The Janus Kinase 2 Is Required for Expression and Nuclear Accumulation of Cyclin D1 in Proliferating

Using a conditional knockout approach, we previ-ously demonstrated that the Janus kinase 2 (Jak2) is crucial for prolactin (PRL) signaling and normal mammary gland development. PRL is suggested to synchronously activate multiple signaling cas-cades that emerge on the PRL receptor (PRLR). This study demonstrates that Jak2 is essential for the activation of the signal transducer and activa-tor of transcription 5 (Stat5) and expression of Cish (cytokine-inducible SH2-containing protein), a Stat5-responsive negative regulator of Jak/Stat signaling. However, Jak2 is dispensable for the PRL-induced activation of c-Src, focal adhesion kinase, and the MAPK pathway. Despite activation of these kinases that are commonly associate