Abstract
Riboflavin is thoroughly established to be indispensable in a multitude of cellular oxidation-reduction reactions through its conversion to coenzyme forms flavin mononucleotide and flavin adenine dinucleotide. Despite its physiological importance, little is known about specific mechanisms or proteins involved in regulating its cellular entry in humans. Studies involving biochemical modulators and immunological inhibition assays have indirectly revealed that riboflavin internalization and trafficking occurs at least in part through a clathrin-dependent receptor-mediated endocytic process. Here, using a two-tiered strategy involving RNA interference and the overexpression of dominant-negative constructs, we directly show the involvement of this endocytic mechanism through the requirement of the pluripotent endocytic vesicle scission enzyme, dynamin 2 GTPase, in human placental trophoblasts. Similar to the endocytic control ligand, transferrin, riboflavin is shown to exhibit 50% dependence on the functional expression of dynamin 2 for its active cellular entry. Furthermore, this reduced vitamin uptake correlates with >2-fold higher riboflavin association at the cell surface. In addition, fluorescent ligand endocytosis assays showing colocalization between rhodamine-riboflavin and the immunostained caveolar coat protein, caveolin 1, suggest that the active absorption of this important nutrient involves multiple and distinct endocytosis pathways.
Footnotes
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This work was supported, in part, by the National Institutes of Health under grant DK56631 (to P.W.S.), a postdoctoral fellowship grant from the Susan G. Komen Foundation (to P.W.S.), and by a predoctoral fellowship sponsored by Eli Lilly and Company (to A.B.F.).
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Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
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doi:10.1124/mol.107.037101.
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ABBREVIATIONS: RME, receptor-mediated endocytosis; B2, riboflavin; CME, clathrin-mediated endocytosis; RNAi, RNA interference; TGF-β, tumor growth factor β; DNM2, dynamin 2 GTPase; CvME, caveolae-mediated endocytosis; siRNA, short-interfering RNA; DLP1, dynamin-like protein 1; TF, transferrin; CAV1, caveolin 1; CTX, cholera toxin subunit B; PC, Pearson's correlation; N, nontargeting; ANOVA, analysis of variance; PBS, phosphate-buffered saline; 3D, three-dimensional;. Rd-RF, rhodamine-riboflavin; DNM2WT, wild-type dynamin 2; DNM2K44A, dominant-negative dynamin 2.
- Received April 13, 2007.
- Accepted June 12, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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