Titolo della tesi: “ID4 protein expression controls tumor microenvironment through paracrine activity in Basal- like Breast Cancer”
Breast cancer (BC) is the most common malignant tumour in women worldwide and contributes as the primary cause of cancer death. As recently emerged, the progression to malignancy in breast cancer is regulated by reciprocal interactions between cancer cells and host stromal and immune-related cells in the tumour microenvironment (TME). The TME comprises mesenchymal stem cells, adipocytes, endothelial cells, fibroblast, extracellular matrix (ECM) and immune system elements, such as tumor-associated macrophages (TAMs) and leukocytes. Among the cell types infiltrated in breast tumors, TAMs have been extensively shown to tightly control angiogenesis, immunosuppressive pathways, tissue remodelling and progression to malignancy.
ID4, a member of the Inhibitors of Differentiation ID family of proteins (ID1 to 4), involved in the inhibition of differentiation-associated bHLH transcription factors, is associated with a stem-like phenotype and poor prognosis in basal-like BC (BLBC).
Here, we show that ID4 protein, previously referred to as regulator of linear isoforms of VEGFA, induces back-splicing of VEGFA exon 7, producing circular RNA circ_0076611. Circ_0076611 is detectable in triple-negative breast cancer (TNBC) cells and tissues, in exosomes released from TNBC cells and in the serum of breast cancer patients. Circ_0076611 interacts with a variety of proliferation-related transcripts, included VEGFA mRNAs, and increases cell proliferation and migration of TNBC cells. Mechanistically, circ_0076611 favors the expression of its target mRNAs by facilitating their interaction with components of the translation initiation machinery.
In addition to the previously reported reprogramming of TAMs in proangiogenic sense, which strictly depends on the levels of ID4 in cancer cells 1, we demonstrate here that ID4-dependent circ_0076611, released from cancer cells, can be transferred to TAMs, where it enhances the expression of VEGFA. Moreover, in TAMs the exogenous circ_0076611 controls immune-related pathways relevant for tumour immune escape. Specifically, in macrophages grown with conditioned medium from BC cells, we observed a circ_0076611-dependent downregulation of CXCL9 and CXCL10, two cytokines regulating T cell recruitment and activation.
In conclusion, this study shows that ID4-dependent circ_0076611 exerts autocrine and paracrine functions in BLBC, leading to the induction of proangiogenic growth factors expression and to the downregulation of key cytokines responsible for efficient recognition of tumor cells by the immune system.