Glucose and Lipid Metabolism during Pregnancy and Lactation in Rats : Role of Undercarboxylated Osteocalcin

Abstract

Energy homeostasis is an important physiological mechanism essential for balancingenergy flow through the living systems by managing overall metabolism in the body. Thus, energy homeostasis is under a tight control by means of extremely well-regulated energy metabolism. One of the most common metabolic disorders that occur following disruption in energy homeostasis mechanisms is obesity. Obese individuals develop insulin resistance in the peripheral tissues (fat and muscle) and may also include non-alcoholic fatty liver disease. Insulin resistance is the primary factor responsible for the development of type 2 diabetes mellitus (T2D). Towards control and management of T2D condition, insulin, drugs that regulate the insulin sensitivity and drugs that regulate glucose metabolism are widely used. Repeated insulin administration is painful, expensive and requires constant glucose monitoring while other drugs have various limitations and side effects. Therefore, there is wide scope development of new anti-diabetic molecules for effective management of T2D. Studies related to energy metabolism are necessary to understand the cause of such disorders and improve existing methods to manage metabolic abnormalities. Animal models to understand such metabolic disorders have been developed by chemical treatments and genetic modifications, but diet-induced obese (DIO) animal models appear to be the better among all the models reported. DIO animal models are known to most closely mimic the physiological situation. Apart from the experimental model system studies have been conducted under physiological conditions to gain knowledge on possible mechanisms behind energy balance maintained and established during extreme situations such as pregnancy and lactation. To support fetal growth and milk synthesis several metabolic adjustments occur during pregnancy and lactation without the major disruption in the maternal energy homeostasis. In the present study, to gain knowledge on the mother’s body glucose, lipid management and insulin responses throughout the gestation and lactation periods analyses were carried out during at different stages of pregnancy and lactation in rats. It was observed that during pregnancy, the dam developed insulin resistance in peripheral tissues with decreased activation of insulin pathway and reduced glucose utilization while the liver remained unaffected. Although, as soon as the lactation began, peripheral tissue such as muscle developed increased insulin sensitivity associated with increased expression of glucose transporter gene and higher glucose metabolism. The reversal of insulin response in the muscle tissue observed during lactation appears to be a suitable model system for understanding the process by which the body undergoes a transition from insulin resistant state to sensitive state under a physiological condition. Interestingly, early lactation period is known to have much lower levels of insulin available to act upon peripheral tissues. Factors involved in this transition could be potential therapeutic agents for control of T2D, since during early stages of T2D muscle appears to be the first metabolic organ to exhibit resistance to insulin. The undercarboxylated osteocalcin (UNOC) has been reported to function as anti-diabetic molecule. UNOC is released from skeletal system during bone turnover, especially due to resorption process. Experiments were carried out to examine the role of UNOC during the transition from insulin resistant state of pregnancy to sensitive state of lactation period. It was observed that UNOC levels were lower during pregnancy, but increased during early lactation (day 3 to 6 of lactation). The increased UNOC levels seen during early lactation was higher than the levels observed in non-pregnant, non-lactating (NPNL) rats and the UNOC levels decreased following removal of pups immediately after parturition. It was noted that altering UNOC levels during early lactation altered the insulin response of the whole body and muscle transporter-4 expression (glut4) of lactating rats. A significant increase in bone turnover was also observed during lactation compared to NPNL and pregnant rats. The data suggest that increased bone turnover leads to increased UNOC levels in blood during lactation. Estrogen is known as bone protector molecule which acts via its receptors, estrogen receptor α and β (ERα and β). It was reported that ERβ is a dominant regulator of estrogen signaling when both the receptors of estrogen i.e. ERα and ERβ coexist in the target tissue and estrogen levels are relatively higher. Compared to NPNL rats estrogen levels have shown to be higher during late pregnancy and lower during early lactation. It was observed that liver and adipose tissues largely express ERα, but the muscle showed expression of both the receptors in NPNL rats indicating that muscle is the metabolic tissue that may be modulated by both the receptors. It has been reported that ERβ suppresses ERα action on glut4 transcription in the myocytes. It is possible that the altered ERs ratio modulates glut4 expression during late pregnancy and early lactation. The receptor expression ratio data indicated that muscle is an ERβ dominant during late pregnancy, while it is ERα dominant during early lactation. Further, alteration in UNOC levels during early lactation changed ERs ratio but not sufficient enough to alter the ER dominance, indicating lack of effect of UNOC on ER dominance during early lactation. Experiments were conducted to alter insulin sensitivity during early lactation to extrapolate physiological findings to a pathological condition of the DIO model by feeding rats with high-fat diet (HFD). During early lactation, HFD dams had lower insulin response, lower circulatory UNOC level and lower UNOC receptor (GPRC6A) expression in the muscle. Gene expression of muscle glut4 was lower in HFD rats and the tissue remained ERα dominant indicating no role of HFD on ERs ratio in muscle during early lactation. UNOC has been found to have negative effect on lipid accumulation. During pregnancy, lipid accumulation is one of the first events essential for proper fetal development. Since UNOC levels were suppressed during pregnancy, experiments were carried out to examine relevance of UNOC suppression on lipid accumulation during early pregnancy. For this purpose, pharmacological approaches were utilized to alter UNOC levels during early pregnancy. It was observed that the transient elevation of UNOC levels caused decrease in maternal fat depots without changing circulatory triacylglyceride (TAG) levels. In experiments that decreased UNOC levels in NPNL state to mimic lower levels of UNOC present during early pregnancy, it was found fat storage was higher and TG was found to be lowered in the circulation. These results indicate that UNOC can cause a reduction in fat accumulation and TG levels but UNOC effects on TG levels, was not observed during pregnancy. The data taken together suggest that suppression of UNOC is required for better fat deposition in the mother’s body. Although, some studies have indicated an insulin response transition occurring during pregnancy to lactation, but the factors involved in this transition have not been reported. This report discusses about the factors such as UNOC and ERs and their involvement in the transition process. UNOC role has been studied in genetically modified models and in metabolic disorders such as obesity model system and evidence for physiological role of UNOC would further support its candidature as anti-diabetic molecule. The present research work is the first report to detail relevance of UNOC in physiological conditions such as pregnancy and lactation for glucose and lipid management.