Borja Guerra

Exercise and Bone Mass in Adults

There is a substantial body of evidence indicating that exercise prior to the pubertal growth spurt stimulates bone growth and skeletal muscle hypertrophy to a greater degree than observed during growth in non-physically active children. Bone mass can be increased by some exercise programmes in adults and the elderly, and attenuate the losses in bone mass associated with aging. This review provides an overview of cross-sectional and longitudinal studies performed to date involving training and bone measurements. Cross-sectional studies show in general that exercise modalities requiring high forces and/or generating high impacts have the greatest osteogenic potential. Several training methods have been used to improve bone mineral density (BMD) and content in prospective studies. Not all exercise modalities have shown positive effects on bone mass. For example, unloaded exercise such as swimming has no impact on bone mass, while walking or running has limited positive effects.It is not clear which training method is superior for bone stimulation in adults, although scientific evidence points to a combination of high-impact (i.e. jumping) and weight-lifting exercises. Exercise involving high impacts, even a relatively small amount, appears to be the most efficient for enhancing bone mass, except in postmenopausal women. Several types of resistance exercise have been tested also with positive results, especially when the intensity of the exercise is high and the speed of movement elevated. A handful of other studies have reported little or no effect on bone density. However, these results may be partially attributable to the study design, intensity and duration of the exercise protocol, and the bone density measurement techniques used. Studies performed in older adults show only mild increases, maintenance or just attenuation of BMD losses in postmenopausal women, but net changes in BMD relative to control subjects who are losing bone mass are beneficial in decreasing fracture risk. Older men have been less studied than women, and although it seems that men may respond better than their female counterparts, the experimental evidence for a dimorphism based on sex in the osteogenic response to exercise in the elderly is weak. A randomized longitudinal study of the effects of exercise on bone mass in elderly men and women is still lacking. It remains to be determined if elderly females need a different exercise protocol compared with men of similar age. Impact and resistance exercise should be advocated for the prevention of osteoporosis. For those with osteoporosis, weight-bearing exercise in general, and resistance exercise in particular, as tolerated, along with exercise targeted to improve balance, mobility and posture, should be recommended to reduce the likelihood of falling and its associated morbidity and mortality. Additional randomized controlled trials are needed to determine the most efficient training loads depending on age, sex, current bone mass and training history for improvement of bone mass.

Plasma membrane oestrogen receptor mediates neuroprotection against β‐amyloid toxicity through activation of Raf‐1/MEK/ERK cascade in septal‐derived cholinergic SN56 cells

Rapid oestrogen neuroprotection against β‐amyloid peptide (Aβ)‐induced toxicity, a main feature of Alzheimers disease, may be partially initiated at the plasma membrane. However, the mechanism by which this oestrogen effect occurs is unknown. In a septal murine cell line (SN56), we observed that short exposures to either 17β‐oestradiol (E2) or membrane impermeant E2 bound to horseradish peroxidase (E‐HRP) induced a biphasic stimulation of extracellular‐signal regulated protein kinase (ERK1/2) phosphorylation, with peak inductions detected around 4–8 min in the early phase and a second maximum around 8 h after treatment. ERK1/2 phosphorylation was abolished by ERK1/2 kinase (MEK) inhibitors PD98059 and U0126. Interestingly, PD98059 was also shown to block rapid E2‐related prevention of death in cells exposed to Aβ fragment 1–40 (Aβ1−40) for 24 h. In contrast, no neuroprotective effects were obtained when MEK inhibitor was used to selectively abolish the late phosphorylation phase. Furthermore, both ERK1/2 activation and E2‐associated protection were blocked by an inhibitor of Raf‐1 kinase. Raf‐1 may be involved in these effects because oestrogen caused the rapid serine 338 (Ser338) phosphorylation of this protein. In addition, the oestrogen receptor (ER) antagonist ICI 182 780 was also observed to block ERK1/2 phosphorylation. We propose a novel mechanism in SN56 cells by which rapid effects of oestrogen leading to neuroprotection are signalled through Raf‐1/MEK/ERK1/2 pathway, possibly by activation of a membrane‐related ER.

Estrogen Activates Classical and Alternative Mechanisms to Orchestrate Neuroprotection

Evidence for a protective role of estradiol in neurodegenerative diseases has steadily increased over the past decade, though the mechanisms of action and the participation of true estrogen receptors (ERs) have proven a complex score. The protective effects of estrogens take place partly through pathways involving canonical ER activation, which is constitutively expressed in many brain regions and is able to initiate gene transcription after specifically binding to estradiol. In addition, non-genomic (or alternative) signalling pathways, involving extranuclear ERs, respond to physiological concentration of estrogens to elicit neuroprotection. Often, rapid activation of intracellular signallers such as mitogen-activated protein kinase (MAPK) or phosphatidylinositol 3-kinase (PI3K) underlie alternative estrogen-induced neuroprotection upon activation of specific binding sites at the plasma membrane. Although the molecular characteristics of these unconventional ERs are still largely unknown, the generally held view maintains that plasma membrane ER (mER) originates from, or is related to, classical nuclear ERs. The present article will review some of the most recent evidence revealing the relevance of alternative mechanisms in estrogen-dependent neuroprotection. Special emphasis will be paid to cellular models of amyloid-beta toxicity where classical and alternative pathways activated by estrogens seem to coexist to orchestrate neuroprotection.

GLUT4 and Glycogen Synthase Are Key Players in Bed Rest–Induced Insulin Resistance

To elucidate the molecular mechanisms behind physical inactivity–induced insulin resistance in skeletal muscle, 12 young, healthy male subjects completed 7 days of bed rest with vastus lateralis muscle biopsies obtained before and after. In six of the subjects, muscle biopsies were taken from both legs before and after a 3-h hyperinsulinemic euglycemic clamp performed 3 h after a 45-min, one-legged exercise. Blood samples were obtained from one femoral artery and both femoral veins before and during the clamp. Glucose infusion rate and leg glucose extraction during the clamp were lower after than before bed rest. This bed rest–induced insulin resistance occurred together with reduced muscle GLUT4, hexokinase II, protein kinase B/Akt1, and Akt2 protein level, and a tendency for reduced 3-hydroxyacyl-CoA dehydrogenase activity. The ability of insulin to phosphorylate Akt and activate glycogen synthase (GS) was reduced with normal GS site 3 but abnormal GS site 2+2a phosphorylation after bed rest. Exercise enhanced insulin-stimulated leg glucose extraction both before and after bed rest, which was accompanied by higher GS activity in the prior-exercised leg than the rested leg. The present findings demonstrate that physical inactivity–induced insulin resistance in muscle is associated with lower content/activity of key proteins in glucose transport/phosphorylation and storage.

Normal mitochondrial function and increased fat oxidation capacity in leg and arm muscles in obese humans

Aim/hypothesis:The aim of this study was to investigate mitochondrial function, fibre-type distribution and substrate oxidation during exercise in arm and leg muscles in male postobese (PO), obese (O) and age- and body mass index (BMI)-matched control (C) subjects. The hypothesis of the study was that fat oxidation during exercise might be differentially preserved in leg and arm muscles after weight loss.Methods:Indirect calorimetry was used to calculate fat and carbohydrate oxidation during both progressive arm-cranking and leg-cycling exercises. Muscle biopsy samples were obtained from musculus deltoideus (m. deltoideus) and m. vastus lateralis muscles. Fibre-type composition, enzyme activity and O2 flux capacity of saponin-permeabilized muscle fibres were measured, the latter by high-resolution respirometry.Results:During the graded exercise tests, peak fat oxidation during leg cycling and the relative workload at which it occurred (FatMax) were higher in PO and O than in C. During arm cranking, peak fat oxidation was higher in O than in C, and FatMax was higher in O than in PO and C. Similar fibre-type composition was found between groups. Plasma adiponectin was higher in PO than in C and O, and plasma leptin was higher in O than in PO and C.Conclusions:In O subjects, maximal fat oxidation during exercise and the eliciting relative exercise intensity are increased. This is associated with higher intramuscular triglyceride levels and higher resting non esterified fatty acid (NEFA) concentrations, but not with differences in fibre-type composition, mitochondrial function or muscle enzyme levels compared with Cs. In PO subjects, the changes in fat oxidation are preserved during leg, but not during arm, exercise.

Leptin receptor 170 kDa (OB-R170) protein expression is reduced in obese human skeletal muscle: a potential mechanism of leptin resistance

To examine whether obesity‐associated leptin resistance could be due to down‐regulation of leptin receptors (OB‐Rs) and/or up‐regulation of suppressor of cytokine signalling 3 (SOCS3) and protein tyrosine phosphatase 1B (PTP1B) in skeletal muscle, which blunt janus kinase 2‐dependent leptin signalling and signal transducer and activator of transcription 3 (STAT3) phosphorylation and reduce AMP‐activated protein kinase (AMPK) and acetyl‐coenzyme A carboxylase (ACC) phosphorylation. Deltoid and vastus lateralis muscle biopsies were obtained from 20 men: 10 non‐obese control subjects (mean ±s.d. age, 31 ± 5 years; height, 184 ± 9 cm; weight, 91 ± 13 kg; and percentage body fat, 24.8 ± 5.8%) and 10 obese (age, 30 ± 7 years; height, 184 ± 8 cm; weight, 115 ± 8 kg; and percentage body fat, 34.9 ± 5.1%). Skeletal muscle OB‐R170 (OB‐R long isoform) protein expression was 28 and 25% lower (both P < 0.05) in arm and leg muscles, respectively, of obese men compared with control subjects. In normal‐weight subjects, SOCS3 protein expression, and STAT3, AMPKα and ACCβ phosphorylation, were similar in the deltoid and vastus lateralis muscles. In obese subjects, the deltoid muscle had a greater amount of leptin receptors than the vastus lateralis, whilst SOCS3 protein expression was increased and basal STAT3, AMPKα and ACCβ phosphorylation levels were reduced in the vastus lateralis compared with the deltoid muscle (all P < 0.05). In summary, skeletal muscle leptin receptors and leptin signalling are reduced in obesity, particularly in the leg muscles.

Gender Dimorphism in Skeletal Muscle Leptin Receptors, Serum Leptin and Insulin Sensitivity

To determine if there is a gender dimorphism in the expression of leptin receptors (OB-R170, OB-R128 and OB-R98) and the protein suppressor of cytokine signaling 3 (SOCS3) in human skeletal muscle, the protein expression of OB-R, perilipin A, SOCS3 and alpha-tubulin was assessed by Western blot in muscle biopsies obtained from the m. vastus lateralis in thirty-four men (age = 27.1±6.8 yr) and thirty-three women (age = 26.7±6.7 yr). Basal serum insulin concentration and HOMA were similar in both genders. Serum leptin concentration was 3.4 times higher in women compared to men (P<0.05) and this difference remained significant after accounting for the differences in percentage of body fat or soluble leptin receptor. OB-R protein was 41% (OB-R170, P<0.05) and 163% (OB-R128, P<0.05) greater in women than men. There was no relationship between OB-R expression and the serum concentrations of leptin or 17β-estradiol. In men, muscle OB-R128 protein was inversely related to serum free testosterone. In women, OB-R98 and OB-R128 were inversely related to total serum testosterone concentration, and OB-R128 to serum free testosterone concentration. SOCS3 protein expression was similar in men and women and was not related to OB-R. In women, there was an inverse relationship between the logarithm of free testosterone and SCOS3 protein content in skeletal muscle (r = −0.46, P<0.05). In summary, there is a gender dimorphism in skeletal muscle leptin receptors expression, which can be partly explained by the influence of testosterone. SOCS3 expression in skeletal muscle is not up-regulated in women, despite very high serum leptin concentrations compared to men. The circulating form of the leptin receptor can not be used as a surrogate measure of the amount of leptin receptors expressed in skeletal muscles.

Increased oxidative stress and anaerobic energy release, but blunted Thr172-AMPKα phosphorylation, in response to sprint exercise in severe acute hypoxia in humans

AMP-activated protein kinase (AMPK) is a major mediator of the exercise response and a molecular target to improve insulin sensitivity. To determine if the anaerobic component of the exercise response, which is exaggerated when sprint is performed in severe acute hypoxia, influences sprint exercise-elicited Thr(172)-AMPKα phosphorylation, 10 volunteers performed a single 30-s sprint (Wingate test) in normoxia and in severe acute hypoxia (inspired Po(2): 75 mmHg). Vastus lateralis muscle biopsies were obtained before and immediately after 30 and 120 min postsprint. Mean power output and O(2) consumption were 6% and 37%, respectively, lower in hypoxia than in normoxia. O(2) deficit and muscle lactate accumulation were greater in hypoxia than in normoxia. Carbonylated skeletal muscle and plasma proteins were increased after the sprint in hypoxia. Thr(172)-AMPKα phosphorylation was increased by 3.1-fold 30 min after the sprint in normoxia. This effect was prevented by hypoxia. The NAD(+)-to-NADH.H(+) ratio was reduced (by 24-fold) after the sprints, with a greater reduction in hypoxia than in normoxia (P < 0.05), concomitant with 53% lower sirtuin 1 (SIRT1) protein levels after the sprint in hypoxia (P < 0.05). This could have led to lower liver kinase B1 (LKB1) activation by SIRT1 and, hence, blunted Thr(172)-AMPKα phosphorylation. Ser(485)-AMPKα(1)/Ser(491)-AMPKα(2) phosphorylation, a known negative regulating mechanism of Thr(172)-AMPKα phosphorylation, was increased by 60% immediately after the sprint in hypoxia, coincident with increased Thr(308)-Akt phosphorylation. Collectively, our results indicate that the signaling response to sprint exercise in human skeletal muscle is altered in severe acute hypoxia, which abrogated Thr(172)-AMPKα phosphorylation, likely due to lower LKB1 activation by SIRT1.

Repeated muscle biopsies through a single skin incision do not elicit muscle signaling, but IL-6 mRNA and STAT3 phosphorylation increase in injured muscle.

To determine if muscle biopsies can be repeated using a single small (5-6 mm) skin incision without inducing immediate MAPK activation or inflammation in the noninjured areas, the phosphorylation of ERK1/2, p38-MAPK, c-Jun NH(2)-terminal kinases (JNKs), IκBα, IKKα, and signal transducer and activator of transcription 3 (STAT3) was examined concurrent with IL-6 mRNA in six muscle biopsies obtained from the vastus lateralis of five men. Four biopsies were obtained through the same incision (5-6 mm) from the right leg (taken at 0, 30, 123, and 126 min) and another two each from new incisions performed in the left leg (at 31 and 120 min), while the subjects rested supine. The first three biopsies from the right leg were taken ∼3 cm apart from prebiopsied areas. The last biopsy was obtained from the same point from which the second biopsy was sampled. The three biopsies performed through the same skin incision from noninjured muscle areas showed similar levels of ERK1/2, p38-MAPK, JNK, IKKα, IκBα, and STAT3 phosphorylation and similar IL-6 mRNA content. There were no significant differences in the levels of ERK1/2, p38-MAPK, JNK, IKKα, and IκBα phosphorylation between the mean of the three biopsies obtained from the same incision and the sixth biopsy obtained from an injured area. STAT3 phosphorylation was increased by ∼3.5-fold in the sixth biopsy compared with the mean the three biopsies obtained from the same incision (P < 0.05), and IL-6 mRNA content was increased by 1.8-fold (P < 0.05). In summary, repeated muscle biopsies can be performed through a single 5- to 6-mm skin incision without eliciting muscle signaling through cascades responding to cellular stress, inflammation, or muscle damage. STAT3 phosphorylation is an early event in the healing response to muscle injury, probably mediated by the autocrine production of IL-6.

Critical role for free radicals on sprint exercise-induced CaMKII and AMPKα phosphorylation in human skeletal muscle.

The extremely high energy demand elicited by sprint exercise is satisfied by an increase in O2 consumption combined with a high glycolytic rate, leading to a marked lactate accumulation, increased AMP-to-ATP ratio, and reduced NAD(+)/NADH.H(+) and muscle pH, which are accompanied by marked Thr(172) AMP-activated protein kinase (AMPK)-α phosphorylation during the recovery period by a mechanism not fully understood. To determine the role played by reactive nitrogen and oxygen species (RNOS) on Thr(172)-AMPKα phosphorylation in response to cycling sprint exercise, nine voluntary participants performed a single 30-s sprint (Wingate test) on two occasions: one 2 h after the ingestion of placebo and another after the intake of antioxidants (α-lipoic acid, vitamin C, and vitamin E) in a double-blind design. Vastus lateralis muscle biopsies were obtained before, immediately postsprint, and 30 and 120 min postsprint. Performance and muscle metabolism were similar during both sprints. The NAD(+)-to-NADH.H(+) ratio was similarly reduced (84%) and the AMP-to-ATP ratio was similarly increased (×21-fold) immediately after the sprints. Thr(286) Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and Thr(172)-AMPKα phosphorylations were increased after the control sprint (with placebo) but not when the sprints were preceded by the ingestion of antioxidants. Ser(485)-AMPKα1/Ser(491)-AMPKα2 phosphorylation, a known inhibitory mechanism of Thr(172)-AMPKα phosphorylation, was increased only with antioxidant ingestion. In conclusion, RNOS play a crucial role in AMPK-mediated signaling after sprint exercise in human skeletal muscle. Antioxidant ingestion 2 h before sprint exercise abrogates the Thr(172)-AMPKα phosphorylation response observed after the ingestion of placebo by reducing CaMKII and increasing Ser(485)-AMPKα1/Ser(491)-AMPKα2 phosphorylation. Sprint performance, muscle metabolism, and AMP-to-ATP and NAD(+)-to-NADH.H(+) ratios are not affected by the acute ingestion of antioxidants.