DNA-PK (Jackson, 1997) is activated by DNA double-stranded breaks (DSBs). It is a trimeric complex composed of the catalytic subunit, DNA-PKcs, and the Ku70/80 heterodimer. DNA-PK mediates non-homologous end joining (NHEJ), which joins programmed DSBs created during V(D)J recombination and class switching recombination in lymphocytes (Critchlow and Jackson, 1998). As a result, severe combined immune deficiency (SCID) mice (Blunt et al., 1996), which carry a leaky mutation in DNA-PKcs, have impaired lymphocyte development. As a DNA DSB sensor, DNA-PK has some unusual properties. For example, DNA-PK is very abundant: it is estimated that HeLa NSC228155 structure contain approximately 100,000 copies of DNA-PKcs per cell (Anderson and Carter, 1996), far in excess of what is probably needed for NHEJ. In addition, DNA-PKcs is present not only in the nucleus but also in the cytoplasm (Huston et al., 2008). In agreement with this, evidence is mounting that DNA-PK has functions beyond genetic stability. One such function is in metabolism: DNA-PK phosphorylates transcription factor USF-1 and promotes fatty-acid synthesis in response to insulin (Wong et al., 2009). One of the proteins DNA-PK interacts with and phosphorylates is HSP90α (Lees-Miller and Anderson, 1989), one of the two isoforms of HSP90 (α and β). HSP90 is unique among chaperone proteins in that it binds to and folds only a small fraction of the total proteome, called "clients," the majority of which are protein kinases (Li and Buchner, 2013). DNA-PK phosphorylates Thr5,7 (T5,7) in the extreme N terminus (ENT) region of HSP90α (a.a. 1-11, Figure S1; Lees-Miller and Anderson, 1989, Quanz et al., 2012, Solier et al., 2012), but DNA-PK does not phosphorylate HSP90β (Lees-Miller and Anderson, 1989). The effect of T5,7 phosphorylation on HSP90α function is not known. Here, we show that T5,7 phosphorylation of HSP90α, which increases with aging in skeletal muscle, disrupts the HSP90α-client complex. AMPK (Taipale et al., 2012) and its upstream activator kinase LKB1 are known HSP90 clients. Preventing T5,7 phosphorylation, either by mutating T5,7 or by genetic loss of DNA-PKcs, increases AMPK activity in skeletal muscle. Decreasing DNA-PK activity protects against diet-induced obesity and insulin resistance and against loss of mitochondria and physical fitness at older age in mice. Therefore, DNA-PK promotes metabolic and physical decline in older age.
Discussion In Western societies, approximately 30% of the population becomes obese by age 60–70, and 40% of the population becomes either prediabetic or diabetic by age 70–80 (Harris, 1993). Moreover, the capacity for physical exercise, which has protective effects against obesity as well as many diseases of aging, also declines with age. Obesity and aging are also dominant risk factors for many common diseases, including type 2 diabetes, coronary artery disease, hypertension, stroke, cancer, and even Alzheimer's disease. Since the percentage of people aged 65 or more is rapidly increasing, understanding how aging negatively affects energy metabolism and exercise capacity is imperative. Cells respond to conditions in which energy demand exceeds respiratory capacity by expanding mitochondrial content, but with increasing age, mitochondrial content and function declines. One clue to the mechanism for this decline was the discovery that aging impairs AMPK-induced mitochondrial biogenesis in skeletal muscle (Lee et al., 2010, Reznick et al., 2007), although the reason for this was not known. Our findings indicate that aging increases DNA DSBs and DNA-PK-mediated phosphorylation of HSP90α, which results in decreased chaperone function for LKB1/AMPK (Figure 7C). The long-held belief has been that aging-associated forms of deterioration, including the mitochondrial decline in skeletal muscle, ar e due to passive accumulation of damage to DNA, proteins and/or lipids with age. While these accumulated damages may contribute to aging-associated metabolic decline, our findings indicate that a genetic program activated by DNA-PK (Figure 7C) contributes to the metabolic and physical decline at older age. Aging is also associated with inflammation, which is a common denominator of most chronic diseases, including type 2 diabetes (Lumeng and Saltiel, 2011). Therefore, it is possible that the pro-inflammatory effects of DNA-PK (Mishra et al., 2015) may also contribute to the metabolic dysfunction associated with obesity and aging.