Chronic Kidney Disease Causes Defects in Signaling through the Insulin Receptor Substrate/Phosphatidylinositol 3-Kinase/Akt Pathway: Implications for Muscle Atrophy James L. Bailey,* Bin Zheng,* Zhaoyong Hu, † S. Russ Price,* and William E. Mitch † *Renal Division, Department of Medicine, Emory University School of Medicine, Atlanta, Georgia; and † Renal Division, Department of Medicine, Baylor College of Medicine, Houston, Texas Complications of chronic kidney disease (CKD) include depressed responses to insulin/IGF-1 and accelerated muscle proteolysis as a result of activation of caspase-3 and the ubiquitin-proteasome system. Experimentally, proteolysis in muscle cells occurs when there is suppression of phosphatidylinositol 3-kinase (PI3-K) activity. Postreceptor signaling through the insulin receptor substrate (IRS)/PI3-K/Akt pathway was evaluated in muscles of acidotic, CKD and pair-fed control rats under physiologic conditions and in response to a dose of insulin that quickly stimulated the pathway. Basal IRS-1–associated PI3-K activity was suppressed by CKD; IRS-2–associated PI3-K activity was increased. The basal level of activated Akt in CKD muscles also was low, indicating that the higher IRS-2–associated PI3-K activity did not compensate for the reduced IRS-1–associated PI3-K activity. Insulin treatment overcame this abnormality. The low IRS-1–associated PI3-K activity in muscle was not due to a decrease in IRS-1 protein, but there was a higher amount of the PI3-K p85 subunit protein without a concomitant increase in the p110 catalytic subunit, offering a potential explanation for the lower IRS-1–associated PI3-K activity. Eliminating the acidosis of CKD partially corrected the decrease in basal IRS-1–associated PI3-K activity and protein degradation in muscle. It is concluded that in CKD, acidosis and an increase in the PI3-K p85 subunit are mechanisms that contribute to suppression of PI3-K activity in muscle, and this leads to accelerated muscle proteolysis. J Am Soc Nephrol 17: 1388 –1394, 2006. doi: 10.1681/ASN.2004100842 C hronic kidney disease (CKD) is frequently associated with muscle atrophy as a result of increased degrada- tion of muscle proteins through activation of the ubiq- uitin-proteasome (Ub-P’some) system (1). Activation of the Ub- P’some system in muscle could arise because of metabolic acidosis or in response to defects in insulin or IGF-1 signaling pathways. The latter possibility is raised because resistance to these hormones occurs frequently in kidney failure, and IGF-1 and insulin are major modulators of protein metabolism in healthy individuals and experimental animals (2–7). Other ev- idence linking reduced insulin/IGF-1 action to accelerated muscle protein turnover includes the finding that acute insulin deficiency, like CKD, stimulates proteolysis by activating caspase-3 and the Ub-P’some system (8 –12). We have found that reduced activity of the class 1A phosphatidylinositol 3-kinase (PI3-K) leads to activation of both proteolytic path- ways in muscle (8,9,13) If PI3-K activity in muscle is depressed by CKD, then this abnormality could contribute to the acceler- ated muscle protein degradation that is present in CKD. The basis of impaired responses to insulin/IGF-1 in CKD remains obscure. Cecchin et al. (14) examined insulin binding and the kinase activity of insulin receptors that were isolated from skeletal muscle of uremic and control rats and found no change in the receptor number, insulin affinity, or tyrosine kinase activity. Tsao et al. (15) performed similar measurements of IGF-1 receptor number, binding affinity, and kinase activity in muscle of CKD and control rats and reported that there were no differences. Both groups attributed the inability of insulin and IGF-1 to ameliorate muscle wasting in CKD to postreceptor defects. In contrast, Ding et al. (6) reported that CKD was associated with an increase in the number of IGF-1 receptors in muscle but lower IGF-1 receptor kinase activity. There are potential explanations for the discrepancies in these reports. All of the studies were performed in vitro with partially purified receptor preparations that may have differed in their purity. Differences in the models (e.g., acute versus chronic kidney failure, degrees of acidosis or azotemia, insulin versus IGF-1) also might have contributed to the inconsistent outcomes. Given the importance of insulin and IGF-1 as modulators of muscle protein metabolism and the commonality in their pos- treceptor signaling pathway, we tested muscles of CKD and pair-fed, sham-operated control rats for abnormalities in the insulin receptor substrate (IRS)/PI3-K/Akt pathway under basal, physiologic conditions and after maximally stimulating the signaling system by injecting a supraphysiologic dose of insulin into the rats. We evaluated the IRS proteins, PI3-K, and a more distal effector of insulin/IGF-1 action, Akt. The results provide insights into the abnormalities that render muscle re- sistant to the anabolic effects of insulin and IGF-1. Received October 11, 2004. Accepted February 20, 2006. Published online ahead of print. Publication date available at www.jasn.org. Address correspondence to: Dr. James L. Bailey, Renal Division, Emory Univer- sity School of Medicine, WMB 338, 1639 Pierce Drive, Atlanta, GA 30322. Phone: 404-727-2525; Fax: 404-727-3425; E-mail: james.l.bailey@emory.edu Copyright © 2006 by the American Society of Nephrology ISSN: 1046-6673/1705-1388