| Abstract |
Invasive lobular carcinoma (ILC) is an understudied, unique subtype of
breast cancer. E-cadherin (CDH1) loss is a hallmark of ILC that
contributes to its many observed epithelial to mesenchymal transition
(EMT)-like features. Though ILCs predominantly express clinically
actionable target estrogen receptor alpha (ERa), common late recurrences
suggest a role for endocrine therapy resistance. We recently identified de
novo resistance to the partial estrogen receptor antagonist/agonist
Tamoxifen (4OHT) in MDA-MB-134-VI ILC cells that was accompanied by
upregulation of EMT transcription factor (EMT-TF) SNAIL (SNAI1). We
therefore hypothesized that 4OHT induction of SNAIL contributes to
endocrine therapy resistance in a subset of ILCs. We demonstrated estrogen
induction of SNAI1 in multiple invasive ductal carcinoma (IDC) and ILC
cell lines. In contrast, 4OHT induction of SNAI1 was restricted to ILC
cells tested, and was associated with recruitment of ERa to the SNAI1
promoter. We observed upregulation of additional EMT genes in ILC cells,
partially translatable to clinical samples. However, 4OHT regulation of
SNAIL was unique, thus we pursued study of SNAIL-mediated phenotypes in
ILC cells. Unfortunately, manipulation of SNAIL levels in ILC cells was
challenging, with expression rebound in stable constitutive
knockdown/overexpression attempts. Inducible SNAIL overexpression revealed
unexpected repression of 2D and 3D proliferation. These data suggest a
role for SNAIL in unexplored, critical ILC phenotypes (e.g. tumor
dormancy). Lack of ILC models for these studies highlighted need for novel
ILC cell lines. We therefore established and characterized ILC cell line
WCRC-25 from a pleural effusion from a patient with ER+ ILC. WCRC-25
maintained phenotypes consistent with other ILC cells (e.g. slow
proliferation, poor soft agar formation). We identified a somatic CDH1
mutation (2240C>T, p.Q706*) in gDNA from WCRC-25 that we confirmed in
liquid biopsies from the patient. DNA/RNA sequencing analyses are being
performed using clinical samples from the patient. While loss of ER
impeded testing endocrine therapy resistance in WCRC-25 cells, additional
drug responses revealed sensitivity to a PI3K inhibitor. In summary, these
studies provided the foundation for better understanding of the role of
EMT-TF SNAIL in endocrine therapy resistance, and led to establishment and
characterization of a novel ILC research tool.
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