive Breast Cancer
Resistance to endocrine therapies remains a major challenge for patients with estrogen receptor-positive (ER+) breast cancer. Adaptive reprogramming of cellular metabolism in response to treatment is one feature of this resistance. Solute carriers (SLCs) transport sugars, amino acids, and other nutrients, regulating their intracellular abundance. We found 109 SLC mRNAs to be differentially expressed between endocrine sensitive and resistant breast cancer cells. In univariate analyses, 55 of these SLC mRNAs were associated with poor outcome in ER+ breast cancer patients treated with an endocrine therapy. Data from tandem mass tag (TMT) and stable isotope labeling with amino acids in cell culture (SILAC) studies identified 20 SLC proteins that were upregulated and 35 that were down regulated in resistant compared with sensitive human breast cancer cells. TMT and SILAC also found an upregulation of heat shock proteins HSPB8 and BAG3 in endocrine resistant cells. We concluded that HSPB8 was upregulated by estrogen in sensitive cells; BAG3 decreased with estrogen treatment. Genetic targeting of HSPB8 reduced S phase, but we could not determine the mechanism by which HSP8 influenced autophagy. We focused our mechanistic studies on SLC7A5 (LAT1) which, in complex with SLC3A2 (CD98), is the primary transporter of large, neutral amino acids including leucine and tyrosine. LAT1 expression was estrogen-regulated in endocrine sensitive cells but this regulation was lost, and basal expression was increased, in resistant cells. Of two EREs we identified in the LAT1 gene, ChIP-qRT-PCR confirmed site Ch16: 87868314-87868373 where levels of H4 acetylation increased, while levels of H3K9 dimethylation decreased, with estrogen treatment. Pharmacologic inhibition or genetic depletion of LAT1 each suppressed global protein translation as measured by total puromycinylated protein levels and cell proliferation in endocrine resistant cells. Overexpression of the LAT1 cDNA increased protein synthesis in cells. This study uncovers a novel LAT1-mediated adaptive response that contributes to the development of endocrine resistance and illuminates a possible role for LAT1 activity, and that of other SLCs, as potential rate limiting steps in determining the proliferative capacity of ER+ breast cancer cells. Blocking LAT1 function may offer a new avenue for effective therapeutic intervention against endocrine resistant ER+ breast cancers.
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