Concise synthesis of polymethoxyflavone sudachitin and its derivatives, and biological evaluations

The flavonoid Sudachitin regulates glucose metabolism via PDE inhibition

Sudachitin, a member of the flavonoid family, reportedly improves glucose metabolism after long-term administration, but details of the underlying mechanisms are unknown. We found that Sudachitin approximately doubles insulin secretion under high glucose concentrations in mouse pancreatic islets and MIN6 cells. When Sudachitin was orally administered to mice, early-phase insulin secretion was increased and a 30 % reduction in blood glucose levels was demonstrated 30 min after glucose loading. Insulin tolerance tests also showed Sudachitin to increase systemic insulin sensitivity. Additionally, we observed that Sudachitin raised intracellular cAMP levels in pancreatic islets. Phosphodiesterase (PDE) activity assays revealed Sudachitin to inhibit PDE activity and computer simulations predicted a high binding affinity between PDEs and Sudachitin. These findings suggest that Sudachitin enhances both insulin secretion and insulin sensitivity via an increase in intracellular cAMP resulting from PDE inhibition. These insights may facilitate understanding the mechanisms underlying the regulation of glucose metabolism by Sudachitin and other isoflavones.

Chronotropic and Inotropic Effects of Sudachitin, a Polymethoxyflavone from the Peel of Citrus sudachi on Isolated Rat Atria and Its Underlying Mechanisms

Sudachitin, a polymethoxyflavone found in sudachi peel, has been reported to improve hyperlipidemia in humans, and is thus attracting research attention. However, its effect on cardiac function remains unclear. We investigated the mechanisms underlying the chronotropic and inotropic effects of sudachitin on rat atria. Sudachitin (0.3-30 µM) produced concentration-dependent positive chronotropic and inotropic effects. Other polymethoxyflavones, including demethoxysudachitin (0.3-30 µM) and nobiletin (0.3-30 µM), also produced positive chronotropic and inotropic effects; however, the maximum efficacy of all polymethoxyflavones, including sudachitin, was lower than that of isoproterenol. Propranolol (0.1 µM) did not affect the positive chronotropic and inotropic effects of sudachitin. The concentration-response curves for the chronotropic and inotropic effects of dibutyryl-cAMP (1-100 µM) were shifted to the left upon pretreatment with sudachitin (3, 10 µM). Phosphodiesterase inhibitors (3-isobutyl-1-methylxanthine 1 µM or milrinone 10 µM) alone, sudachitin alone (10, 30 µM), and a combination of phosphodiesterase inhibitors and sudachitin exhibited positive chronotropic and inotropic effects, whereas the lack of any interaction between each phosphodiesterase inhibitor and sudachitin indicated an additive effect of the two substances. These results suggest that sudachitin-induced positive chronotropic and inotropic effects similar to those of other polymethoxyflavones, but its maximum efficacy was lower than that of isoproterenol. Both demethoxysudachitin and nobiletin exhibited similar positive chronotropic and inotropic effects, indicating that these effects are not specific to sudachitin, but are common to polymethoxyflavones. The mechanism of action of sudachitin was associated with the enhancement of cAMP-dependent pathways, without the involvement of β-adrenoceptors.

Expression and functional analysis of the nobiletin biosynthesis-related gene CitOMT in citrus fruit

Nobiletin, a polymethoxy flavone (PMF), is specific to citrus and has been reported to exhibit important health-supporting properties. Nobiletin has six methoxy groups at the 3′,4′,5,6,7,8-positions, which are catalyzed by O-methyltransferases (OMTs). To date, researches on OMTs in citrus fruit are still limited. In the present study, a novel OMT gene (CitOMT) was isolated from two citrus varieties Satsuma mandarin (Citrus unshiu Marc.) and Ponkan mandarin (Citrus reticulata Blanco), and its function was characterized in vitro. The results showed that the expression of CitOMT in the flavedo of Ponkan mandarin was much higher than that of Satsuma mandarin during maturation, which was consistent with the higher accumulation of nobiletin in Ponkan mandarin. In addition, functional analysis showed that the recombinant protein of CitOMT had methylation activity to transfer a methyl group to 3′-hydroxy group of flavones in vitro. Because methylation at the 3′-position of flavones is vital for the nobiletin biosynthesis, CitOMT may be a key gene responsible for nobiletin biosynthesis in citrus fruit. The results presented in this study will provide new strategies to enhance nobiletin accumulation and improve the nutritional qualities of citrus fruit.

[Citrus flavonoids as a target for the prevention of pancreatic β-cells dysfunction in diabetes.]

The number of patients with type 2 diabetes mellitus (T2DM) has rapidly increased, especially in East and Southeast Asia. In these areas, in general, people have especially vulnerable β-cells and insulin secretion deficiency and reduced β-cell mass are the primary cause of T2DM. Therefore, the alleviation of such β-cell dysfunction would provide therapeutic approaches to prevent the development of T2DM. Nobiletin, a polymethoxylated flavonoid found in citrus fruits, has been shown to improve obesity and insulin resistance in T2DM model mice. We focused on β-cells and investigated the effects of nobiletin on insulin secretion and β-cell apoptosis. In β-cell line INS-1, nobiletin increased glucose-induced insulin secretion (GSIS) in a concentration-dependent manner, which was inhibited by an Epac inhibitor. In addition, nobiletin at 10 μM inhibited thapsigargin-induced apoptosis, which was inhibited by a PKA inhibitor. Nobiletin also suppressed thapsigargin-induced increases in cleaved caspase-3 and phosphorylated JNK. Thus, nobiletin is suggested to promote GSIS and prevent ER stress-induced β-cell apoptosis, which are mediated via Epac and PKA-dependent pathways, respectively. In summary, nobiletin is suggested to exhibit insulinotropic and anti-apoptotic effects on β-cells, which are one of the causes of its anti-diabetic effect. Moreover, nobiletin seems to be able to alleviate the development of T2DM by protecting β-cells from apoptosis.