Postnatal loss of Cyp26a1 and Cyp26b1 in mice causes impaired retinoid homeostasis in multiple organs

Abstract

Retinoic acid (RA), the active metabolite of vitamin A (retinol), is essential to various biological processes such as maintenance of epithelia and regulation of cell division, proliferation and differentiation. Cyp26a1 and Cyp26b1 enzymes catalyze hydroxylation of RA and are believed to be responsible for RA clearance in all chordates. Global knockout of either Cyp26a1 or Cyp26b1 is embryonic lethal, demonstrating a critical role of Cyp26 enzymes in regulating RA signaling during fetal development. However, whether Cyp26 activity is required in postnatal life to maintain vitamin A homeostasis is still not clear. We hypothesized that postnatal loss of Cyp26a1 and Cyp26b1 would decrease RA clearance, and consequently alter tissue retinoid concentrations and cause retinoid toxicity. A tamoxifen‐inducible Cre‐loxP system was used to induce a conditional global knockout of Cyp26a1 and Cyp26b1 (double‐knockout; DKO). The knock‐out was induced in female and male mice with tamoxifen injections at 21 days postpartum and then these animals were monitored (body weight and condition) biweekly for 8 weeks. Animals were euthanized if they received a body condition score ≤ 2 or had lost > 15% of their pre‐tamoxifen injection body weight. Tissues were harvested for histological analysis and retinoid measurements. All WT animals were healthy and euthanized after the 8‐week follow‐up but 45% of male and 41% of female DKO animals were euthanized before the completion of the 8‐week follow‐up according to the criteria described above. Compared to WT mice, DKO animals showed significant decrease in weight gain. DKO mice had moderate to severe blepharitis and dermatitis with skin thickening and inflammation. Spleens of DKO mice were significantly enlarged and showed extramedullary hematopoiesis. Cell proliferation in the epidermis and in the epithelium of nonglandular stomach was increased in DKO animals. In general, retinoid homeostasis was altered in DKO animals. In comparison to WT animals, skin, spleen and liver RA concentrations were significantly increased in DKO mice while tissue retinol and retinyl ester concentrations were unchanged. In contrast, serum retinol and retinol binding protein 4 (RBP4) concentrations were significantly decreased in DKO mice with no change in RA concentrations. When DKO animals were dosed with exogenous RA (1mg/kg; i.p.), animals showed signs of toxicity and the exposure to RA was significantly greater in DKO animals than in WT animals. Collectively these data show that Cyp26a1 and Cyp26b1 are required for maintaining vitamin A homeostasis and retinoid signaling also in postnatal life.