Angiogenesis : from tumor initiation to therapeutic resistance

Cancer is a leading cause of morbidity and mortality worldwide. In 2012 approximately 14 million new cases were diagnosed and 8.2 million cancer-related deaths were recorded. A better understanding of the strategies employed by cancer cells to grow and disseminate through the body is still required. Precise characterization of the signaling pathways involved in these processes will allow us to propose new diagnostic and prognostic markers but also to improve therapeutic strategies.
Angiogenesis, the formation of new blood vessels from a pre-existing vasculature, has been proposed as a suitable target in order to curtail cancer. In particular, it has been proposed that preventing the supply of nutrients and oxygen supply to the tumor would starve it to death. However, the clinical outcome of anti-angiogenic therapy has been sobering; despite initial therapeutic effects, patients relapse with cancers that have developed resistance to the therapy. Tumors treated with bevacizumab, a monoclonal antibody targeting the master regulator of angiogenesis, Vascular Endothelial Growth Factor-A (VEGF-A), have been found to activate alternative pro-angiogenic signaling pathways in order to revascularize and resume growth. Therefore, it becomes critical to decipher the molecular mechanisms implicated in tumor angiogenesis in general but also the mechanisms underlying the development of resistance to anti-angiogenic therapies.
In my Ph.D. thesis, I first aimed to decipher the mechanisms of resistance to anti-angiogenic therapy. In order to overcome revascularization through activation of alternative pro-angiogenic signaling pathways, several pan-tyrosine kinase inhibitors have been developed. They demonstrated increased efficacy compared with bevacizumab. Here, we assessed the efficacy of nintedanib, a multikinase inhibitor targeting VEGFRs, FGFRs and PDGFRs in a mouse model of breast cancer. While tumors primarily responded to nintedanib treatment and demonstrated decreased tumor mass after short-term treatment, prolonged nintedanib treatment was associated with tumor regrowth. However, angiogenesis was still repressed in tumors escaping therapy and no revascularization was observed. Microarray analysis of FAC-sorted tumor cells revealed a metabolic shift towards anaerobic glycolysis. Moreover, tumors established metabolic symbiosis as suggested by the alternation between
highly hypoxic, glycolytic and normoxic areas. Indeed, the inhibition of glycolysis or the disruption of metabolic symbiosis by genetically ablating MCT4 expression, a protein involved in metabolic symbiosis, efficiently overcame resistance to anti-angiogenic therapy.
In order to reach blood vessels and to metastasize, epithelial cancer cells have to gain motile properties. The first step of the metastatic cascade consists of an epithelial-mesenchymal transtition (EMT). Epithelial cells undergoing this program lose apico-basal polarity and their epithelial markers and cell-cell and cell-matrix contacts, yet express mesenchymal markers and gain migratory capacity. Moreover, cells undergoing an EMT acquire cancer stem cell (CSC) traits. Mesenchymal cells are, for example, able to initiate tumor formation in a more efficient way compared to epithelial cells. While this feature is expected to rely on increased self-renewal capacity in mesenchymal cells, our laboratory identified VEGF-A as a causal agent in tumor initiation. By secreting VEGF-A, mesenchymal cells induce a precocious angiogenic switch, therefore sustaining tumor growth.
In a second project, I aimed to identify the upstream regulator of VEGF-A in cells undergoing an EMT. Here, by performing a low throughput siRNA screen for transcription factors possessing a binding site on the VEGF-A gene promoter, I could identify JunB as the main regulator of VEGF-A expression in mesenchymal cells. JunB inhibition in diverse mesenchymal cell lines led to decreased VEGF-A expression, suggesting a key role for JunB in EMT-induced angiogenesis and thus tumor growth.
In summary, my Ph.D. work provided new insights into tumor angiogenesis as:
- I identified a new mechanism of resistance to anti-angiogenic therapy, in which tumor cells resumed growth despite lack of blood vessels by switching their metabolism towards glycolysis. This work highlighted the use of glycolysis inhibitors to overcome anti-angiogenic resistance;
- I highlighted a new role for JunB as a regulator of EMT-induced angiogenesis and tumor growth.