Evidence that annexin II Is not a putative membrane receptor for 1α,25(OH)2‐vitamin D3

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Abstract
The seco-steroid hormone 1α,25(OH)2-vitamin D3 (1,25-D3) is known to generate biological responses via both genomic and non-genomic rapid signal transduction pathways. The calcium regulated annexin II/p11 heterotetramer (AII2/p112] was proposed by Baran and co-authors to be the membrane receptor responsible for mediating non-genomic, rapid actions of 1,25-D3, based on ligand affinity labeling, competition, and saturation analysis experiments. Given the cytosolic presence of both the monomeric and heterotetrameric form of AII and their functional regulation by intracellular calcium concentrations, which are known to be affected by 1,25-D3 rapid, non-genomic activities, we investigated in vitro the affinity of [3H]1,25-D3 for the AII monomer and AII2/p112 in the absence and presence of calcium using saturation analysis and gel-filtration chromatography. Using two different techniques for separating bound from free ligand (perchlorate and hydroxylapatite (HAP)) over a series of 30 experiments, no evidence for specific binding of [3H]1,25-D3 was obtained with or without the presence of 700 nM exogenous calcium, using either the AII monomer or AII2/p112. However saturable binding of [3H]1,25-D3 to the lipid raft/caveolae enriched rat intestinal fraction was consistently observed (Kd = 3.0 nM; Bmax = 45 fmols/mg total protein). AII was detected in lipid raft/caveolae enriched fractions from rat and mouse intestine and ROS 17/2.8 and NB4 cells by Western blot, but incubation in the presence of exogenous calcium did not ablate 1,25-D3 binding as reported by Baran et al. Our results suggest that AII does not bind 1,25-D3 in a physiologically relevant manner; however, recent studies linking AII2/p112 phosphorylation to vesicle fusion and its calcium regulated localization may make AII a possible down-stream substrate for 1,25-D3 induced rapid cellular effects.