Prolactin prevents hepatocellular carcinoma by restricting innate immune activation of c-Myc in mice

Significance Hormonally active women are better protected from the primary liver cancer hepatocellular carcinoma (HCC) than men. In rodent models, the pituitary gland governs sexually dimorphic HCC risk. We found that the estrogen-responsive pituitary hormone prolactin (PRL) inhibits HCC by binding liver short-form prolactin receptors, resulting in accelerated degradation of a multimolecular “trafasome” involved in tumor-promoting innate immune signaling and preventing activation of the HCC-associated gene c-Myc. Mouse models confirmed sex-dependent HCC regulation upstream of c-Myc and a requirement for PRL but not estrogen in female tumor resistance. Importantly, a preclinical trial in mice showed that pharmacologic PRL mobilization using the human drug domperidone protected males from HCC. PRL-targeted therapy may help prevent liver cancer in high-risk men and women. Women are more resistant to hepatocellular carcinoma (HCC) than men despite equal exposure to major risk factors, such as hepatitis B or C virus infection. Female resistance is hormone-dependent, as evidenced by the sharp increase in HCC incidence in postmenopausal women who do not take hormone replacement therapy. In rodent models sex-dimorphic HCC phenotypes are pituitary-dependent, suggesting that sex hormones act via the gonadal-hypophyseal axis. We found that the estrogen-responsive pituitary hormone prolactin (PRL), signaling through hepatocyte-predominant short-form prolactin receptors (PRLR-S), constrained TNF receptor-associated factor (TRAF)-dependent innate immune responses invoked by IL-1β, TNF-α, and LPS/Toll-like receptor 4 (TLR4), but not TRIF-dependent poly(I:C)/TLR3. PRL ubiquitinated and accelerated poststimulatory decay of a “trafasome” comprised of IRAK1, TRAF6, and MAP3K proteins, abrogating downstream activation of c-Myc–interacting pathways, including PI3K/AKT, mTORC1, p38 MAPK, and NF-κB. Consistent with this finding, we documented exaggerated male liver responses to immune stimuli in mice and humans. Tumor promotion through, but regulation above, the level of c-Myc was demonstrated by sex-independent HCC eruption in Alb-Myc transgenic mice. PRL deficiency accelerated liver carcinogenesis in Prl−/− mice of both sexes. Conversely, pharmacologic PRL mobilization using the dopamine D2 receptor antagonist domperidone prevented HCC in tumor-prone C3H/HeN males. Viewed together, our results demonstrate that PRL constrains tumor-promoting liver inflammation by inhibiting MAP3K-dependent activation of c-Myc at the level of the trafasome. PRL-targeted therapy may hold promise for reducing the burden of liver cancer in high-risk men and women.