Sex-dependent effects of adipocyte STAT3 inhibition on the inflammatory response during severe sepsis.

INTRODUCTION: Sepsis is a dysregulated host response to infection that can lead to life-threatening organ dysfunction. Clinical and animal studies consistently demonstrate that female subjects are less susceptible to the adverse effects of sepsis, demonstrating the importance of understanding how sex influences sepsis outcomes. The signal transducer and activator of transcription 3 (STAT3) pathway is a major signaling pathway that facilitates inflammation during sepsis. STAT3 is abundantly expressed in white adipose tissue, however little is known about the contribution of white adipose tissue STAT3 activation during sepsis. We hypothesize that adipocyte STAT3 inhibition during severe sepsis will exaggerate the inflammatory response and impact organ injury, in a sex-dependent manner.

METHODS: We generated STAT3 flox/flox (WT) and adipocyte STAT3 knock out (A-STAT3 KO) mice using Cre-lox technology. Studies were done in 12-16-week-old male and female mice. Polymicrobial sepsis was induced by cecal ligation and puncture (CLP). Control non-septic mice did not undergo CLP (0 h CLP). Tissues were harvested 18 h after CLP. Body composition was determined by Echo magnetic resonance imaging (MRI). Energy metabolism was determine by indirect calorimetry. White adipose tissue morphology was determined by hematoxylin and eosin staining, while STAT3 activation in the white adipose tissue was determined by Western blot analysis and immunohistochemistry staining of STAT3 activation/phosphorylation at tyrosine 705. Plasma cytokines (TNF-α, IL-6, and leptin) were determined by luminex assay. Neutrophil infiltration of the lung and liver were assessed by myeloperoxidase activity assay. Histological signs of organ injury on lung and liver tissue were assessed by hematoxylin and eosin staining. Liver injury was further assessed by measuring plasma alanine and aspartate aminotransferase. In a separate cohort of mice, sepsis was induced by CLP and mice were monitored every 6-12 hours over a 7-day period to assess survival rate.

RESULTS: We demonstrate that neither body composition nor energy metabolism is altered with adipocyte STAT3 inhibition in male or female mice, under non-septic conditions. Sepsis was associated with reduced adipocyte size in female WT and A-STAT3 KO mice, suggesting that this event is STAT3 independent. Sepsis did not alter adipocyte size in male WT and A-STAT3 KO mice, suggesting that this event is also sex-dependent. Although STAT3 phosphorylation at tyrosine 705 expression is negligible in male and female A-STAT3 KO mice, septic female WT and A-STAT3 KO mice have higher white adipose tissue STAT3 activation than male WT and A-STAT3 KO mice. Adipocyte STAT3 inhibition did not alter the proinflammatory cytokine response during sepsis in male or female mice, as measured by plasma TNF-α, IL-6, and leptin levels. Adipocyte STAT3 inhibition reduced lung neutrophil infiltration and histological signs of lung injury during sepsis in male mice. On the contrary, adipocyte STAT3 inhibition had no effect on lung neutrophil infiltration or lung injury in female mice. We further demonstrate that neither liver neutrophil infiltration nor histological signs of liver injury is altered by adipocyte STAT3 inhibition during sepsis, in male or female mice. Lastly, adipocyte STAT3 inhibition did not affect survival rate of male or female mice during sepsis.

CONCLUSIONS: Our study demonstrates that sex influences white adipose tissue STAT3 activation and morphology during sepsis, which is not dependent on the presence of functional STAT3 in mature adipocytes. Furthermore, genetic inhibition of adipocyte STAT3 activation in male, but not female mice, results in reduced lung neutrophil infiltration and lung injury during sepsis. The results from our study demonstrate the importance of considering biological sex and the white adipose tissue as potential sources and targets of inflammation during sepsis.