Protein expressions of branched-chain keto acid dehydrogenase subunits are selectively and posttranscriptionally altered in liver and skeletal muscle of starved rats

Tissue-specific and nutrient regulation of the branched-chain α-keto acid dehydrogenase phosphatase, protein phosphatase 2Cm (PP2Cm)

Branched-chain amino acid (BCAA) homeostasis is maintained through highly regulated catabolic activities where the rate-limiting step is catalyzed by branched-chain α-keto dehydrogenase (BCKD). Our previous study has identified a mitochondria-targeted protein phosphatase, PP2Cm, as the BCKD phosphatase and thus serves as a key regulator for BCAA catabolism. In this report, we performed comprehensive molecular and biochemical studies of PP2Cm regulation using both in vivo and in vitro systems. We show that PP2Cm expression is highly enriched in brain, heart, liver, kidney, and diaphragm, but low in skeletal muscle. The PP2Cm expression is regulated at the transcriptional level in response to nutrient status. Furthermore, we have established that PP2Cm interacts with the BCKD E2 subunit and competes with the BCKD kinase in a substrate-dependent and mutually exclusive manner. These data suggest that BCAA homeostasis is at least in part contributed by nutrient-dependent PP2Cm expression and interaction with the BCKD complex. Finally, a number of human PP2Cm single nucleotide polymorphic changes as identified in the public data base can produce either inactive or constitutive active mutant phosphatases, suggesting that putative PP2Cm mutations may contribute to BCAA catabolic defects in human.

Decreased enzyme activity and contents of hepatic branched-chain alpha-keto acid dehydrogenase complex subunits in a rat model for type 2 diabetes mellitus

The mitochondrial branched-chain alpha-keto acid dehydrogenase complex (BCKDC) is responsible for the committed step in branched-chain amino acid catabolism. In the present study, we examined BCKDC regulation in Otsuka Long-Evans Tokushima Fatty (OLETF) rats both before (8 weeks of age) and after (25 weeks of age) the onset of type 2 diabetes mellitus. Long-Evans Tokushima Otsuka (LETO) rats were used as controls. Plasma branched-chain amino acid and branched-chain alpha-keto acid concentrations were significantly increased in young and middle-aged OLETF rats. Although the hepatic complex was nearly 100% active in all animals, total BCKDC activity and protein abundance of E1alpha, E1beta, and E2 subunits were markedly lower in OLETF than in LETO rats at 8 and 25 weeks of age. In addition, hepatic BCKDC activity and protein amounts were significantly decreased in LETO rats aged 25 weeks than in LETO rats aged 8 weeks. In skeletal muscle, E1beta and E2 proteins were significantly reduced, whereas E1alpha tended to increase in OLETF rats. Taken together, these results suggest that (1) whole-body branched-chain alpha-keto acid oxidation capacity is extremely reduced in OLETF rats independently of diabetes development, (2) the aging process decreases BCKDC activity and protein abundance in the liver of normal rats, and (3) differential posttranscriptional regulation for the subunits of BCKDC may exist in skeletal muscle.

Markers of inflammation, proteolysis, and apoptosis in ESRD

BACKGROUND

Hemodialysis (HD) is associated with protein catabolism and augmented apoptosis. Although the effect of metabolic acidosis and inflammatory cytokines on activation of the ubiquitin-proteasome pathway and branched-chain keto acid dehydrogenase (BCKAD) is well known, the effect of HD on these pathways remains unexplored.

METHODS

Twelve patients with end-stage renal disease were studied before and during HD. Eight controls also were studied. Plasma levels of complement components and cytokines, interleukin-1 (IL-1), IL-6, and tumor necrosis factor-alpha (TNF-alpha) were measured. Messenger RNA (mRNA) levels of caspase-3, a mediator of apoptosis; ubiquitin, a marker of proteolysis; and BCKAD-E2, an enzyme regulating branched-chain amino acid oxidation, were estimated in skeletal muscle biopsy specimens by means of reverse-transcriptase polymerase chain reaction. Annexin-V expression was quantified by DNA array. Before the study, participants were placed on a 1.2-g/kg/d protein diet, and metabolic acidosis was corrected.

RESULTS

During HD, plasma IL-6 levels increased from 7.54 +/- 2.24 to 27.86 +/- 4.94 pg/dL (P < 0.001). Complement component, IL-1, and TNF-alpha levels did not change significantly during HD. mRNA levels of caspase-3 (0.50 +/- 0.01 versus 0.81 +/- 0.04), annexin-V (0.94 +/- 0.06 versus 1.48 +/- 0.05; P < 0.001), ubiquitin (1.10 +/- 0.03 versus 1.44 +/- 0.03), and BCKAD-E2 (0.47 +/- 0.01 versus 0.81 +/- 0.04) increased in muscle during HD compared with pre-HD values (P < 0.001). mRNA levels of ubiquitin (0.62 +/- 0.03) and BCKAD-E2 (0.58 +/- 0.02) were greater in controls than pre-HD values (P < 0.05). There were significant positive correlations between plasma IL-6 levels and expression of caspase-3, ubiquitin, and BCKAD-E2 genes.

CONCLUSION

HD causes activation of cytokines, which may mediate the increase in gene expression of caspase-3, ubiquitin, and BCKAD-E2 in skeletal muscles.