摘要：

Metformin is a widely prescribed drug for Type II diabetes mellitus. Previous studies have shown that this highly hydrophilic and charged compound traverses predominantly paracellularly across the Caco-2 cell monolayer, a well-established model for human intestinal epithelium. However, oral bioavailability of metformin is significantly higher than that of the paracellular probe, mannitol (~60% vs ~16%). Based on these observations, the Thakker laboratory proposed a sponge hypothesis (Proctor et al., 2008) which states that the functional synergy between apical (AP) transporters and paracellular transport enhances the intestinal absorption of metformin. This dissertation work aims to identify AP uptake transporters of metformin, determine their polarized localization, and elucidate their roles in the intestinal absorption and adverse effects of metformin. Chemical inhibition and transporter-knockdown studies revealed that four transporters, namely, organic cation transporter 1 (OCT1), plasma membrane monoamine transporter (PMAT), serotonin reuptake transporter (SERT) and choline high-affinity transporter (CHT) contribute to AP uptake of metformin in Caco-2 cells. Although AP intestinal localization of PMAT, SERT and CHT has been established, the intestinal membrane localization of OCT1 is ambiguous in the literature. In this dissertation work, functional and immunostaining studies showed that OCT1 is localized in the AP membrane of Caco-2 cells and human and mouse intestinal epithelia. Since this dissertation data showed for the first time that metformin is a substrate of SERT, the in vivo role of this transporter in mediating metformin oral absorption was investigated. The SERT-selective chemical inhibitor, paroxetine, decreased metformin systemic AUC0-6h by 9% (p<0.05), and in mSert-/- mice, metformin systemic AUC0-6h was lowered by 6% (p<0.05). Because SERT mediates the reuptake of serotonin which modulates gastrointestinal (GI) functions, its role in metformin-mediated GI adverse effects, such as diarrhea, was investigated in a mouse model. Results showed that metformin induced a 4-fold increase (p<0.001) in intestinal motility, and a 50% increase (p<0.05) in water content of the large intestine. These effects were diminished by serotonin receptor 4 antagonist GR113808, suggesting that metformin could cause GI adverse effects, such as diarrhea, by inhibiting SERT-mediated serotonin re-uptake and subsequent serotonin-induced increased GI motility and water-retention.

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