Apoptosis and myostatin mRNA are upregulated in the skeletal muscle of patients with chronic kidney disease.

Apoptosis and myostatin are major mediators of muscle atrophy and might therefore be involved in the wasting of uremia. To examine whether they are expressed in the skeletal muscle of patients with chronic kidney disease (CKD), we measured muscle apoptosis and myostatin mRNA and their related intracellular signal pathways in rectus abdominis biopsies obtained from 22 consecutive patients with stage 5 CKD scheduled for peritoneal dialysis. Apoptotic loss of myonuclei, determined by anti-single-stranded DNA antibody and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assays, was significantly increased three to fivefold, respectively. Additionally, myostatin and interleukin (IL)-6 gene expressions were significantly upregulated, whereas insulin-like growth factor-I mRNA was significantly lower than in controls. Phosphorylated JNK (c-Jun amino-terminal kinase) and its downstream effector, phospho-c-Jun, were significantly upregulated, whereas phospho-Akt was markedly downregulated. Multivariate analysis models showed that phospho-Akt and IL-6 contributed individually and significantly to the prediction of apoptosis and myostatin gene expression, respectively. Thus, our study found activation of multiple pathways that promote muscle atrophy in the skeletal muscle of patients with CKD. These pathways appear to be associated with different intracellular signals, and are likely differently regulated in patients with CKD.

Androgen-mediated apoptosis of kidney tubule cells: role of c-Jun amino terminal kinase.

The incidence and the rate of progression of chronic kidney diseases (CKD) are for most diseases greater in men than in age-matched women. We have previously shown that testosterone (T) promotes the apoptosis of proximal tubule kidney cells. To better understand the downstream signaling process associated with T-induced apoptosis, we examined the involvement of c-Jun amino terminal kinase (JNK) in a human proximal tubule cell line (HK-2) exposed to T: JNK and its downstream effector c-Jun were rapidly phosphorylated. By blocking androgen receptor, JNK phosphorylation was reduced and 17beta-Estradiol treatment had no effect on it. Similarly, pre-treatment with the JNK inhibitor SP600125 prevented the T-induced apoptosis, the phosphorylation of c-Jun and the upregulation of the Fas/FADD pathway. These data show that the JNK/c-Jun pathway is directly regulated by androgens in vitro and highlight a potential mechanism explaining the reported gender differences in the progression of renal diseases.

Kidney and splanchnic handling of interleukin-6 in humans.

Chronic elevation of circulating Interleukin-6 (IL-6) is observed in elderly individuals as well as in several illnesses, including chronic kidney diseases. A number of cells and tissues possess the ability to metabolize significant amounts of IL-6 in vitro. However, information on signals and mechanisms by which IL-6 is removed from blood in humans is still incomplete. To assess the individual role of splanchnic organs and kidney on IL-6 inter-organ exchange we used the IL-6 mass-balance technique across the hepato-splanchnic bed and kidney in six subjects with normal renal and liver function undergoing diagnostic venous catheterizations. Both in the hepatic and renal veins IL-6 levels were significantly lower (p=0.041 and 0.038, respectively), than in the artery. The fractional extraction of IL-6, i.e., the percentage of arterial IL-6 extracted after a single pass, was greater across the splanchnic organs (18%) than across the kidney (8%). Accordingly, IL-6 plasma clearance across splanchnic organs was greater than across the kidney and the sum of kidney and splanchnic removal accounted for as much as 63% of the estimated adipocyte IL-6 release. Our data demonstrate that, although the individual contribution to removal is different, both splanchnic organs and kidneys affect in a significant way the disposal of IL-6 in humans. According, both liver and kidney dysfunction could affect the handling of this proinflammatory cytokine and favour a chronic inflammatory response.

Causes of hyperhomocysteinemia in patients with chronic kidney diseases.

Plasma homocysteine (Hcy) levels are increased significantly in patients with moderate renal failure and increase markedly in patients with end-stage renal disease. An increase in plasma Hcy level theoretically could be caused by an increased production rate (ie, transmethylation), a decreased rate of removal by transsulfuration or remethylation, or a decrease in the excretion of Hcy. Current evidence indicates that the major mechanism for hyperhomocysteinemia in renal failure is a decrease in Hcy removal from the body. However, it is debated whether this effect is the result of a decrease in the renal metabolic clearance or a result of extrarenal metabolic changes. The human kidney plays a major role in the removal of several aminothiols or Hcy-related compounds from the circulation (eg, cysteine-glycine, glutathione, AdoMet, and AdoHcy). However, the glomerular filtration of Hcy seems to be restricted because of protein binding. Besides glomerular filtration, the normal kidney can remove Hcy by plasma flow and peritubular uptake. Although in the low normal range in absolute terms, the flow through the transsulfuration pathway is reduced if related to Hcy levels in uremia; in addition, the remethylation pathway also is impaired. Besides the potential effect of the reduced renal mass on Hcy removal, available evidence suggests the occurrence of a generalized down-regulation of the methionine cycle and catabolism in uremia. AdoHcy, sulfate, and dimethylglycine currently are being investigated as retained solutes that can inhibit 1 or more pathways of Hcy metabolism. In addition, the high Hcy levels decrease in malnourished end-stage renal disease patients and change according to nutrient intake and several other nutritional parameters, indicating that circulating Hcy levels become an expression of nutritional status.

Kidney protein dynamics and ammoniagenesis in humans with chronic metabolic acidosis.

To evaluate the effects of chronic metabolic acidosis on protein dynamics and amino acid oxidation in the human kidney, a combination of organ isotopic ((14)C-leucine) and mass-balance techniques in 11 subjects with normal renal function undergoing venous catheterizations was used. Five of 11 studies were performed in the presence of metabolic acidosis. In subjects with normal acid-base balance, kidney protein degradation was 35% to 130% higher than protein synthesis, so net protein leucine balance was markedly negative. In acidemic subjects, kidney protein degradation was no different from protein synthesis and was significantly lower (P < 0.05) than in controls. Kidney leucine oxidation was similar in both groups. Urinary ammonia excretion and total ammonia production were 186% and 110% higher, respectively, and more of the ammonia that was produced was shifted into urine (82% versus 65% in acidemic subjects versus controls). In all studies, protein degradation and net protein balance across the kidney were inversely related to urinary ammonia excretion and to the partition of ammonia into urine, but not to total ammonia production, arterial pH, [HCO(-)(3)], urinary flow, the uptake of glutamine by the kidney, or the ammonia released into the renal veins. The data show that response of the human kidney to metabolic acidosis includes both changes in amino acid uptake and suppression of protein degradation. The latter effect, which is likely induced by the increase in ammonia excretion and partition into the urine, is potentially responsible for kidney hypertrophy.