Location: Children's Nutrition Research CenterTitle: Ascorbate synthesis pathway, dual role of ascorbate in bone homeostasis Author
Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2010
Publication Date: 6/18/2010
Citation: Gabbay, K.H., Bohren, K.M., Morello, R., Bertin, T., Liu, J., Vogel, P. 2010. Ascorbate synthesis pathway, dual role of ascorbate in bone homeostatis. Journal of Biological Chemistry. 285(25):19510-19520. Interpretive Summary: We used mouse gene knock-out models to identify the route and mechanism of vitamin C (ascorbate) synthesis in mice. Unlike humans, these rodents are able to synthesize vitamin C, and are not dependent on vitamin C intake. By genetically altering the levels of vitamin C synthesis, we were able to produce several mouse models with graded deficiencies of vitamin C. A partially deficient mouse model (~85% vitamin C deficit) develops and grows normally when fed regular feed, but suffer severe osteopenia and spontaneous bone fractures. We were able to identify two modes of vitamin C action on bone. This is the first demonstration of two independent roles for vitamin C as suppressing osteoclast activity needed for bone changes. Although humans have lost the ability to synthesize vitamin C, our mouse models suggest the mechanisms by which suboptimal vitamin C availability facilitates the development of osteoporosis, which has important implications for human osteoporosis.
Technical Abstract: Using mouse gene knock-out models, we identify aldehyde reductase (EC 22.214.171.124, Akr1a4 (GR)) and aldose reductase (EC 126.96.36.199, Akr1b3 (AR)) as the enzymes responsible for conversion of D-glucuronate to L-gulonate, a key step in the ascorbate (ASC) synthesis pathway in mice. The gene knock-out (KO) mice show that the two enzymes, GR and AR, provide ~85 and ~15% of L-gulonate, respectively. GRKO/ARKO double knock-out mice are unable to synthesize ASC (>95% ASC deficit) and develop scurvy. The GRKO mice (~85% ASC deficit) develop and grow normally when fed regular mouse chow (ASC content = 0) but suffer severe osteopenia and spontaneous fractures with stresses that increase ASC requirements, such as pregnancy or castration. Castration greatly increases osteoclast numbers and activity in GRKO mice and promotes increased bone loss as compared with wild-type controls and additionally induces proliferation of immature dysplastic osteoblasts likely because of an ASC-sensitive block(s) in early differentiation. ASC and the antioxidants pycnogenol and resveratrol block osteoclast proliferation and bone loss, but only ASC feeding restores osteoblast differentiation and prevents their dysplastic proliferation. This is the first in vivo demonstration of two independent roles for ASC as an antioxidant suppressing osteoclast activity and number as well as a cofactor promoting osteoblast differentiation. Although humans have lost the ability to synthesize ASC, our mouse models suggest the mechanisms by which suboptimal ASC availability facilitates the development of osteoporosis, which has important implications for human osteoporosis.