R‐spondin‐mediated WNT signaling potentiation in mammary and breast cancer development

The mammary gland is a secretory organ, which develops as a network of growing epithelial ducts composed of luminal and basal cells that invade the surrounding adipose tissue through a series of developmental cycles. Mammary stem cells (MaSCs) maintain an accurate tissue homeostasis, and their proliferation and cell fate determination are regulated by multiple hormones and local factors. The WNT pathway plays a critical role in controlling the enormous tissue expansion and remodeling during mammary gland development through the maintenance and differentiation of MaSCs, and its deregulation has been implicated in breast cancer (BC) initiation and progression. The R‐spondins (RSPOs) are four secreted proteins that strongly enhance target cell sensitivity to WNT ligands. Moreover, leucine‐rich repeat‐containing G‐protein‐coupled receptors (LGRs) 4–6 are considered obligate high‐affinity receptors for RSPOs and have been described as stem cell markers. Importantly, elevated RSPO expression has been recently identified in several tumor types from patients, including BC, and it has been reported that they play a significant role in mammary tumor progression in experimental models. In this review, exploring our present knowledge, we summarize the role of the RSPO‐LGR axis as a WNT‐enhancing signaling cascade in the MaSC compartment and during the normal and neoplastic mammary gland development. In addition, we include an updated expression profile of the RSPOs and their action mediators at the cell membrane, the LGRs, and the ubiquitin‐ligases ZNRF3/RNF43, in different BC subtypes. Finally and based on these data, we discuss the significance of tumor‐associated alterations of these proteins and their potential use as molecular targets for detection and treatment of BC.