Signaling Mechanisms Regulation Muscle Protein Turnover

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Loss of muscle protein has been shown to be due to activation of the ubiquitin-proteasome proteolytic system in both patients and experimental rats with catabolic conditions like kidney failure, sepsis, diabetes and in each case, glucocorticoid production is increased. Our group was the first to demonstrate that glucocorticoids are necessary for the increase in protein degradation and levels of mRNAs encoding components of this system. We used nuclear run-on assays to show that this increase in pathway mRNAs results from increased transcription in muscle of rats with diabetes or renal insufficiency. It is important to define the mechanisms for the transcriptional responses because several reports indicate that the level of specific pathway proteins (e.g., proteasome C2 subunit, ubiquitin) is closely linked to or determines the rate of protein degradation. We have identified links among glucocorticoid action, activation of two novel cellular signaling pathways, and transcription factors that regulate the expression of genes for the C3 proteasome subunit and ubiquitin in muscle: 1) glucocorticoids upregulate the expression of the proteasome C3 subunit by a mechanism that antagonizes NF-kB (we showed earlier that NF-kB suppresses transcription of the C3 subunit in muscle cells). Our Preliminary Results indicate that this glucocorticoid-dependent antagonism is achieved through inhibition of the kinase (IkB kinase or IKK) that initiates the destruction of IkB (the inhibitor of NF-kB translocation to the nucleus). This response results in sequestration of NF-kB in the cytosol, relieving suppression of C3 subunit transcription. There also are Preliminary Results that the serum- and glucocorticoid-regulated kinase (SGK) is involved in regulating NF-kB activation. Our hypothesis is that activated SGK stimulates IKK activity leading to degradation of IkB and that glucocorticoids inhibit IKK by preventing the activation of SGK. 2) Other Preliminary Results indicate the increase in ubiquitin transcription induced by glucocorticoids involves Sp1 and the MAP kinase pathway. Our hypothesis is that glucocorticoids activate ERK1/2 which then phosphorylates Sp1, increasing Sp1 binding to the ubiquitin promoter. Our strategy for examining these hypothesis will be to determine if glucocorticoids: 1) inhibit SGK activity; and 2) activate the ERK MAPK and increase Sp1 phosphorylation. Next, we will determine if regulating SGK or MEK1 (the upstream activator of ERK1/2) will change protein degradation and expression of ubiquitin-proteasome pathway genes in L6 muscle cells. Lastly, we will study adrenalectomized rats to determine if glucocorticoids can be linked to inhibition of SGK activity or activation of the MEK/ERK MAPK pathway in skeletal muscle.