Modulating the metabolic response to injury and infection

Induction of a hypermetabolic state in cultured hepatocytes by glucagon and H2O2

Stress hormones and pro-inflammatory cytokines are putative signals triggering increased energy expenditure or "hypermetabolism" commonly observed in inflammatory states. Cytokines also cause the release of reactive oxidants by immune cells resident in tissues in vivo. Therefore, we hypothesized that oxidative stress plays a role in the induction of hypermetabolism. We examined the effect of glucagon (1.0 nM), a catabolic stress hormone, and the oxidant H(2)O(2) (1.0 mM) on the metabolism of stable hepatocyte cultures for 4 days. Combined H(2)O(2) and glucagon treatment, but not H(2)O(2) or glucagon used alone, increased the hepatocyte oxygen uptake rate 25% above control untreated cells after a lag-time of 72 h. The same treatment also increased the expression of mitochondrial uncoupling protein-2 (UCP2). These effects were significantly inhibited by the antioxidant N-acetylcysteine (5mM) and the pentose phosphate pathway (PPP) inhibitor dehydroepianderosterone (200 microM). Glucagon alone induced urea synthesis and H(2)O(2) alone induced the PPP. These findings show, for the first time, that oxidative stress, in combination with glucagon, increases metabolic energy expenditure in cultured cells, and that this effect may be mediated by UCP-2. Furthermore, the results implicate the PPP in the induction of the hypermetabolic response.

Nutrition and muscle loss in humans during spaceflight

The protein loss in humans during spaceflight is partly due to a normal adaptive response to a decreased work load on the muscles involved in weight bearing. The process is mediated by changes in prostaglandin release, secondary to the decrease in tension on the affected muscles. On missions, where there is a high level of physical demands on the astronauts, there tends to be an energy deficit, which adds to the muscle protein loss and depletes the body fat reserves. While the adaptive response is a normal part of homeostasis, the additional protein loss from an energy deficit can, in the long run, have a negative effect on health and capability of humans to live and work in space and afterward return to Earth.

Tumor necrosis factor-alpha confers resistance to hypoxic injury in the adult mammalian cardiac myocyte

BACKGROUND

Previous studies in isolated cardiac myocytes have shown that tumor necrosis factor (TNF)-alpha provokes increased expression of 27- and 70-kD stress proteins as well as manganese superoxide dismutase, suggesting that TNF-alpha might play a role in mediating stress responses in the heart.

METHODS AND RESULTS

To determine whether TNF-alpha stimulation would protect isolated cardiac myocytes against environmental stress, myocyte cultures were pretreated with TNF-alpha for 12 hours and then subjected to continuous hypoxic injury (O2 content, 3 to 5 ppm) for 12 hours, followed by reoxygenation. Cell injury was assessed in terms of lactic dehydrogenase (LDH) release, 45Ca2+ uptake, and MTT metabolism. Pretreatment with TNF-alpha concentrations > or = 50 U/mL significantly attenuated LDH release by hypoxic cells compared with diluent-treated hypoxic cells. Similar findings were observed with respect to 45Ca2+ uptake and MTT metabolism in TNF-alpha-pretreated cells that were subjected to prolonged hypoxia. To determine the mechanism for the TNF-alpha-induced protective effect, the cells were pretreated with heat shock protein (HSP) 72 antisense oligonucleotides. These studies showed that the protective effect of TNF-alpha was not inhibited by antisense oligonucleotides, despite use of a concentration of antisense that was sufficient to attenuate the TNF-alpha-induced increase in HSP 72 expression. Subsequent studies using mutated TNF ligands showed that activation of both types 1 and 2 TNF receptors was sufficient to confer a protective response in isolated cardiac myocytes through an as yet unknown pathway(s).

CONCLUSIONS

Taken together, the above observations demonstrate that TNF-alpha pretreatment confers resistance to hypoxic stress in the adult cardiac myocyte through a novel mechanism that appears to be different from but not necessarily exclusive of the protective response conferred by HSP 72 expression.

Interleukin-6, but not tumour necrosis factor-alpha, increases lipogenesis in rat hepatocyte primary cultures

The Kupffer-cell products interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha) have been shown to stimulate hepatic lipogenesis in vivo. Studies were performed to define the direct effects of these cytokines on lipogenesis in primary-culture rat hepatocytes. Hepatocytes were cultured in the presence of IL-6 or TNF-alpha for periods of 24-72 h. IL-6 increased hepatocyte protein content per microgram of DNA. IL-6 also caused a dose- and time-dependent induction of hepatocyte capacity for incorporation of [2-14C]pyruvate into fatty acids (56% increase by 12.5 ng/ml IL-6 after 72 h of cytokine exposure). This increase in cellular lipogenic capacity was confirmed by using 3H2O incorporation into fatty acids as tracer. TNF-alpha did not increase hepatocyte lipogenesis. In contrast with studies in vivo, neither IL-6 nor TNF-alpha had any acute (2 h of exposure) effects on rates of lipogenesis. Both IL-6 and TNF-alpha are known to increase macrophage prostaglandin synthesis acutely. The prostaglandin E agonist misoprostol free acid (0.1 microM) acutely increased hepatocyte lipogenic rates by 14%. Thus, IL-6 can directly induce hepatocyte lipogenic capacity, and E-series prostaglandins can antagonize the acute inhibition of lipogenesis by glucagon. The observations provide further evidence for the role of Kupffer-cell products in the regulation of hepatocyte metabolism.

Postoperative changes in resting energy expenditure and interleukin 6 level in infants

The relationship was determined between resting energy expenditure (REE) and interleukin (IL)6 level following surgery in infants. Sixteen infants of mean(s.e.m.) weight 3.2(0.2) kg undergoing uncomplicated surgery were studied. The operative stress score was recorded and plasma IL-6 levels were measured before and 4, 8, 12 and 24 h after surgery. Respiratory gas exchange was measured by indirect calorimetry before and at 2, 4, 6, 8, 10, 12, 24 and 48 h. Urinary nitrogen excretion was measured and calculated. IL-6 levels increased significantly after surgery (P = 0.006) and were maximal at 12 h. There was a linear correlation between the increase in IL-6 level and operative severity score (r = 0.61, P = 0.012). REE increased significantly after operation (P < 0.001), peaking at 2-4 h, and returned to baseline levels by 12 h. The changes in REE preceded those in IL-6. IL-6 is a marker of the stress response in infants and may play a role in the subsequent modulation of this response.

Cytokine changes in the postoperative period

The trauma of elective surgery leads to the release of cytokines, many of which act primarily at local level having been released from tissue macrophages, circulating monocytes or other activated tissues. There is also a reproducible response in plasma IL-6, the magnitude of which relates to the magnitude of operative trauma. It seems likely that cytokines have an important role in mediating the anorexia associated with the postoperative period, and this may be associated with increased release of CRF from the hypothalamus. Further studies on the interrelationship between cytokine and neuroendocrine systems will lead to a greater understanding of the control of food intake following injury, and will permit the development of new strategies to improve the effectiveness of nutrition at this time.