Regulation of the prorenin - angiotensin system by oxygen and miRNAs; parallels between placentation and tumour development?

Tissue renin-angiotensin systems (RASs) are involved in tissue growth and development as they are important regulators of angiogenesis, cell proliferation and migration. The placental RAS is most highly expressed in early gestation, at a time when the oxygen tension within the conceptus is reduced, and plays a key role in placental growth and development. Similar to the placenta, tumour development relies on proliferation, angiogenesis and invasion in order to grow and metastasize. The RAS is known to be upregulated in a variety of solid tumours, including ovarian, endometrial, cervical, breast and prostate. This review explores the roles of oxygen and microRNAs in regulating the normal expression of the placental RAS, providing insight into regulation of its development as well as the development of disease states in which the RAS is overexpressed. We propose that the placental RAS is downregulated by microRNAs that are suppressed during the physiologically normal 'hypoxic' phase of early placentation. Suppression of these miRNAs allows the placental RAS to stimulate placental growth and angiogenesis. We propose that similar mechanisms may be at play in solid tumours, which are characterised by hypoxia.