Interleukin (IL)-6, one of the cytokines released from inflammatory cells, stimulates insulin secretion in a physiological concentration (1-100 pg/ml), but the exact mechanism is still unknown. The present studies were undertaken to investigate the mechanism of IL-6-induced stimulation of insulin secretion in HIT-T 15 cells. The effects of the addition of nifedipine on the IL-6 (100 pg/ml)-induced stimulation of insulin secretion were investigated. We also examined the possibility that IL-6 (1-100 pg/ml) may stimulate insulin messenger ribonucleic acid (mRNA) expression, using the reverse transcription-polymerase chain reaction method. The addition of 100 and 1000 nM nifedipine significantly attenuated the stimulatory effects of 100 pg/ml IL-6 on insulin secretion. The addition of 1-100 pg/ml IL-6 dose-dependently increased preproinsulin mRNA expression relative to beta-actin mRNA. IL-6 increased insulin gene promoter activity of fragments A (-2188 to +337 bp) and B (-1782 to +270 bp) but not fragments C (-1275 to +270 bp), D (-1138 to +270 bp), E (-880 to +236 bp) or F (-356 to +252 bp). The addition of 10 nM nifedipine completely abolished the stimulatory effect of 10-100 pg/ml IL-6 on relative preproinsulin mRNA expression. These data raised the possibility that IL-6 increased preproinsulin mRNA expression via the stimulation of Ca(2+) influx which enhances insulin gene expression.
H Shimizu, K Ohtani, Y Kato and M Mori
ZW Fu, T Kubo, K Sugahara, T Noguchi and H Kato
We investigated the effects of vitamin A (VA) nutritional status on the levels of expression of retinoic acid (RA) receptor-beta (RARbeta) gene in the various tissues of Japanese quail. VA deficiency caused a significant decrease in the mRNA levels of brain, liver, heart, lung and kidney RARbeta2/beta4, whereas no change was observed in the level of testis RARbeta2 transcript. In contrast, reduction in the RARbeta1 transcript caused by VA depletion was observed only in the lung, remaining unchanged in the other tissues. The administration of RA to the VA-deficient quail rapidly induced the expression of RARbeta2/beta4 mRNAs in all the tissues examined, but RA increased the expression of RARbeta1 transcript in the liver, heart, lung and kidney at a lower magnitude. RA could not change the expression of the brain RARbeta1 transcript, while it induced the expression of the testis RARbeta1 mRNA in a temporal way. These results clearly indicate that VA nutritional status differently regulates the expression of RARbeta1 and RARbeta2/beta4 transcripts in a tissue-specific manner.
N Hirayama, K Kitamura, T Imamura, J Kato, Y Koiwaya, T Tsuji, K Kangawa and T Eto
In the biosynthesis of adrenomedullin (AM), an intermediate form, AM(1-52)-glycine-COOH (iAM), is cleaved from proAM and subsequently processed to a biologically active mature form, AM(1-52)-NH2 (mAM), by enzymatic amidation. We recently reported that immunoreactive AM in human plasma consists of mAM and iAM. To clarify the pathophysiological roles of mAM and iAM in heart failure, we established an assay method to specifically detect mAM, and we determined the plasma concentrations of mAM and iAM in 68 patients with congestive heart failure (CHF). The plasma mAM concentrations of the CHF patients classified as being class I or II of New York Heart Association (NYHA) functional classification were significantly greater than those of the 28 healthy controls, and a further increase was noted in the class III or IV patients. Similar increases in plasma iAM were also observed in these patients compared with controls. The increased plasma mAM and iAM in 12 patients with exacerbated CHF were significantly reduced by treatment of their CHF for 7 days. In addition, the plasma concentrations of both mAM and iAM were significantly correlated with pulmonary capillary wedge pressure, pulmonary artery pressure, right atrial pressure, cardiothoracic ratio, heart rate, and the plasma concentrations of atrial and brain natriuretic peptides in the CHF patients. Thus the plasma concentrations of both mAM and iAM were increased progressively in proportion to the severity of CHF. These results suggest that, though the role of iAM remains to be clarified, mAM acts against the further deterioration of heart failure in patients with CHF.
H Tokuda, O Kozawa, M Niwa, H Matsuno, K Kato and T Uematsu
We investigated the effect of prostaglandin E2 (PGE2) on the induction of heat shock protein 27 (HSP27) and HSP70, and the mechanism behind the induction in osteoblast-like MC3T3-E1 cells. PGE2 time-dependently increased the level of HSP27 without affecting the level of HSP70. PGE2 stimulated the accumulation of HSP27 dose-dependently in the range between 10 nM and 10 microM. PGE2 stimulated the increase in the level of the mRNA for HSP27. Staurosporine and calphostin C, inhibitors of protein kinase C (PKC), suppressed the PGE2-induced HSP27 accumulation. The effect of PGE2 on HSP27 accumulation was reduced in the PKC down-regulated cells. BAPTA/AM, a chelator of intracellular Ca2+, or TMB-8, an inhibitor of intracellular Ca2+ mobilization, reduced the accumulation of HSP27 induced by PGE2. Dibutyryl cAMP had little effect on the basal level of HSP27. PGE2 induced the phosphorylation of both p44/p42 mitogen-activated protein (MAP) kinase and p38 MAP kinase. PD98059 and U-0126, inhibitors of the upstream kinase of p44/p42 MAP kinase, reduced the accumulation of HSP27 induced by PGE2. SB203580, a specific inhibitor of p38 MAP kinase, suppressed the HSP27 accumulation induced by PGE2. U-73122, an inhibitor of phospholipase C, and calphostin C reduced the PGE2-induced phosphorylation of both p44/p42 MAP kinase and p38 MAP kinase. These results indicate that PGE2 stimulates the induction of HSP27 through PKC-dependent activations of both p44/p42 MAP kinase and p38 MAP kinase in osteoblasts.
M Makino, N Oda, N Miura, S Imamura, K Yamamoto, T Kato, K Fujiwara, Y Sawai, K Iwase, A Nagasaka and M Itoh
Thyroid hormones affect reactions in almost all pathways of lipid metabolism. It has been reported that plasma free fatty acid (FFA) concentration in hypothyroidism is generally within the normal range. In this study, however, we show that plasma FFA concentration in some hypothyroid patients is higher than the normal range. Symptoms of thyroid dysfunction in these individuals were less severe than those of patients with lower plasma FFA concentrations. From these findings we hypothesized that the change in FFA concentration must correlate with thyroid function. Using an animal model, we then examined the effect of highly purified eicosapentaenoic acid ethyl ester (EPA-E), a n-3 polyunsaturated fatty acid derived from fish oil, on thyroid function in 1-methyl-2-imidazolethiol (MMI)-induced hypothyroid rats. Oral administration of EPA-E inhibited reduction of thyroid hormone levels and the change of thyroid follicles in MMI-induced hypothyroid rats. These findings suggest that FFA may affect thyroid functions and EPA-E may prevent MMI-induced hypothyroidism.
T Takahashi, K Sato, S Kato, T Yonezawa, Y Kobayashi, Y Ohtani, S Ohwada, H Aso, T Yamaguchi, S G Roh and K Katoh
Ghrelin is a multifunctional peptide that promotes an increase of food intake and stimulates GH secretion. Ghrelin secretion is regulated by nutritional status and nutrients. Although a high-protein (HP) diet increases plasma ghrelin secretion in mammals, the mechanisms and the roles of the elevated ghrelin concentrations due to a HP diet have not been fully established. To clarify the roles of elevated acylated ghrelin upon intake of a HP diet, we investigated the regulation of ghrelin concentrations in plasma and tissues in wethers fed with either the HP diet or the control (CNT) diet for 14 days, and examined the action of the elevated plasma ghrelin by using a ghrelin-receptor antagonist. The HP diet gradually increased the plasma acylated-ghrelin concentrations, but the CNT diet did not. Although the GH concentrations did not vary significantly across the groups, an injection of ghrelin-receptor antagonist enhanced insulin levels in circulation in the HP diet group. In the fundus region of the stomach, the ghrelin levels did not differ between the HP and CNT diet groups, whereas ghrelin O-acyltransferase mRNA levels were higher in the group fed with HP diet than those of the CNT diet group were. These results indicate that the HP diet elevated the plasma ghrelin levels by increasing its synthesis; this elevation strongly suppresses the appearance of insulin in the circulation of wethers, but it is not involved in GH secretion. Overall, our findings indicate a role of endogenous ghrelin action in secretion of insulin, which acts as a regulator after the consumption of a HP diet.
Y Itoh, S Imamura, K Yamamoto, Y Ono, M Nagata, T Kobayashi, T Kato, M Tomita, A Nakai, M Itoh and A Nagasaka
Endothelin-1 (ET-1) concentrations are increased in patients with diabetes mellitus, particularly those with diabetic retinopathy, or essential hypertension. We hypothesized that ET-1 might participate in the development and progression of diabetic microangiopathy. In this study, the effects of the angiotensin converting enzyme (ACE) inhibitor, enalapril maleate, on diabetic angiopathy were examined in streptozotocin (STZ)-induced diabetic (STZ-DM) rats by monitoring variations in renal function and ET-1 concentrations in blood and organ tissues. Significant increases in kidney weight and in concentrations of urinary albumin, N-acetyl-fl-d-glucosamidase (NAG) and serum ET-1 were observed in the STZ-DM rats as compared with the non-diabetic rats, and the concentration of ET-1 in the kidneys tended to be increased. Microscopic and electron microscopic analyses showed increased mesangial cell proliferation, matrix expansion and enlarged mesangial area in the kidney of the diabetic rats. After administration of the ACE inhibitor, increased concentrations of urinary albumin and NAG in the STZ-DM rats were reduced to the control values with a slight improvement in the electron microscopic changes. These data suggest that ET-1 may be involved in the development and progression of diabetic nephropathy and may explain, in part, why diabetes is liable to complicate hypertension. ACE inhibitor may help to restore diabetic nephropathy in the STZ-induced diabetic rats.