Studies on transcriptional control of metabolic changes in the liver induced by a high protein diet

概要

Protein is the crucial macronutrient for liveliness among other two components: fat and carbohydrate. Cellular compounds are made of protein complexes that are formed on amino acids as building blocks. Cellular metabolism and metabolites are mostly formed by amino acids too. While despite two other macronutrients there is no specified storage of protein in body, body protein is used in crisis situations to serve critical needs through autophagy, protein turn over, protein synthesis restriction and control of ureagenesis. Regarding the fact that some amino acids could not be built in the body, sufficient nutritional supply of protein is necessary for maintaining health [1].

　Obesity and linked comorbidities such as metabolic disorder, cardiovascular diseases, nonalcoholic fatty liver and diabetes have grown to an epidemic extent that have created a major public health problem in developed and developing countries. Effective new approaches are needed to prevent progression and treat it [2]. To reach this goal some researchers have visited the previously tested theory of macronutrient alteration effect on energy balance in hope to achieve new profits from dietary manipulation [3].

　Current investigations show that diets with increased protein amount are very good strategies for the treatment of obesity related disoreders. In fact, high protein diets have become a lifestyle increasingly. Reduced appetite and food intake with no induced food aversion are preliminary effects of higher dietary protein intake. In researches done on rodents, high protein diets were shown to induce satiety and consequently decrease energy intake and body weight [4].

　A high protein diet induced body weight loss and body weight maintenance can reverse metabolic issues. It has been proved that as an effect of body weight loss, insulin sensitivity and lipid profile improve immensely. This result indicates that the mentioned metabolic problems were probably made by extra weight and are not the reason behind it [2]. Blood pressure also reduces when body fat mass goes down. Hyperglycemia, hypercholesterolemia and hypertriglyceridemia are some other metabolic risk factors that get normalized or at least get controlled in response to high protein diets, especially in obese people [5]. High protein diet also stimulates hepatic gluconeogenesis [6].

　Klaus et al showed that improved glucose homeostasis, increased energy expenditure, reduced blood lipids and blood pressure, and preserved lean body mass are some positive effects of high-protein diets [7]. In one large European study, a modest elevation in protein content and a mild reduction in the glycemic index resulted to a progress in study completion and maintenance of weight loss [8]. Based on several studies, high protein diet may have positive effects on reducing cardiovascular disease risk factors. Besides, achieved weight loss is closely associated with reduction in the risk of diabetes development [9-10].

　Skeletal muscle mass is the main protein reservoir of the body containing 50-75 percent of all proteins. Physiological anorexia, reduced calorie intake and loss of weight are very common in elderly people which in turn end to a decline in muscle mass. This involuntary loss of muscle is called sarcopenia. Development of sarcopenia results in mortality, disability, dependency, increased risk of falls and fracture and huge health costs.

　Up to half of the population of 80 years old and those who are older, suffer from sarcopenia and after 30 years of age, loss of skeletal muscle happens at a rate of 3-8 % percent per decade. It is expected that by 2050, 1.5 billion of human population fit in the description of aging range and demand health care resources. Therefore, it is crucial to find methods that improve muscle mass maintenance in elderly people [11].

　As mentioned before, non-sufficient protein intake and altered anabolic reaction to dietary protein are contributors of muscle mass loss in elderly people. The synthesis of muscle protein is stimulated via protein and amino acid intake with emphasis on essential amino acids as major regulators of this process. Thus, it is logical to assume nutritional intervention can be a candidate therapy for sarcopenia. High protein diets can stimulate muscle synthesis and preserve the muscle mass. Previous studies have shown that essential amino acid supplementation especially leucine could improve physical function and muscle strength in old patients [12].

　Muscle synthesis stimulations with leucine and other essential amino acids happens through mammalian target of rapamycin (mTOR) pathway. This effect is synergic with exercise effect on muscle mass synthesis. Studies suggest a high protein diet enriched by essential amino acids can have a declining effect on muscle mass loss especially in exercising elderlies. Also, it was shown previously that protein supplementation in combination with anabolic agents such as testosterone and growth hormone can elevate the positive effects [13].

　In sum, high protein diet provides anabolic properties of amino acids which in combination with physical activity optimizes the muscle mass synthesis in old age and proves the important role of nutritional intervention in sarcopenia prevention and management [14].

　Dietary nutrient handling is majorly controlled in liver. Diet induced metabolic alterations in liver have key role in pathologies. Metabolic analysis of liver with focus on gene expression in case of diets with macronutrient manipulation are very important [10]. Gene expression profile of liver after high protein diet channels the excess of amino acids to indirect oxidation with metabolic advantages [15]. Metabolic disease may emerge as a result of alteration in gene expression controlling mechanisms. Expanding our knowledge on the underlying mechanisms of nutrients effect on gene expression may lead to new therapeutic methods based on nutritional interventions and individual genetic makeup [16]. Transcriptional regulation is considered one of the main contributors of gene expression regulation process [17].

　Recent studies have showed the importance of transcriptional gene families in metabolic regulation. They are key players in metabolic regulation by binding to specific DNA sequence motifs and assisting with coregulators in a context-dependent fashion [18].

　It was reported as early as in 1939 that dietary protein intake can change liver arginase activity in rats [19]. Schimke reported that activities of all urea cycle enzymes are directly associated with protein consumption in rats [20]. Although it is known that in some animal species like the cat, a carnivore, adaptation of urea cycle enzyme activities in response to varying levels of protein intake does not occur [21], many mammals including primates show similar adaptation responses to high protein intake [22]. Furthermore, the regulations of these enzyme activities are largely coordinate in degree for the corresponding mRNA levels [23]. As these enzyme activities [24-25] as well as mRNA expressions [23] are regulated also by agents such as glucagon, insulin and glucocorticoids, it has long been postulated that dietary responses are mediated primarily by hormones [26]. However, the precise regulatory mechanisms including those of the transcriptional control remain unclear.

　Krüppel-like factor 15 is a member of a zinc finger family of transcription factors. It has high expression levels in skeletal and cardiac muscle, liver and adipose tissue with major roles in biological processes from pluripotency to carcinogenesis that is induced by glucocorticoids and fasting. It regulates energy metabolism through regulation of glucose, lipids and amino acid metabolism. Previously it was suggested that Klf15 may be involved in regulation of amino acid utilization in a rhythmic fashion. Also, Klf15 plays a critical role in the regulation of gluconeogenesis. Moreover, it’s malfunction is closely associated with cardiovascular diseases such as hypertension, atherosclerosis and coronary heart disease [27-29].

　With wide-reaching effects, the circadian glucocorticoid-Krüppel-like factor 15- branched-chain amino acid (GC-KLF15-BCAA) signaling pathway exists in muscle. Isoleucine, leucine and valine consist 35 percent of essential amino acids in muscle and are involved in survival and repair of sarcopenia induced damage.
The major catabolic enzymes of these amino acids are proven to be controlled by Klf15 activity [30].

　Regarding the CACCC box and its variants as preferred DNA binding sequences of Klf15 and the limited information they provide, it is unknown how KLF family members achieve transcriptional specificity [31]. Moreover, much less is known about the details of KLf15 mediated transcriptional regulation of amino acid catabolism, as a necessary element of gluconeogenesis process [32].

　Due to crucial role of Klf15 in euglycemia maintenance, systemic KLF15-deficient mice exhibit hypoglycemia following an overnight fast. Besides, impaired absorption, uptake, and utilization of lipids in skeletal muscle of KLF15−/− mice leads to damaged exercise endurance. Moreover, amino acid metabolism and its circadian rhythmicity and nitrogen detoxification are compromised in a Klf15 deficient mice due to Klf15 dependent nature of these concepts and thus Klf15 deficient mice manifest hyperammonemia. The expression of some amino acid degradation enzymes reduce significantly in a Klf15−/− mice while serum levels of proline, tyrosine, leucine and valine increase. On the other hand, serum concentration of glutamine which is a substrate of renal gluconeogenesis decreases in this type of mice. The disrupted Klf15-mediated branched chain amino acid catabolism is an important participant to cardiac pathology, and this pathway may serve as a potential therapeutic target for cardiovascular disease. Finally, Klf15−/− mice demonstrate some levels of cognitive disfunction [33-34].

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