The serine/threonine kinase Akt/PKB plays key roles in the regulation of cell growth, survival, and metabolism. It remains unclear, however, whether the functions of individual Akt/PKB isoforms are distinct. To investigate the function of Akt2/PKBβ, mice lacking this isoform were generated. Both male and female Akt2/PKBβ-null mice exhibit mild growth deficiency and an age-dependent loss of adipose tissue or lipoatrophy, with all observed adipose depots dramatically reduced by 22 weeks of age. Akt2/PKBβ-deficient mice are insulin resistant with elevated plasma triglycerides. In addition, Akt2/PKBβ-deficient mice exhibit fed and fasting hyperglycemia, hyperinsulinemia, glucose intolerance, and impaired muscle glucose uptake. In males, insulin resistance progresses to a severe form of diabetes accompanied by pancreatic β cell failure. In contrast, female Akt2/PKBβ-deficient mice remain mildly hyperglycemic and hyperinsulinemic until at least one year of age. Thus, Akt2/PKBβ-deficient mice exhibit growth deficiency similar to that reported previously for mice lacking Akt1/PKBα, indicating that both Akt2/PKBβ and Akt1/PKBα participate in the regulation of growth. The marked hyperglycemia and loss of pancreatic β cells and adipose tissue in Akt2/PKBβ-deficient mice suggest that Akt2/PKBβ plays critical roles in glucose metabolism and the development or maintenance of proper adipose tissue and islet mass for which other Akt/PKB isoforms are unable to fully compensate.This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

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Fasting elicits a progressive increase in lipid metabolism within skeletal muscle. To determine the effects of fasting on the transcriptional regulation of genes important for metabolic control in skeletal muscle composed of different fiber types, nuclei from control and fasted (24 and 72 h) rats were subjected to nuclear run-on analysis using an RT-PCR-based technique. Fasting increased (P < 0.05) transcription rate of the muscle-specific uncoupling protein-3 gene (UCP3) 14.3- to 21.1-fold in white gastrocnemius (WG; fast-twitch glycolytic) and 5.5- to 7.5-fold in red gastrocnemius (RG; fast-twitch oxidative) and plantaris (PL; mixed) muscles. No change occurred in soleus (slow-twitch oxidative) muscle. Fasting also increased transcription rate of the lipoprotein lipase (LPL), muscle carnitine palmitoyltransferase I (CPT I), and long-chain acyl-CoA dehydrogenase (LCAD) genes 1.7- to 3.7-fold in WG, RG, and PL muscles. Transcription rate responses were similar after 24 and 72 h of fasting. Surprisingly, increasing metabolic demand during the initial 8 h of starvation (two 2-h bouts of treadmill running) attenuated the 24-h fasting-induced transcriptional activation of UCP3, LPL, CPT I, and LCAD in RG and PL muscles, suggesting the presence of opposing regulatory mechanisms. These data demonstrate that fasting elicits a fiber type-specific coordinate increase in the transcription rate of several genes involved in and/or required for lipid metabolism and indicate that exercise may attenuate the fasting-induced transcriptional activation of specific metabolic genes.

Hildebrandt, Audrey L., Henriette Pilegaard, and P. Darrell Neufer. Differential transcriptional activation of select metabolic genes in response to variations in exercise intensity and duration. Am J Physiol Endocrinol Metab 285: E1021-E1027, 2003. First published August 5, 2003 10.1152/ajpendo.00234.2003.-Cellular adaptations to endurance training are influenced by the intensity and duration of exercise. To examine the impact of exercise intensity and duration on the acute transcriptional regulation of metabolic genes in red (RG) and white (WG) gastrocnemius muscle, rats completed either low-intensity [ϳ50% maximal O 2 uptake (V O2 max)] treadmill exercise (LIE) for 45 min, LIE for 180 min, or high-intensity (ϳ75% V O2 max) exercise (HIE) for 45 min. LIE for 45 min activated (P Ͻ 0.05) transcription of the pyruvate dehydrogenase kinase-4 (PDK4), uncoupling protein-3 (UCP3), heme oxygenase-1 (HO-1), and hexokinase II (HK II) genes in RG within 1 h after exercise. In WG, transcription of PDK4, UCP3, HKII, and lipoprotein lipase (LPL) was also induced, whereas transcription of the HO-1 gene did not change. In RG, extending LIE duration from 45 to 180 min elicited a similar activation of PDK4 and UCP3 (ϳ15-fold) but a far greater increase in HO-1 (Ͼ30-fold) and HKII transcription (Ͼ25-fold). In WG, extending LIE for 180 min induced a much greater and prolonged (through 2-to 4-h recovery) activation of PDK4, UCP3 (both Ͼ200-fold), and HO-1 (Ͼ10-fold). HIE elicited a similar pattern of gene activation to LIE in both RG and WG, with the exception that HIE triggered Ͼ10-fold activation of HO-1 in WG. These data provide evidence that both the intensity and the duration of exercise affect the transcriptional regulation of metabolic genes in muscle in a fiber type-specific manner, possibly reflecting the relative stress imposed by the exercise bout. endurance training; mitochondrial biogenesis; gene regulation ENDURANCE EXERCISE TRAINING induces a number of adaptations in skeletal muscle, including increased expression of key metabolic enzymes and an overall increase in mitochondrial content (16,19). At the biochemical level, these adaptations improve the efficiency of substrate utilization during exercise, enhance the sensitivity of the respiratory control system, and increase the overall oxidative capacity of skeletal myofibers (5,8,19). The magnitude of change is directly related to the intensity, duration, and frequency of exercise during the training program (7), whereas the total time required to achieve the "trained" state is a function of first-order kinetics, i.e., a function of the turnover rate constants of the specific proteins involved in the adaptive response (43). Thus, under conditions in which all three training parameters are held constant, the training program must be of sufficient length for the cellular proteins to reach their new concentration and the biochemical adaptations to fully develop.The biochemical adaptations to endurance training however, do not represent a change in the true "steadystat...

. AMP-activated protein kinase activates transcription of the UCP3 and HKII genes in rat skeletal muscle. Am J Physiol Endocrinol Metab 283: E1239-E1248, 2002. First published September 11, 2002 10.1152/ajpendo. 00278.2002.-AMP-activated protein kinase (AMPK) has recently emerged as a key signaling protein in skeletal muscle, coordinating the activation of both glucose and fatty acid metabolism in response to increased cellular energy demand. To determine whether AMPK signaling may also regulate gene transcription in muscle, rats were given a single subcutaneous injection (1 mg/g) of the AMP analog 5-aminoimidazole-4-carboxamide-1-␤-D-ribonucleoside (AICAR). AICAR injection activated (P Ͻ 0.05) AMPK-␣2 (ϳ2.5-fold) and transcription of the uncoupling protein-3 (UCP3, ϳ4-fold) and hexokinase II (HKII, ϳ10-fold) genes in both red and white skeletal muscle. However, AICAR injection also elicited (P Ͻ 0.05) an acute drop (60%) in blood glucose and a sustained (2-h) increase in blood lactate, prompting concern regarding the specificity of AICAR on transcription. To maximize AMPK activation in muscle while minimizing potential systemic counterregulatory responses, a single-leg arterial infusion technique was employed in fully conscious rats. Relative to saline-infused controls, single-leg arterial infusion of AICAR (0.125, 0.5, and 2.5 g ⅐ g Ϫ1 ⅐ min Ϫ1 for 60 min) induced a dose-dependent increase (2-to 4-fold, P Ͻ 0.05) in UCP3 and HKII transcription in both red and white skeletal muscle. Importantly, AICAR infusion activated transcription only in muscle from the infused leg and had no effect on blood glucose or lactate levels. These data provide evidence that AMPK signaling is linked to the transcriptional regulation of select metabolic genes in skeletal muscle.5-aminoimidazole-4-carboxamide ribonucleoside; single-leg arterial infusion; rat; AMP kinase phosphorylation AMP-ACTIVATED PROTEIN KINASE (AMPK) is a highly conserved metabolite-sensing protein kinase found in all eukaryotic cells (12). In mammals, the AMPK enzyme is composed of a catalytic ␣-subunit and two regulatory subunits, ␤ and ␥, each of which is encoded for by either two (␣ 1 , ␣ 2 , ␤ 1 , ␤ 2 ) (40, 42) or three (␥ 1 , ␥ 2 , ␥ 3 ) (6) genes. Although not completely defined, it is thought that AMP activates AMPK by binding to the interface between the ␣-and ␥-subunits, disrupting an autoinhibitory domain within the ␣-subunit (13). AMPK is also stimulated by phosphorylation of a regulatory site (Thr 172 ) in the catalytic domain of the ␣-subunit, a reaction catalyzed by an upstream kinase, AMPK kinase (AMPKK), which is also activated by AMP (14,15,41). Thus full activation of AMPK is achieved by a combination of AMP-mediated allosteric activation of AMPK and its upstream kinase AMPKK, and by covalent phosphorylation of the AMPK ␣-subunit by AMPKK. Although the resting concentration of AMP is typically 100-fold lower than ATP (a competitive inhibitor of AMPK), AMP concentration increases dramatically under conditions of accelerated ATP utilization,...

The serine/threonine kinase Akt/PKB plays key roles in the regulation of cell growth, survival, and metabolism. It remains unclear, however, whether the functions of individual Akt/PKB isoforms are distinct. To investigate the function of Akt2/PKBβ, mice lacking this isoform were generated. Both male and female Akt2/PKBβ-null mice exhibit mild growth deficiency and an age-dependent loss of adipose tissue or lipoatrophy, with all observed adipose depots dramatically reduced by 22 weeks of age. Akt2/PKBβ-deficient mice are insulin resistant with elevated plasma triglycerides. In addition, Akt2/PKBβ-deficient mice exhibit fed and fasting hyperglycemia, hyperinsulinemia, glucose intolerance, and impaired muscle glucose uptake. In males, insulin resistance progresses to a severe form of diabetes accompanied by pancreatic β cell failure. In contrast, female Akt2/PKBβ-deficient mice remain mildly hyperglycemic and hyperinsulinemic until at least one year of age. Thus, Akt2/PKBβ-deficient mice exhibit growth deficiency similar to that reported previously for mice lacking Akt1/PKBα, indicating that both Akt2/PKBβ and Akt1/PKBα participate in the regulation of growth. The marked hyperglycemia and loss of pancreatic β cells and adipose tissue in Akt2/PKBβ-deficient mice suggest that Akt2/PKBβ plays critical roles in glucose metabolism and the development or maintenance of proper adipose tissue and islet mass for which other Akt/PKB isoforms are unable to fully compensate.This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

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