Oral ingestion of carbohydrate triggers glucagon-like peptide 1 (GLP1) secretion, but the molecular mechanism remains elusive. By measuring GLP1 concentrations in murine portal vein, we found that the ATP-sensitive K+ (KATP) channel is not essential for glucose-induced GLP1 secretion from enteroendocrine L cells, while the sodium-glucose co-transporter 1 (SGLT1) is required, at least in the early phase (5 min) of secretion. By contrast, co-administration of the α-glucosidase inhibitor (α-GI) miglitol plus maltose evoked late-phase secretion in a glucose transporter 2-dependent manner. We found that GLP1 secretion induced by miglitol plus maltose was significantly higher than that by another α-GI, acarbose, plus maltose, despite the fact that acarbose inhibits maltase more potently than miglitol. As miglitol activates SGLT3, we compared the effects of miglitol on GLP1 secretion with those of acarbose, which failed to depolarize the Xenopus laevis oocytes expressing human SGLT3. Oral administration of miglitol activated duodenal enterochromaffin (EC) cells as assessed by immunostaining of phosphorylated calcium–calmodulin kinase 2 (phospho-CaMK2). In contrast, acarbose activated much fewer enteroendocrine cells, having only modest phospho-CaMK2 immunoreactivity. Single administration of miglitol triggered no GLP1 secretion, and GLP1 secretion by miglitol plus maltose was significantly attenuated by atropine pretreatment, suggesting regulation via vagal nerve. Thus, while α-GIs generally delay carbohydrate absorption and potentiate GLP1 secretion, miglitol also activates duodenal EC cells, possibly via SGLT3, and potentiates GLP1 secretion through the parasympathetic nervous system.
Eun Young Lee, Shuji Kaneko, Promsuk Jutabha, Xilin Zhang, Susumu Seino, Takahito Jomori, Naohiko Anzai and Takashi Miki
Yusuke Seino, Takashi Miki, Wakako Fujimoto, Eun Young Lee, Yoshihisa Takahashi, Kohtaro Minami, Yutaka Oiso and Susumu Seino
Glucose-induced insulin secretion from pancreatic β-cells critically depends on the activity of ATP-sensitive K+ channels (KATP channel). We previously generated mice lacking Kir6.2, the pore subunit of the β-cell KATP channel (Kir6.2 −/−), that show almost no insulin secretion in response to glucose in vitro. In this study, we compared insulin secretion by voluntary feeding (self-motivated, oral nutrient ingestion) and by forced feeding (intra-gastric nutrient injection via gavage) in wild-type (Kir6.2 + / +) and Kir6.2 −/− mice. Under ad libitum feeding or during voluntary feeding of standard chow, blood glucose levels and plasma insulin levels were similar in Kir6.2 + / + and Kir6.2 −/− mice. By voluntary feeding of carbohydrate alone, insulin secretion was induced significantly in Kir6.2 −/− mice but was markedly attenuated compared with that in Kir6.2 + / + mice. On forced feeding of standard chow or carbohydrate alone, the insulin secretory response was markedly impaired or completely absent in Kir6.2 −/− mice. Pretreatment with a muscarine receptor antagonist, atropine methyl nitrate, which does not cross the blood–brain barrier, almost completely blocked insulin secretion induced by voluntary feeding of standard chow or carbohydrate in Kir6.2 −/− mice. Substantial glucose-induced insulin secretion was induced in the pancreas perfusion study of Kir6.2 −/− mice only in the presence of carbamylcholine. These results suggest that a KATP channel-independent mechanism mediated by the vagal nerve plays a critical role in insulin secretion in response to nutrients in vivo.
Takayoshi Inoue, Yasuhiro Miki, Shingo Kakuo, Akira Hachiya, Takashi Kitahara, Setsuya Aiba, Christos C Zouboulis and Hironobu Sasano
Androgens are well known to influence sebum synthesis and secretion. Various factors related to androgen biosynthesis are expressed in human sebaceous glands. In this study, immunohistochemical analysis of human skin specimens from 43 subjects indicated that various androgen-producing and -metabolizing enzymes were functionally localized to sebocytes accumulating lipid droplets and that the exclusive expression of 17β-hydroxysteroid dehydrogenase type 2 (17β-HSD2 (HSD17B2)) in sebaceous glands was negatively correlated with that of peroxisome proliferator-activated receptor gamma (PPARγ (PPARG)), which also significantly changed in an age-dependent manner. We also demonstrated that the changes of 17β-HSD2 expression in human immortalized sebocytes (SZ95) influenced the expressions of sebogenesis-related factors. In addition, the overexpression of 17β-HSD2 in SZ95 significantly increased the androstenedione production and markedly decreased the amounts of testosterone and dihydrotestosterone when DHEA was added externally. On the other hand, the phosphorylation of mammalian target of rapamycin, which is well known to induce sebum secretion and the onset and/or aggravation of acne, was increased by the addition of testosterone in the presence of IGF1 in hamster sebocytes. These results all indicated that local androgen biosynthesis and metabolism in human sebaceous glands could play a pivotal role in sebum synthesis and secretion.
Eun-Young Lee, Xilin Zhang, Junki Miyamoto, Ikuo Kimura, Tomoaki Taknaka, Kenichi Furusawa, Takahito Jomori, Kosuke Fujimoto, Satoshi Uematsu and Takashi Miki
Mechanisms of carbohydrate-induced secretion of the two incretins namely glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are considered to be mostly similar. However, we found that mice exhibit opposite secretory responses in response to co-administration of maltose plus an α-glucosidase inhibitor miglitol (maltose/miglitol), stimulatory for GLP-1, as reported previously, but inhibitory for GIP. Gut microbiota was shown to be involved in maltose/miglitol-induced GIP suppression, as the suppression was attenuated in antibiotics (Abs)-treated mice and abolished in germ-free mice. In addition, maltose/miglitol administration increased plasma levels of short-chain fatty acids (SCFAs), carbohydrate-derived metabolites, in the portal vein. GIP suppression by maltose/miglitol was not observed in mice lacking a SCFA receptor Ffar3, but it was normally seen in Ffar2-deficient mice. Similar to maltose/miglitol administration, co-administration of glucose plus a sodium glucose transporter inhibitor phloridzin (glucose/phloridzin) induced GIP suppression, which was again cancelled by Abs treatment. In conclusion, oral administration of carbohydrates with α-glucosidase inhibitors suppresses GIP secretion through a microbiota/SCFA/FFAR3 pathway.
Hidetada Ogata, Yusuke Seino, Norio Harada, Atsushi Iida, Kazuyo Suzuki, Takako Izumoto, Kota Ishikawa, Eita Uenishi, Nobuaki Ozaki, Yoshitaka Hayashi, Takashi Miki, Nobuya Inagaki, Shin Tsunekawa, Yoji Hamada, Susumu Seino and Yutaka Oiso
Glucose-dependent insulinotropic polypeptide (GIP), a gut hormone secreted from intestinal K-cells, potentiates insulin secretion. Both K-cells and pancreatic β-cells are glucose-responsive and equipped with a similar glucose-sensing apparatus that includes glucokinase and an ATP-sensitive K+ (KATP) channel comprising KIR6.2 and sulfonylurea receptor 1. In absorptive epithelial cells and enteroendocrine cells, sodium glucose co-transporter 1 (SGLT1) is also known to play an important role in glucose absorption and glucose-induced incretin secretion. However, the glucose-sensing mechanism in K-cells is not fully understood. In this study, we examined the involvement of SGLT1 (SLC5A1) and the KATP channels in glucose sensing in GIP secretion in both normal and streptozotocin-induced diabetic mice. Glimepiride, a sulfonylurea, did not induce GIP secretion and pretreatment with diazoxide, a KATP channel activator, did not affect glucose-induced GIP secretion in the normal state. In mice lacking KATP channels (Kir6.2 −/− mice), glucose-induced GIP secretion was enhanced compared with control (Kir6.2 + / +) mice, but was completely blocked by the SGLT1 inhibitor phlorizin. In Kir6.2 −/− mice, intestinal glucose absorption through SGLT1 was enhanced compared with that in Kir6.2 + / + mice. On the other hand, glucose-induced GIP secretion was enhanced in the diabetic state in Kir6.2 + / + mice. This GIP secretion was partially blocked by phlorizin, but was completely blocked by pretreatment with diazoxide in addition to phlorizin administration. These results demonstrate that glucose-induced GIP secretion depends primarily on SGLT1 in the normal state, whereas the KATP channel as well as SGLT1 is involved in GIP secretion in the diabetic state in vivo.