Page 104 - SRPSKO DRUŠTVO ISTRAŽIVAČA RAKA

Serbian Association for Cancer Research SDIRSACR

P06

Foxo3a mediates Sirt3-dependent regulation of ERα signaling in MCF-7 breast cancer cells

Ena Šimunić, Marija Pinterić, Iva I. Podgorski, Marijana Popović Hadžija, Vedrana Filić, Ana Tadijan, Ivan Ciganek,
Denis Pleše, Tihomir Balog, Sandra Sobočanec

Ruđer Bošković Institute, Zagreb, Croatia

Keywords: Sirtuin 3, Foxo3a, MCF-7, breast cancer, estrogen receptor

Background: Breast cancer remains the most frequently diagnosed cancer among women, with approximately 70%
of cases being estrogen receptor alpha (ERα) positive. Estrogen (E2) plays a critical role in the development and
progression of these hormone-dependent tumors. Sirtuin 3 (Sirt3), a major mitochondrial deacetylase, has shown
context-dependent roles in cancer, acting as either an oncogene or a tumor suppressor. In estrogen/progesterone
receptor-positive MCF-7 breast cancer cells, we previously demonstrated that Sirt3 overexpression increases ERα
levels while reducing tumorigenic properties through upregulation of p53 and attenuation of E2 signaling. In this study,
we explore the potential involvement of transcription factor Foxo3a as a mediator in the Sirt3–p53–ERα regulatory
axis. Foxo3a acts as a tumor suppressor in multiple cancer types, although its specific role in cancer remains unclear.
Moreover, it shows promise as a potential marker for tumor diagnosis and prognosis in breast cancer patients.
Materials and methods: We use MCF-7 breast cancer cells transfected with the FLAG-tagged Sirt3 (MCF-7S3) or empty
pcDNA3.1 plasmid (MCF-7C). Following a combination of Sirt3 overexpression, E2 and fulvestrant (ICI) treatments,
we analyzed the morphology and survival of the cells, protein expression and interaction by Western blot and co-
immunoprecipitation and examined localization using confocal microscopy.
Results: Besides observed tumor-suppressive effects of Sirt3 in these cells, we revealed that Foxo3a expression and
localization are modulated by both Sirt3 and hormonal stimuli. Co-immunoprecipitation experiments confirmed an
interaction between Foxo3a and ERα, which was significantly reduced in Sirt3-overexpressing clones compared to
controls, and almost completely reduced in both clones upon E2 treatment.
Conclusions: Sirt3 expression induces p53 and Foxo3a expression and impacts their localization in MCF-7 breast cancer
cells, coordinately stimulating tumor suppressor cellular processes in tumor cells. Both Sirt3 and E2 disrupt Foxo3a–ERα
interaction, potentially shifting the function of Foxo3a away from ERα-regulated transcription toward p53-mediated
tumor suppression. All these results indicate that Foxo3a acts as a functional mediator of Sirt3-driven changes in ERα
signaling in MCF-7 cells, highlighting the Sirt3–Foxo3a–p53 axis as a potential target in the treatment of hormone-
dependent breast cancer.

P07

Exploring HPV E6 interaction network: new insights into HPV-induced tumorigenesis

Katarina Soža, Toni Rendulić, Josipa Skelin Ilić, Vjekoslav Tomaić

Laboratory of Molecular Virology and Bacteriology, Division of Molecular Medicine, Ruđer Bošković Institute, Zagreb, Croatia

Keywords: HPV, cancer, interactome

Background: While most people will be infected with Human papillomaviruses (HPVs) at some stage of their lives, only
persistent infection with high-risk (HR) types, such as HPV 16 and 18, can lead to the development of various cancers.
Although HR types are predominantly linked to cervical cancer, they are also associated with a subset of anogenital
and head and neck cancers. In contrast, low-risk (LR) HPV types, such as HPV 6 and 11, are not considered tumorigenic,
as they primarily cause benign warts. The hallmark of HPV-induced tumorigenesis is uncontrolled expression of two
main viral oncoproteins, E6 and E7, which target multiple cellular regulators, including p53 and pRB, the major tumor
suppressor proteins involved in the control of the cell cycle and apoptosis. Furthermore, HR E6 oncoproteins also
interact with a number of PDZ-domain containing proteins, which act as cellular tumor suppressors and are involved
in cell polarity regulation. E6s bind to these targets via their PDZ-binding motifs (PBMs), present on their extreme
C-termini, ultimately driving their degradation in the proteasome/dependent manner. In this study we identified a
novel E6 interacting partner, which has been recently shown to be strongly associated with promotion and progression

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