Vitamin D Calbindin D 9k is not required for 1,25-dihydroxyvitamin D 3 -mediated Ca 2+ absorption in small intestine Shirin Akhter, Galina D. Kutuzova, Sylvia Christakos 1 , Hector F. DeLuca * Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, WI 53706-1544, USA Received 6 September 2006, and in revised form 5 December 2006 Available online 12 December 2006 Abstract The exact role of calbindin D 9k in vitamin D-mediated calcium absorption has been debated but remains unsettled. In 129/OlaHsd mice, calbindin D 9k was found highest in duodenum (36–50%) and kidney (24–34%) followed by stomach, lung and uterus. Age does not affect the relative distribution of calbindin D 9k but it does decline with age in duodenum of both male and female 129/Ola mice. Recently, we produced a null calbindin D 9k mutant 129/OlaHsd mouse; this mouse proved to be indistinguishable from the wild-type in phenotype and in a serum calcium level regardless of age or gender. We have now examined directly whether the mutant mouse can absorb calcium from the intestine in response to the active form of vitamin D. The calbindin D 9k null mutant mouse is fully able to absorb calcium from the intestine in response to 1,25-dihydroxyvitamin D 3 . It is, therefore, clear that calbindin D 9k is not required for vitamin D-induced intestinal calcium absorption. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Calbindin D 9k ; Vitamin D 3 ; Transcellular calcium absorption; 1,25-(OH) 2 D 3 ; Knockout mice Although Orr et al. [1] first showed that vitamin D improves utilization of dietary calcium, it was Nicolaysen who provided the first evidence that vitamin D directly improves intestinal calcium and phosphorus absorption [2]. Using everted intestinal sacs ex vivo, Schachter and Rosen discovered that vitamin D stimulates an active calci- um transport mechanism in the small intestine [3], which has been confirmed many times using various techniques [4–7]. In a search for a mechanism of vitamin D action on cal- cium absorption, Wasserman and Taylor [6] discovered in chick intestine a vitamin D-induced calcium binding protein known as calbindin D 28k [8]. However, in mammals, the 28k protein is not found in the intestine and instead Kallfelz et al. discovered a vitamin D-induced calbindin D 9k [9]. Later, cal- bindin D 9k from vitamin D 3 -responsive rat intestinal mucosa was isolated, purified and characterized [10]. Further it was established that calbindin D 9k is regulated both at the tran- scriptional and post-transcriptional levels by the active form of vitamin D 3 , 1,25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 ) or calcitriol , and a vitamin D 3 response element in rat calbindin D 9k gene was identified [11]. Calbindin D 9k is found in intestine of most mammals and, as was shown in rats, its content is the highest in the duodenal mucosa (2% of the soluble protein): 6-fold higher than in jejunum and cecum, and at least 42-fold higher than in any other organ [12]. Calbindin D 9k belongs to the superfamily of EF-hand helix-loop-helix Ca 2+ -bind- ing proteins, S100 subfamily, which are small acidic proteins containing two Ca 2+ -binding sites [13]. It is accepted that the primary regulator of a transepi- thelial calcium transport in the small intestine is the hor- monal form of vitamin D 3 ,1,25-(OH) 2 D 3 2 [14–16]. The 0003-9861/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.abb.2006.12.005 * Corresponding author. Fax: +1 608 262 7122. E-mail address: deluca@biochem.wisc.edu (H.F. DeLuca). 1 Present address: Department of Biochemistry and Molecular Biology, UMDNJ-New Jersey Medical School, 185 S. Orange Avenue, Newark, NJ 07103-2714, USA. 2 Abbreviations used: 1; 25-ðOHÞ 2 D 3 , 1,25-dihydroxyvitamin D 3 ; VDR, vitamin D receptor. www.elsevier.com/locate/yabbi ABB Archives of Biochemistry and Biophysics 460 (2007) 227–232