Belén Alvarez

Interleukin-15 antagonizes muscle protein waste in tumour-bearing rats.

Tissue protein hypercatabolism (TPH) is an important feature in cancer cachexia, particularly with regard to the skeletal muscle. The Yoshida AH-130 rat ascites hepatoma is a model system for studying the mechanisms involved in the processes that lead to tissue depletion, since it induces in the host a rapid and progressive muscle wasting, primarily due to TPH. The present study was aimed at investigating if IL-15, which is known to favour muscle fibre hypertrophy, could antagonize the enhanced muscle protein breakdown in this cancer cachexia model. Indeed, IL-15 treatment partly inhibited skeletal muscle wasting in AH-130-bearing rats by decreasing (8-fold) protein degradative rates (as measured by14C-bicarbonate pre-loading of muscle proteins) to values even lower than those observed in non-tumour-bearing animals. These alterations in protein breakdown rates were associated with an inhibition of the ATP-ubiquitin-dependent proteolytic pathway (35% and 41% for 2.4 and 1.2 kb ubiquitin mRNA, and 57% for the C8 proteasome subunit, respectively). The cytokine did not modify the plasma levels of corticosterone and insulin in the tumour hosts. The present data give new insights into the mechanisms by which IL-15 exerts its preventive effect on muscle protein wasting and seem to warrant the implementation of experimental protocols involving the use of the cytokine in the treatment of pathological states characterized by TPH, particularly in skeletal muscle, such as in the present model of cancer cachexia.

Interleukin-15 mediates reciprocal regulation of adipose and muscle mass: a potential role in body weight control

Interleukin (IL)-15 is a cytokine which is highly expressed in skeletal muscle. Cell culture studies have indicated that IL-15 may have an important role in muscle fiber growth and anabolism. However, data concerning the metabolic effects of this cytokine in vivo are lacking. In the present study, IL-15 was administered to adult rats for 7 days. While IL-15 did not cause changes in either muscle mass or muscle protein content, it induced significant changes in the fractional rates of both muscle protein synthesis and degradation, with no net changes in protein accumulation. Additionally, IL-15 administration resulted in a 33% decrease in white adipose tissue mass and a 20% decrease in circulating triacylglycerols; this was associated with a 47% lower hepatic lipogenic rate and a 36% lower plasma VLDL triacylglycerol content. The decrease in white fat induced by IL-15 was in adipose tissue. No changes were observed in the rate of lipolysis as a result of cytokine administration. These findings indicate that IL-15 has significant effects on both protein and lipid metabolism, and suggest that this cytokine may participate in reciprocal regulation of muscle and adipose tissue mass.

Different cytokines modulate ubiquitin gene expression in rat skeletal muscle

Intravenous administration of different cytokines caused important changes in the expression of ubiquitin genes in skeletal muscle. Tumour necrosis factor-alpha caused a 2.2- and 1.9-fold increase in the expression of the 2.4 and 1.2 kb transcripts, respectively. Administration of interferon-gamma also caused a 2.2- and 1.8-fold increase in the 2.4 and 1.2 kb transcripts, respectively. While administration of leukaemia inhibitory factor and interleukin-6 resulted in no changes in ubiquitin gene expression, interleukin-1 administration also caused an increase in both ubiquitin gene transcripts (2.8- and 1.9-fold for the 2.4 and 1.2 kb transcripts, respectively). The results suggest that some of the cytokine effects on the ubiquitin system gene expression could be related to the enhanced skeletal muscle proteolysis found during cancer cachexia and other pathological states.

In the rat, tumor necrosis factor α administration results in an increase in both UCP2 and UCP3 mRNAs in skeletal muscle: a possible mechanism for cytokine-induced thermogenesis?

Since the discovery of the new members of the UCP (uncoupling protein) family, UCP2 and UCP3, very few studies have dealt with the regulation of their expression. Bearing this in mind, administration of a single intravenous injection of TNF‐α (100 μg/kg body weight) to rats resulted in a significant increase in UCP2 (242%) and UCP3 (113%) gene expression in skeletal muscle. The results suggest a possible role for UCP2 and UCP3 in the increase of energy expenditure associated with cytokine treatment.

Effects of interleukin-15 (IL-15) on adipose tissue mass in rodent obesity models: evidence for direct IL-15 action on adipose tissue.

Interleukin-15 (IL-15) is a proinflammatory cytokine with multifunctional effects outside the immune system. Previous studies have indicated that treatment of normal rats with IL-15 reduces white adipose tissue (WAT) mass, but it was unclear if these effects were direct or indirect. In the present study, the effects of IL-15 on WAT mass and lipid metabolism were studied in two genetic models of obesity: the leptin receptor-negative fa/fa Zucker rat and the leptin-deficient ob/ob mouse. Lean Zucker rats, lean (+/+), and obese mice (ob/ob) responded to IL-15 with reductions in WAT mass and lipoprotein lipase activity (LPL), with no decreases in food intake. In contrast, fa/fa Zucker rats did not respond to IL-15 administration by any of the above measures of fat mass or lipid metabolism. In addition, ribonuclease protection assays (RPAs) were used to demonstrate that all three subunits (gamma(c), beta and alpha) of the IL-15 receptor complex are expressed by rat and mouse WAT, suggesting that the effects of IL-15 on adipose tissue metabolism could be direct. Additionally, the fa/fa rats expressed 84% lower levels of the gamma(c) signaling receptor subunit than lean Zucker rats, suggesting this decrease may play a role in the lack of adipose tissue response to IL-15 in the fa/fa genotype and lending further support for a direct action of IL-15 on adipose tissue.

TNF-α is involved in activating DNA fragmentation in skeletal muscle

Intraperitoneal administration of 100 μg kg−1 (body weight) of tumour necrosis factor-α to rats for 8 consecutive days resulted in a significant decrease in protein content, which was concomitant with a reduction in DNA content. Interestingly, the protein/DNA ratio was unchanged in the skeletal muscle of the tumour necrosis factor-α-treated animals as compared with the non-treated controls. Analysis of muscle DNA fragmentation clearly showed enhanced laddering in the skeletal muscle of tumour necrosis factor-α-treated animals, suggesting an apoptotic phenomenon. In a different set of experiments, mice bearing a cachexia-inducing tumour (the Lewis lung carcinoma) showed an increase in muscle DNA fragmentation (9.8-fold) as compared with their non-tumour-bearing control counterparts as previously described. When gene-deficient mice for tumour necrosis factor-α receptor protein I were inoculated with Lewis lung carcinoma, they were also affected by DNA fragmentation; however the increase was only 2.1-fold. These results suggest that tumour necrosis factor-α partly mediates DNA fragmentation during experimental cancer-associated cachexia.

Tumor necrosis factor-α exerts interleukin-6-dependent and -independent effects on cultured skeletal muscle cells

In vivo studies have shown that cancer-associated skeletal muscle wasting (cachexia) is mediated by two cytokines, tumor necrosis factor-alpha (TNF) and interleukin-6 (IL-6). It has been unclear from these studies whether TNF exerts direct effects on skeletal muscle and/or whether these effects are mediated via IL-6. Previous studies from our laboratory have shown that TNF induces IL-6 mRNA expression in cultured skeletal muscle cells. To further investigate the relationship between TNF and IL-6, the effects of TNF and IL-6 on protein and DNA dynamics in murine C2C12 skeletal myotube cultures were determined. At 1000 U/ml, TNF induced 30% increases in protein and DNA content. The effects of TNF on protein accumulation were inhibited by aphidicolin, an inhibitor of DNA synthesis. IL-6 mimicked the effects of TNF on C2C12 cultures, inducing a 32% increase in protein accumulation and a 71% increase in the rate of protein synthesis. IL-6 also decreased expression of mRNA for several proteolytic system components, including ubiquitin 2.4 kb (51%) and 1.2 kb (63%), cathepsin B (39%) and m-calpain (47%), indicating that IL-6 acts on both protein synthesis and degradation. Incubation of murine C2C12 myotube cultures with TNF (1000 U/ml) in the presence of a polyclonal mouse anti-IL-6 antibody resulted in an abolishment of the effects of TNF on protein synthesis, but did not inhibit TNF-induced stimulation of DNA synthesis. These findings indicate that the effects of TNF on muscle protein synthesis are mediated by IL-6, but that TNF exerts IL-6-independent effects on proliferation of murine skeletal myoblasts.

Skeletal muscle UCP2 and UCP3 gene expression in a rat cancer cachexia model

Rats bearing the Yoshida AH‐130 ascites hepatoma showed an increased expression of both uncoupling protein‐2 (UCP2) (194%) and UCP3 (189%) mRNA levels in skeletal muscle 7 days after tumour inoculation. Interestingly, an even greater increase was observed in mRNA for both UCP2 (278%) and UCP3 (797%) in the pair‐fed animals, suggesting that the increase in gene expression was the result of the anorexia associated with tumour burden. The results constitute the first report of UCP2 and UCP3 gene expression during cancer cachexia and agree to their possible role in the increase of energy expenditure associated with tumour growth.

Branched‐chain amino acids inhibit proteolysis in rat skeletal muscle: mechanisms involved

Incubation of isolated rat soleus and EDL muscles in the presence of 10 mM leucine resulted in a decreased proteolytic rate as measured by the release of tyrosine into the incubation medium. The effect of this branched‐chain amino acid (BCAA) is associated with a decreased activity of the lysosomal proteases and a decreased expression of the genes of the ATP‐ubiquitin–dependent proteolysis (ubiquitin and C8). Incubation of muscles in the presence of actinomycin D revealed that the effects of the amino acid can be accounted for by an inhibition of the transcription rate. The presence of leucine did not influence the gene expression of other nonlysosomal (m‐calpain) and lysosomal (cathepsin B) proteolytic systems. It is concluded that the well‐known effect of BCAA on muscle proteolysis is mediated, in the short term, by the inhibition of lysosomal proteolysis. In a longer period, based on the inhibition of gene transcription observed, an involvement of the ATP‐dependent proteolytic system is also likely to occur. J. Cell. Physiol. 184:380–384, 2000.

Branched-chain amino acids: a role in skeletal muscle proteolysis in catabolic states?

A 48‐h starvation period resulted in a great increase in muscle proteolysis—as measured following the release of tyrosine into the medium—in incubated isolated rat extensor digitorum longus (EDL) muscles. We have quantified the contribution of the different proteolytic systems to the increased protein degradation and observed a considerable activation in the ATP‐dependent proteolytic (60%) and in the calcium‐dependent (125%) systems, while no increases were observed in lysosomal proteolysis. The addition of 10 mM leucine to the incubation medium did not result in any changes in either total proteolytic rate or the activity rates of any of the different systems studied. In addition, the presence of the amino acid did not influence the levels of mRNA for the different genes studied—ubiquitin, C8 proteasome subunit, E2 conjugating enzyme, m‐calpain, and cathepsin B. In a similar way, as observed during starvation, tumor growth resulted in increased protein degradation in incubated isolated EDL muscles from animals bearing the Yoshida AH‐130 ascites hepatoma. The increased rate of protein degradation affected all the proteolytic systems studied: ATP‐ and calcium‐dependent and lysosomal. Finally, leucine addition (10 mM), although not able to revert the increased proteolytic rate, resulted in a decrease in the gene expression for ubiquitin, C8 proteasome subunit and cathepsin B.