World-wide, men get more liver cancer than women. A
hot new paper out of Michael Karin's lab provides new intriguing evidence that may help to explain why.
For several years, Karin's group has been interested in examining
inflammation's role in hepatocellular carcinoma development. Now, their focus turns to estrogen-mediated suppression of the multifunctional cytokine IL-6 - a cytokine associated with systemic inflammation. Using IL-6-null mice, they demonstrate the requirement for IL-6 for development of hepatocellular carcinoma (HCC) in male mice.
DEN, a chemical carcinogen, damages murine hepatocytes and can cause the development of HCC in mice. While HCC incidence in DEN-treated wild-type males is 100% under this specific DEN administration protocol, the incidence in wild-type females is under 20%. However, HCC incidence in both IL-6-null males and females was less than 10%.
HCC is strongly associated with a background of chronic liver damage. IL-6 is thought to contribute to the stimulation of hepatocyte proliferation in response to liver damage and subsequent hepatocyte death. This study found profound differences in IL-6 expression in response to DEN between males and females.
The authors found that the administration of DEN resulted in higher levels of DEN-induced IL-6 production in male mice than in female mice. Furthermore, the administration of estrogen (E2) to male mice treated with DEN reduced DEN-induce IL-6 mRNA production and ovariectomy increased IL-6 mRNA levels in females. Using estrogen receptor alpha- and beta-null mice, they narrowed the E2-mediated suppression of IL-6 to estrogen receptor alpha.
IL-6 signals through STAT3, so it was not surprising that higher levels of activated STAT3 were detected in males than females after DEN treatment. Notably, growth hormone, also a prospective mediator of the sexual dimorphism of liver cancer, signals through STAT1, STAT3, and STAT5b - though
STAT5b is considered to be the master regulator of sex-specific liver gene expression.
Estrogens are thought to decrease IL-6 expression by decreasing the activity of two transcription factors - nuclear factor kappa B (NF-kB) and C/EBP beta. Signaling through these factors involves IKK beta and Toll-like receptor (TLR) adaptor protein MyD88. MyD88-null male mice were found to develop fewer and smaller HCC tumors than wild-type males.
Due to these finding, the authors propose the use of estrogen-like compounds to prevent progression of HCC in men.
Naugler et al. 2007 [
Science]