Sex-dependent lifespan extension of ApcMin/+ FAP mice by chronic mTOR inhibition

Background:  Apc Min/+  mice model familial adenomatous polyposis (FAP), a disease that causes numerous colon polyps leading to colorectal cancer. We previously showed that chronic treatment of Apc Min/+  females with the anti-aging drug, rapamycin, restored a normal lifespan through reduced polyposis and anemia prevention. Lifespan extension by chronic rapamycin in wildtype UM-HET3 mice is sex-dependent with females gaining the most benefit. Whether Apc Min/+  mice have a similar sex-dependent response to chronic mTOR inhibition is not known. Methods:  To address this knowledge gap and gain deeper insight into how chronic mTOR inhibition prevents intestinal polyposis, we compared male and female Apc Min/+  mice responses to chronic treatment with a rapamycin-containing diet. Animals were fed a diet containing either 42 ppm microencapsulate rapamycin or empty capsules, one group was used to determine lifespan and a second group with similar treatment was harvested at 16 weeks of age for cross-sectional studies. Results:  We found that the survival of males is greater than females in this setting (P < 0.0197). To explore the potential basis for this difference we analyzed factors affected by chronic rapamycin. Immunoblot assays showed that males and females exhibited approximately the same level of mTORC1 inhibition using phosphorylation of ribosomal protein S6 (rpS6) as an indirect measure. Immunohistochemistry assays of rpS6 phosphorylation showed that rapamycin reduction of mTORC1 activity was on the same level, with the most prominent difference being in intestinal crypt Paneth cells in both sexes. Chronic rapamycin also reduced crypt depths in both male and female Apc Min/+  mice (P < 0.0001), consistent with reduced crypt epithelial cell proliferation. Finally, chronic rapamycin prevented anemia equally in males and females. Conclusions:  In males and females, these findings link rapamycin-mediated intestinal polyposis prevention with mTORC1 inhibition in Paneth cells and concomitant reduced epithelial cell proliferation. Keywords:  Rapamycin, small intestine, polyposis, mTORC1, Paneth cells, crypt stem cells