ROLE OF NF-kB IN STEM CELLS AGING

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Aging is thought to arise as a consequence of the accumulation of stochastic molecular and cellular damage^. The precise nature of the damage responsible for aging-related degeneration remains poorly-defined, but may consist of mitochondrial damage, telomere attrition, nuclear dysmorphology, accumulation of genetic mutations, or DNA, protein, and membrane damage. There are several lines of evidence to support the notion that DNA damage is one type of molecular damage that contributes to aging. First, both DNA lesions^ and genetic mutations caused by DNA damage accumulate in tissues of aged organisms. Second, mice harboring germ-line mutations that confer resistance to genotoxic stress are long-lived 1¿'". Third, the majority of human progerias (or syndromes of accelerated aging) are caused by inherited mutations in genes required for genome maintenance^. For example, deficiency of the DNA repair endonuclease ERCCi-XPF causes progeria^^. A mouse model of this disease (Erccrl- mice) was generated and used by Dr. Laura Niedernhofer, a collaborator on this application, to probe the similarity between progeria due to vmrepaired DNA damage and natural aging29. Comparison of the transcriptome of the DNA repair deficient mice and old wdld type mice revealed a highly significant correlation. This was recapitulated in young wild type (wt) mice exposed to genotoxic stress. Many types of cellular damage lead to activation of the transcription factor NF-kB including oxidative, physical, and chemical stress. NF-kB activation in response to stress may be mediated, at least in part, by DNA damage. Genotoxic stress caused by UV irradiation, y-irradiation, and topoisomerase poisons activate NF-kB^^'i^. Furthermore, DNA double strand breaks have been demonstrated to activate NF-kB in an ATM-dependent manner. In addition, NF-kB is up-regulated in a variety of tissues of aged rodents relative to young animals, including the skin, liver, kidney, cerebellum, cardiac muscle and gastric mucosa^s-is. Recently, a study determined that NF-kB was the transcription factor most associated with mammalian aging, and subsequently showed that local NF-kB inhibition in skin promoted "younger" skin histologically, with increased Ki67 and reduced pi6 staining'^. This study also demonstrated that expression of a subset of NF-kB regulated genes is increased during aging. Additionally, overexpression of C-rel (an NF-kB subimit) induces hallmark features of cellular scenescence including decreased proliferation, resistance to apoptosis, and morphologic changes, such as enlarged, multinucleated cells that are granular in appearance^o. NF-kB is also implicated in the pathogenesis of numerous age-associated diseases, including atherosclerosis, osteoarthritis^^, dementia^^, osteoporosis, cancer^s and cardiovascular disease. NF-kB is traditionally thought of as an immunologic transcription factor that is activated by pathogenic ligands such as LPS and cytokines such as TNF-a and IL-iB. NF-kB activation is critical for the stimulation of antigen presenting cells (APCs), which produce cytokines, chemokines and reactive oxygen species (ROS), leading to activation of the adaptive unmune system to control the infection or pathogenic insult. However, NF-kB is also activated by other types of stress including genotoxic, oxidative, mechanical and other environmental insults that cause cell damage3¿'3i. Specifically, DNA damaging agents such as UV, y-irradiation, topoisomerase poisons and reactive oxygen species activate NF-kB and both genotoxic and oxidative stress are implicated in the pathogenesis of aging. Therefore, NF-kB-dependent signaling mechanisms that regulate the cellular response to stress could contribute to aging. NF-kB is a family of transcription factors consisting of homo- or heterodimers of p65, p50, c-rel, relB and p52. The dimers of immunologic relevance are the inflammatory p65/p50 heterodimer and the repressor p50 homodimer. Two main pathways of NF-kB activation exist, but of importance for this proposal is the canonical pathway that signals via the IKK complex. The IKK complex is comprised of two catalytic subimits, IKKa and 13, and a reguatory subunit IKKy or NEMO. Once activated by IKKy, IKKJ3 and a phosphorylate IkB, the cytoplasmic inhibitor of NF-kB^^. After phosphorylation, IkBa is ubiquitinated and undergoes proteosomal degradation. This releases NF-kB, allowing it to translocate to the nucleus where it promotes transcription of mmierous pro-inflammatory, antiapoptotic and cellular growth genes. IKK assembly requires the binding of the N-terminus of IKKy/NEMO to the C-terminus of IKKa and IKK13 in a region of these subunits called the NEMO binding domain (NBD).25 The NBD region of IKKB consists of i i amino acids designed from the C-terminus, which can then be delivered to cells using a protein transduction domain (PTD).26.27 Addition of the wild type, but not mutant NBD peptide has been shown to interfere with IKK assembly and inhibits cytokine-induced activation of NF-kB. Notably, NBD peptide does not block basal IKK activity, but only modulates the induced activation of IKK in response to inflammatory signals. In vivo, systemic administration of a PTD-NBD peptide is not associated with any described toxicity in mice or rats and inhibition of NF-kB correlates with therapeutic responses in an evergrowing list of diseases.