Stimulation of c-Src by prolactin is independent of Jak2

Abstract

Interaction of prolactin (PRL) with its receptor (PRLR) leads to activation of Jak and Src family tyrosine kinases. The PRL/growth hormone/cytokine receptor family conserves a proline-rich sequence in the cytoplasmic juxtamembrane region (Box 1) required for association and subsequent activation of Jaks. In the present work, we studied the mechanisms underlying c-Src kinase activation by PRL and the role that Jak2 plays in this process. PRL addition to chicken embryo fibroblasts (CEF) expressing the rat PRLR long form resulted in activation of c-Src and Jak2 and in tyrosine phosphorylation of the receptor. Receptor phosphorylation was due to associated Jak2, since in cells expressing either a Box 1 mutated PRLR (PRLR4P-A), which is unable to interact with Jak2, or a kinase-domain-deleted Jak2 (Jak2∆k), PRL did not stimulate receptor phosphorylation. Interestingly, addition of PRL to cells expressing PRLR4P-A resulted in an activation of c-Src equivalent to that observed with the wild-type receptor. These findings indicate that PRL-mediated stimulation of c-Src was independent of Jak2 activation and of receptor phosphorylation. Our results suggest that PRL-activated Src could send signals to downstream cellular targets independently of Jak2.

A 5′ distal palindrome within the mouse mammary tumor virus-long terminal repeat recruits a mammary gland-specific complex and is required for a synergistic response to progesterone plus prolactin

“…Although several groups have implicated the MAP and PI3 kinases as downstream effectors of crosstalk between PR and c-Src (Lange 2004), our present results indicate that these pathways do not participate in the synergistic effect of PC PRL on the MMTV-LTR. It is also well established that Jak2 (Campbell et al 1994, Rui et al 1994 and Src family kinases (Clevenger & Medaglia 1994, Al-Sakkaf et al 1997, Fresno Vara et al 2000 can converge during PRL signaling, where the Src family kinase p59 (Fyn) can preform with all PRLR isoforms (Clevenger & Medaglia 1994). While a role for STAT1 or STAT3 in the synergistic response by the MMTV-LTR to PCPRL cannot be ruled out, our data do indicate that STAT5a and STAT5b are not required.…”

Prolactin stimulates ubiquitination, initial internalization, and degradation of its receptor via catalytic activation of Janus kinase 2

“…It is plausible that a second, Src-dependent pathway might be responsible for downregulation of PRLr in Ser349 phosphorylation-and ubiquitination-independent manner. In this regard, it is worth noting that c-Src can be activated by PRL independent of Jak2 in chicken embryo fibroblasts (Fresno Vara et al 2000). These pathways might either synergize to facilitate optimal/efficient PRLr downregulation or might differentially contribute to PRLr endocytosis and degradation in a cell-type-specific manner.…”

Stat5 Promotes Survival of Mammary Epithelial Cells through Transcriptional Activation of a Distinct Promoter in Akt1

“…Signaling through the PRLR has been shown to phosphorylate Akt1 via Src-mediated signal transduction to PI3 kinase but not through a Stat5-dependent mechanism in lymphoid and breast cancer cells (for references, refer to Clevenger et al [13]). Since our recent study and that of Fresno Vara et al (19) suggested that the functional ablation of Jak2 did not abolish the PRLR-induced activation of Src, it was surprising to note that Akt1 expression and activation was significantly reduced in Jak2 conditional knockout cells (42). We therefore reasoned that mammary epithelial cells might possess alternative mechanisms to regulate the expression of Akt1 in a Jak2/Stat5-dependent manner.…”