Negative Regulation of Prolactin Receptor Stability and Signaling Mediated by SCF^β-TrCP E3 Ubiquitin Ligase

Ubiquitin-dependent degradation of hormone receptors is emerging as a key mechanism that regulates the magnitude and duration of hormonal effects on cells and tissues. The pituitary hormone prolactin (PRL) is involved in regulating cell differentiation, proliferation, and survival. PRL engages its receptor (PRLR) to initiate various signaling cascades, including the phosphorylation and activation of Stat5. We found that PRL promotes interaction between PRLR and the F-box protein β-TrCP2, which functions as a substrate recognition subunit of the SCF^β-TrCP E3 ubiquitin ligase. This interaction requires PRLR phosphorylation and the integrity of serine 349 within a conserved motif, which is similar to conserved motifs present in other substrates of SCF^β-TrCP. The PRLR^S349A mutant is resistant to ubiquitination and is more stable than its wild-type counterpart. Phosphorylated PRLR undergoes ubiquitination by SCF^β-TrCP in vitro. Knockdown of β-TrCP expression inhibits the ubiquitination and degradation of PRLR and promotes PRL-dependent phosphorylation of Stat5 as well as Stat5-dependent transcription in cells. Furthermore, the activation of Stat5 and the stimulation of cell growth by PRL are augmented in cells expressing the PRLR^S349A mutant. These data indicate that PRLR is a novel SCF ^β-TrCP substrate and implicate β-TrCP as an important negative regulator of PRL signaling and cellular responses to this hormone.