Experimental Biology and Medicine

Numerous peripheral signals contribute to the regulation of food intake and energy homeostasis. Mechano- and chemoreceptors signaling the presence and energy density of food in the gastrointestinal (GI) tract contribute to satiety in the immediate postprandial period. Changes in circulating glucose concentrations appear to elicit meal initiation and termination by regulating activity of specific hypothalamic neurons that respond to glucose. Other nutrients (e.g., amino acids and fatty acids) and GI peptide hormones, most notably cholecystokinin, are also involved in short-term regulation of food intake. However, the energy density of food and short-term hormonal signals by themselves are insufficient to produce sustained changes in energy balance and body adiposity. Rather, these signals interact with long-term regulators (i.e., insulin, leptin, and possibly the orexigenic gastric peptide, ghrelin) to maintain energy homeostasis. Insulin and leptin are transported into the brain where they modulate expression of hypothalamic neuropeptides known to regulate feeding behavior and body weight. Circulating insulin and leptin concentrations are proportional to body fat content; however, their secretion and circulating levels are also influenced by recent energy intake and dietary macronutrient content. Insulin and leptin concentrations decrease during fasting and energy-restricted diets, independent of body fat changes, ensuring that feeding is triggered before body energy stores become depleted. Dietary fat and fructose do not stimulate insulin secretion and leptin production. Therefore, attenuated production of insulin and leptin could lead to increased energy intake and contribute to weight gain and obesity during long-term consumption of diets high in fat and/or fructose. Transcription of the leptin gene and leptin secretion are regulated by insulin-mediated increases of glucose utilization and appear to require aerobic metabolism of glucose beyond pyruvate. Other adipocyte-derived hormones and proteins that regulate adipocyte metabolism, including acylation stimulating protein, adiponectin, diacylglycerol acyltransferase, and perilipin, are likely to have significant roles in energy homeostasis.

Evidence has existed for more than 50 years in support of the hypothesis that body energy stored in the form of fat is homeostatically regulated. Implicit in this concept is the existence of a biological system that operates dynamically over time to match cumulative energy intake to energy expenditure. For example, to compensate for weight loss induced by energy restriction, animals must enter a period of positive energy balance (i.e., energy intake greater than energy expenditure) that is sustained for as long as it takes to correct the deficit in body fat stores. Having reached this point, the animal must return to a state of neutral energy balance if stable fat mass is to be maintained. The identification of neuronal circuits in the hypothalamus that, when activated, exert potent, unidirectional effects on energy balance provides a cornerstone of support for this model. The additional finding that these central effector pathways are regulated by humoral signals generated in proportion to body fat stores, including the hormones insulin and leptin, helps to round out the picture of how energy homeostasis is achieved. The goal of this overview is to highlight the evidence that specific subsets of hypothalamic neurons containing specific signaling molecules participate in this dynamic regulatory process, and to put these observations in the larger context of a biological system that controls body adiposity.

Interest in the biology of adipose tissue has undergone a revival in recent years with the discovery of a host of genes that contribute to the regulation of satiety and metabolic rate. The catecholamines have long been known to be key modulators of adipose tissue lipolysis and the hydrolysis of triglyceride energy stores. However, more recent efforts to understand the role of Individual adrenergic receptor subtypes expressed in adipocytes and their signal transduction pathways have revealed a complexity not previously appreciated. Combined with this interest in the modulation of adipocyte metabolism is a renewed focus upon brown adipose tissue and the mechanisms of whole body thermogenesis in general. The discovery of novel homologs of the brown fat uncoupling protein (UCP) such as UCP2 and UCP3 has provoked intensive study of these mitochondrial proteins and the role that they play in fuel metabolism. The story of the novel UCPs has proven to be intriguing and still incompletely understood. Here, we review the status of adipose tissue from inert storage depot to endocrine organ, Interesting signal transduction pathways triggered by β-adrenergic receptors in adipocytes, the potential of these receptors for discriminating and coordinated metabolic regulation, and current views on the role of UCP2 and UCP3 based on physiological studies and gene knockout models.

Although rapid globalization of the Westernized way of life is responsible for the large rise in the number of obesity cases (about 1 billion individuals are now overweight or frankly obese), obesity is a typical common multifactorial disease in that environmental and genetic factors interact, resulting in a disease state (1). There is strong evidence for a genetic component to human obesity: e.g., the familial clustering (the relative risk among siblings being 3–7) (2) and the high concordance of body composition in monozygotic twins (3). However, the role of genetic factors in many human obesities (referred to as “common obesity” in this review) is complex, being determined by interaction of several genes (polygenic), each of which may have relatively small effects (i.e., they are “susceptibility” genes and work in combination with each other as well as with environmental factors such as nutrients, physical activity, and smoking).

Recent advances regarding the biology of adipose tissue have demonstrated that white adipose tissue (WAT) plays a central role in the regulation of energy balance and acts as a secretory/endocrine organ that mediates numerous physiological and pathological processes. Dysregulation of WAT mass causes obesity or lipoatrophy, two disorders associated with life-threatening pathologies, including cardiovascular diseases and diabetes. Alterations in WAT mass result from changes in adipocyte size and/or number. Change in adipocyte number is achieved through a complex interplay between proliferation and differentiation of preadipocytes. Adipocyte differentiation or adipogenesis is a highly controlled process that has been extensively studied for the last 25 years. In vitro preadipocyte culture systems that recapitulate most of the critical aspects of fat cell formation in vivo have allowed a meticulous dissection of the cellular and molecular events involved in the adipogenesis process. The adipogenic transcription factors peroxisome proliferator-activated receptor-γ and CCAAT/enhancer binding protein-α play a key role in the complex transcriptional cascade that occurs during adipogenesis. Hormonal and nutritional signaling affects adipocyte differentiation in a positive or negative manner, and components involved in cell-cell or cell-matrix interactions are also pivotal in regulating the differentiation process. This knowledge provides a basis for understanding the physiological and pathophysiological mechanisms that underlie adipose tissue formation and for the development of novel and sound therapeutic approaches to treat obesity and its related diseases.

Inflammation of the mucosal layer of the gastrointestinal (GI) tract is not only a feature almost always associated with ulceration of those tissues, but it also plays an important role in both the production and healing of the lesions. The mediators that coordinate inflammatory responses also have the capability to alter the resistance of the mucosa to injury induced by noxious substances, while others render the mucosa more susceptible to injury. In this article, we provide a review of the inflammatory mediators that modulate GI mucosal defense. Among the mediators discussed are nitric oxide, the eicosanoids (prostaglandins, leukotrienes, and thromboxanes), neuropeptides, cytokines, and proteinases. Many of these mediators are considered potential therapeutic targets for the treatment of ulcerative diseases of the digestive tract.

Nephropathy, interstitial pneumopathy, and renal and lung fibrosis are major complications of bone marrow transplantation (BMT). This study evaluated the antifibrotic property of an angiotensin II (A₂) type-1 receptor blocker (L-159,809) and compared it with those of Captopril and Enalapril, two angiotensin-converting enzyme (ACE) inhibitors, in a rat model of BMT. Male WAG/Rij/MCW rats received a preparative regimen of 60 mg/kg body wt of cytoxan (i.p., Days 9 and 8) and 18.5 Gy of total body irradiation (TBI) in six twice daily fractions (Days 2, 1, and 0) followed immediately (Day 0) by BMT. Modifiers were given in drinking water from Day 10 until autopsy, 8 weeks after BMT. Rats treated with TBI plus cytoxan alone developed severe nephropathy. Trichrome staining showed marked collagen deposition in glomeruli, renal interstitium, and renal arteries and arterioles (especially in their adventitia). Collagen deposition and renal damage were markedly reduced by the three modifiers. Of the three, L-158,809-treated rats had slightly thinner vessels and slightly less collagen than nonirradiated normal controls. The study shows the effectiveness of these drugs in the protection of the renal parenchyma from the development of radiation-induced fibrosis. It also indicates a role for angiotensin II in the modulation of collagen synthesis.

Long-term potentiation of sympathetic ganglia (gLTP), a unique form of synaptic plasticity, is serotonin dependent and can be blocked with 5-HT₃ receptor antagonists. Long-lasting enhancement of the basal tone of ganglionic transmission (as with gLTP) is expected to result in sustained increase in peripheral resistance that would lead to elevated blood pressure. We examined the possibility that in sympathetic ganglia, gLTP may be involved in the expression of stress-induced (neurogenic) form of hypertension. High blood pressure in spontaneously hypertensive rat (SHR), known to show exaggerated cardiovascular defense reactions to environmental stimuli, is partly due to a neurogenic factor. Chronic treatment of SHR and their normotensive counterpart, the Wistar Kyoto (WKY) rats with the 5-HT₃ receptor antagonist tropisetron (ICS; 5 mg/kg/day), caused a marked decrease in the blood pressure of the SHR but not of WKY rats. Increasing the daily dose of ICS cumulatively (7 and 10 mg/kg) did not result in significant additional decrease in blood pressure of SHR, indicating that the drug blocks only the neurogenic component of hypertension in the SHR. electrophysiological procedures for indirectly testing for the presence of gLTP in ganglia excised from SHR suggest that gLTP has been previously expressed in these ganglia in vivo. This contrasts with the absence of gLTP in ganglia from normotensive rats. The results support contribution of gLTP to the expression of neurogenic hypertension.

We previously demonstrated that a peptic hydrolysate of guanidinated casein strongly stimulates exocrine pancreatic secretion in chronic bile-pancreatic juice-diverted rats and cholecystokinin (CCK) release from dispersed rat intestinal mucosal cells. These results reveal that the chemically modified protein hydrolysate stimulates CCK secretion and increases pancreatic secretion by a luminal trypsin-independent direct action on the small intestine. In the present study, we examined the direct effect of peptic hydrolysates of naturally occurring dietary proteins, casein, soybean protein isolate (SPI), egg white, and wheat gluten on CCK release under in vitro trypsin-independent conditions. All protein hydrolysates significantly stimulated CCK release from dispersed rat intestinal mucosal cells. Among the hydrolysates treated, SPI hydrolysate was the most effective in stimulating CCK release. The potential of SPI hydrolysate to stimulate CCK release was increased by long-term peptic digestion. However, an SPI-like amino acid mixture did not effect CCK release. In conclusion, peptic hydrolysates of commonly ingested dietary proteins stimulate CCK release via trypsin-independent direct sensing by intestinal mucosal cells.

Morning serum leptin values in humans are inconsistently altered by diet, and the molecular mechanisms controlling the diurnal leptin pattern remain unexplained. We determined whether leptin values after meals or the leptin diurnal pattern was altered by the type of carbohydrate (CHO) ingested in diets containing either 20% or 30% fat. In a randomized, crossover study design, nine healthy lean adults ate one of four isocaloric diets for 8 days. Diets contained 15% protein: A, high glycemic index (GI) CHO, 30% fat; B, low GI CHO, 30% fat; C, high GI CHO, 20% fat; and D, low GI CHO, 20% fat. Serum glucose, insulin, and leptin were measured at intervals on Day 8 for 24 hr, and on Day 9 during an oral glucose tolerance test (GTT). Although the 24-hr glucose and insulin profiles did not differ with the diets, diets A and C altered the serum leptin diurnal pattern. In contrast to the usual evening rise in leptin concentration, which begins after 2200 hr, diets A and C caused a rise in leptin beginning at 1300 hr. The area under the curve for leptin between 1230 and 2400 hr was 17% greater for diets A and C. During the GTT, leptin concentrations were similar for each diet. These results suggest that the pattern and amount of leptin secretion may be altered by high GI CHO or the simple sugar content of the diet, unrelated to differences in insulin concentration, that high GI foods may have little or no effect on serum insulin in the context of a mixed meal, and that a single 0800-hr leptin value may not be sufficient to reveal a diet-induced change in leptin secretion

In laboratory animals, dietary restriction prolongs life span, improves physiologic function, and prevents or lessens severity of several diseases including some experimental inflammatory states. We investigated the effect of dietary restriction on a spontaneously occurring mouse model of atopic dermatitis, an inflammatory skin disease. NC/Nga mice were assigned to a group fed ad libitum or to a restricted diet group receiving 60% of the amount of food consumed by the other group. Dermatitis was characterized according to extent, intensity, and scratching time. We then used computer-assisted image analysis to quantify immunologic findings in skin sections. Extent, intensity score, and scratching time in mice with restriction increased more gradually than in mice fed ad libitum. Infiltrating Inflammatory cells (CD4-positive T cells, CD8-positive T cells, eosinophils, and mast cells) as well as interleukin-4 and −5 secreted into tissue were reduced in mice with restriction. In conclusion, dietary restriction delayed onset and progression of spontaneous dermatitis in NC/Nga mice, an effect possibly involving inhibition of inflammatory infiltration cell and cytokine secretion.

Although dehydroepiandrosterone (DHEA) has long been considered as a precursor for steroid hormones, it has also been shown to have regulatory effects in immune homeostasis. We have examined the effect of high DHEA doses on T cell proliferation, differentiation, and cytokine secretion patterns following stimulation with mitogens and soluble antigens. DHEA profoundly inhibited T cell receptor-mediated T cell proliferation in the upstream of IL-2R signaling. Addition of DHEA to KLH-primed splenocytes stimulated Th2 response, indicated by an increase of IL-4 or a decrease of IFN-γ production in the cultures. Further studies showed that DHEA enhanced IL-4, but inhibited IL-12-mediated T cell proliferation and IL-12 production in antigen-presenting cells (APCs). Our data demonstrated that supraphysiologic levels of DHEA favored Th2 immune responses in vitro by inhibition of IL-12 production from APCs and/or stimulation of Th2 proliferation during the interactions of T cells with APCs.

Calorie restriction without essential nutrient deficiency (calorie restriction, CR) abrogates experimental carcinogenesis and extends healthful life span. To test whether CR influences cell-cycle protein expression during the hepatocellular proliferation induced by 70% partial hepatectomy (PH), BALB/c mice were separated into two groups, fed comparable semi-purified diets for 10 weeks that differed 40% in caloric offering, and were then subjected to PH. When PH was performed, CR mice weighed 36% less than ad libitum (AL)-fed mice (P < 0.01), but liver-to-body weight ratios were similar. During the regenerative hyperplasia, hepatocytes of CR mice demonstrated evidence of accelerated entrance and passage through G1 and S phases, and an earlier exit from the cell cycle. The first peak of DNA synthesis occurred 6 hr earlier, and the second peak was significantly greater among CR mice with 38% ± 13% bromodeoxyuridine (BrdU)-positive hepatocytes, compared with 14% ± 4% in AL mice (P < 0.01). More E2F-1 expression was induced at the hepatic G1/S boundary just prior to each peak of DNA synthesis in regenerating livers of CR mice (P < 0.01), and 8 hr earlier among CR mice. More hyperphosphorylated retinoblastoma p110 was detected during hepatic G1 and the G1-S transition among CR mice, coincident with the early hepatocellular proliferative wave. Cyclin A was induced during the first peak of DNA synthesis 4 hr earlier among CR mice, and it continued 4 hr longer in AL mice, indicating an earlier post-replicative exit by hepatocytes in CR mice. p21 was induced during the G1 phase at 4 hr post-PH, and was maximally expressed during and after peak DNA synthesis in both dietary groups. These results indicate that CR influences cell cycle protein expression levels, causing hepatocytes to enter into S phase earlier and exit abruptly from the cell cycle, and they support the premise that CR enhances induced cell responsiveness by influencing cell cycle regulatory controls.