Project Summary- Triple-negative breast cancer (TNBC), the most aggressive subtype of breast cancer (BC),
evades hormonal treatment modalities, and patients with TNBC experience high rates of metastasis and have
a poor prognosis. Therefore, there is a critical need to find better approaches to treat TNBC. Sirtuin 1 (SIRT1)
represents an interesting target in this regard, as SIRT1 has been shown to be implicated in cancer as a
tumor suppressor, and in fact, SIRT1 mRNA and protein are significantly downregulated in TNBC. Previous
work has shown that decreasing SIRT1 in human TNBC cells promotes the generation of a secretome
containing soluble hydrolases and a large number of exosomes, a specific class of extracellular vesicles, with
unique cargo. Furthermore, the hydrolases and exosomes produced by TNBC cells depleted of SIRT1 were
shown to promote the aggressive phenotype of TNBC cells by promoting cell survival, invasive activity, and
metastasis. Thus, it is important to determine whether decreasing SIRT1 expression or activity in vivo in
mouse models of breast cancer promotes tumorigenesis and metastasis. Additionally, the mechanism for
decreased expression of SIRT1 in TNBC is largely unknown, and a better understanding of SIRT1 regulation
will uncover changes in cancer-promoting pathways that make TNBC so aggressive. In this proposal, the
effects of decreased SIRT1 expression or activity on tumorigenesis and metastasis will be elucidated (Aim 1),
and the transcriptional and post-transcriptional regulatory factors that decrease SIRT1 expression in TNBC
will be identified (Aim 2). In Aim 1, three distinct, yet complementary, mouse models of breast cancer will be
used to determine the effects of altered SIRT1 expression or pharmacologic modulation of SIRT1 activity on
cancer progression in the tumor microenvironment, including tumor growth, invasiveness, and angiogenesis,
as well as rate of metastasis. In Aim 2, precision nuclear run-on RNA sequencing (PRO-Seq) and small RNA
sequencing will be performed to elucidate transcriptional and post-transcriptional regulators of SIRT1,
respectively. For this aim, a system of three human BC cell lines with varying levels of SIRT1 expression will
be used. Changes in transcriptional regulation and microRNA expression with PRO-Seq and small RNA-Seq,
respectively, will be determined by comparing across these three cell lines. Due to the highly aggressive
nature of and difficulty to treat TNBC, it is imperative to develop new therapeutic strategies that slow
tumorigenesis and metastasis. Through the outlined six-year training plan in Drs. Richard Cerione and Robert
Weiss’s labs, which have expertise in elucidating underlying molecular mechanisms and their effects in vivo
through mouse models, by determining the effects of low SIRT1 levels in mouse models of BC and the
regulatory factors that mediate this downregulation, this proposal holds promise for clarifying the role of
SIRT1 as a tumor suppressor in TNBC.