Jak2 is required for the initiation but not maintenance of prolactin-induced mammary cancer

The prolactin receptor (PRLR), its associated kinase Jak2, and Stat5 are essential for normal mammary gland development. Due to the upregulation of the PRLR and the local synthesis of its ligand in neoplastic cells, it has been proposed that PRL can act as a local growth factor in human breast cancers. This notion is supported by experimental evidence in transgenic mice that demonstrated that the mammary-specific expression of PRL contributes to carcinogenesis in vivo. To assess the importance of Jak2/Stat5 signaling during mammary cancer initiation and progression, we generated a PRL-induced mammary cancer model that allows the functional ablation of the Jak2 gene in the mammary epithelium prior to and after neoplastic transformation. Collectively, the results of this study show that the functional ablation of Jak2 protects against the onset of PRL-induced mammary tumorigenesis, suggesting that targeting this Janus kinase is a relevant strategy for mammary cancer prevention. Surprisingly, Jak2 deficiency did not affect the growth and survival of PRL-induced mammary cancer cells in culture and in vivo. Consequently, Jak2 cannot be a sole therapeutic target to treat the established disease. PRL-induced mammary cancers exhibited an upregulation of ErbB2 and other ErbB receptor tyrosine kinases that may supersede the functionality of PRLR signaling through Jak2.