Abstract
Prolactin influences mammary development and carcinogenesis through endocrine and autocrine/paracrine mechanisms. In virgin female mice, pro-lactin overexpression under control of a mammary selective nonhormonally responsive promoter, neu-related lipocalin, results in estrogen receptor alpha (ERalpha)-positive and ERalpha-negative adenocarcinomas. However, disease in vivo occurs in the context of dysregulation of multiple pathways. In this study, we investigated the ability of prolactin to modulate carcinogenesis when co-expressed with the potent oncogene transforming growth factor alpha (TGFalpha) in bitransgenic mice. Prolactin and TGFalpha cooperated to reduce dramatically the latency of mammary macrocyst development, the principal lesion type induced by TGFalpha. In combination, prolactin and TGFalpha also increased the incidence and reduced the latency of other preneoplastic lesions and increased cellular turnover in structurally normal alveoli and ducts compared with single transgenic females. Bitransgenic glands contained higher levels of phosphorylated ERK1/2 compared with single TGFalpha transgenic glands, suggesting that this kinase may be a point of signaling crosstalk. Furthermore, transgenic prolactin also reversed the decrease in ERalpha induced by neu-related lipocalin-TGFalpha. Our findings demonstrate that locally produced prolactin can strikingly potentiate the carcinogenic actions of another oncogene and modify ovarian hormone responsiveness, suggesting that prolactin signaling may be a potential therapeutic target.