Consumption of a high-fat diet (HFD) in experimental animal models initiates a series of molecular events and outcomes, including insulin resistance and obesity, that mimic the metabolic syndrome in humans. The relationship among, and order of, the molecular events linking a diet high in fat to pathologies is often unclear. In the present study, we provide several novel insights into the relationship between a HFD and AMP-activated protein kinase (AMPK), a key regulator of cellular metabolism and whole-body energy balance. HFD substantially decreased the activities of both iso-forms of AMPK in white adipose tissue, heart, and liver. These decreases in AMPK activity occurred in the absence of decreased AMPK transcription, systemic inflammation, hyperglycemia, or elevated levels of free fatty acids. The HFD-induced decrease in AMPK activity was associated with systemic insulin resistance and hyperleptinemia. In blood, >98 % of AMPK activity was localized in agranulocytes as the α1 isoform. In contrast to the solid tissues studied, AMPK activities were not altered by HFD in granulocytes or agranulocytes. We conclude that HFD-induced obesity causes a broad, non-tissue, or isoform-specific lowering of AMPK activity. Given the central position AMPK plays in whole-body energy balance, this decreased AMPK activity may play a previously unrecognized role in obesity and its associated pathologies.

Bauwens JD, Schmuck EG, Lindholm CR, Ertel RL, Mulligan JD, Hovis I, Viollet B, Saupe KW. Cold tolerance, cold-induced hyperphagia, and nonshivering thermogenesis are normal in ␣ 1-AMPK Ϫ/Ϫ mice.

The aged heart displays a loss of cardiomyocyte number and function, possibly due to the senescence and decreased regenerative potential that has been observed in some cardiac progenitor cells. An important cardiac progenitor that has not been studied in the context of aging is the cardiac side population (CSP) cell. To address this, flow cytometry-assisted cell sorting was used to isolate CSP cells from adult (6-10 months old) and aged (24-32 months old) C57Bl/6 mice that were fed either a control diet or an anti-aging diet (caloric restriction, CR). Aging caused a 2.3-fold increase in the total number of CSP cells and a 3.2-fold increase in the cardiomyogenic sca1(+)/CD31(-) subpopulation. Aging did not affect markers of proliferation or senescence, including telomerase activity and expression of cell cycle genes, in sca1(+)/CD31(-) CSP cells. In contrast, the aged cells had reduced expression of genes associated with differentiation, including smooth muscle actin and cardiac muscle actin (5.1- and 3.2-fold, respectively). None of these age effects were altered by CR diet. Therefore, it appears that the manner in which CSP cells age is distinct from the aging of post-mitotic tissue (and perhaps other progenitor cells) that can often be attenuated by CR.

The aged heart displays a loss of cardiomyocyte number and function, possibly due to decreased regenerative potential of cardiac progenitor cells. An important cardiac progenitor population that has not been studied in the context of aging is cardiac side population cells (CSPC). To address this, flow cytometry‐assisted cell sorting was used to isolate CSPC from adult (~5‐months old) and aged (~25‐months old) C57Bl/6 mice that were fed either a control diet or an anti‐aging (caloric restriction, CR) diet. Aging caused a 2.0‐fold increase in both the total number of CSPC and in the highly cardiomyogenic sca1+/CD31‐ subpopulation (n=8–10 mice/group). Telomerase activity (a marker of proliferation) in CSPC was not altered by aging (n=4 mice/group). In contrast, aged CSPC showed reduced expression of several genes associated with differentiation, including smooth muscle actin, cardiac muscle actin, and bone collagen type I (5.1‐, 3.2‐, and 2.2‐fold respectively; n=4/group). These data suggest that any loss of regenerative potential in aged CSPC is likely due to inefficiency of differentiation into cardiac lineages rather than poor proliferation rate. None of these age effects were altered by CR diet. Therefore it appears that the manner in which CSPC (and perhaps other types of progenitor cells) age is distinct from the post‐mitotic tissue aging that can often be attenuated by CR.

While activity of α1 isoform of AMP‐activated protein kinase (AMPK) is higher in brown adipose tissue (BAT) than any tissue yet reported, its role in BAT is unknown. Our goal was to elucidate AMPK's in vivo role in BAT, specifically in acute non‐shivering thermogenesis (NST) and chronic (14 days at 4° C) cold‐induced hyperphagia and UCP1 upregulation. C57Bl/6 wild type (WT) and α1AMPK knockout (KO) mice were studied (6–8/group). During acute NST (IP injection of the β3‐adrenergic agonist CL 316,243) core temperature increased similarly (≈0.8°C) in WT and KO mice. Thus α1AMPK is not obligatory for acute NST. In a separate study, chronic cold‐induced hyperphagia was nearly identical in WT and KO mice (≈ 2.5 fold increase in food intake). Chronic cold caused less upregulation of UCP1 in KO (1.9‐fold) than in WT (4‐fold) mice, but did not cause a lower body temperature in KO mice. Therefore α1AMPK is not obligatory for hyperphagia or other adaptations that maintain body temperature near normal during chronic cold exposure. There was little if any compensatory upregulation of α2AMPK in α1AMPK KO mice. Overall our data indicate that high levels of α1AMPK activity in BAT do not play a key role in major aspects of thermoregulation, and instead suggest a currently undefined role for AMPK in BAT physiology.