Mechanism and regulation of vitamin B2 (riboflavin) uptake by mouse and human pancreatic β-cells/islets: physiological and molecular aspects

Riboflavin (RF) is essential for the normal metabolic activities of pancreatic beta-cells and provides protection against oxidative stress. Very little is known about the mechanism of RF uptake by these cells and how the process is regulated. We addressed these issues using mouse-derived pancreatic beta-TC-6 cells and freshly isolated primary mouse and human pancreatic islets. Our results showed H-3-RF uptake by beta-TC-6 cells is Na+ independent, cis inhibited by RF-related compounds, trans stimulated by unlabeled RF, and saturable as a function of concentration (apparent K-m of 0.17 +/- 0.02 mu M). The latter findings suggest involvement of a carrier-mediated process. Similarly, RF uptake by primary mouse and human pancreatic islets was via carrier-mediated process. RF transporters 1, 2, and 3 (RFVT-1, -3, and -2) were all expressed in mouse and human pancreatic beta-cells/islets, with RFVT-1 being the predominant transporter expressed in the mouse and RFVT-3 in the human. Specific knockdown of RFVT-1 with gene-specific small interfering RNA leads to a significant inhibition in RF uptake by beta-TC-6 cells. RF uptake by beta-TC-6 cells was also found to be adaptively upregulated in RF deficiency via a transcriptional mechanism(s). Also, the process appears to be under the regulation of a Ca2+/calmodulin-mediated regulatory pathway. Results of these studies demonstrate, for the first time, the involvement of a carrier-mediated process for RF uptake by mouse and human pancreatic beta-cells/islets. Furthermore, the process appears to be regulated by extracellular and intracellular factors.