Effect of Age on Glucose and Lipid Metabolism

Aging is a major risk factor for the development of type 2 diabetes (T2DM). Approximately
50% of subjects aged ≥65 have diabetes or impaired glucose tolerance, a pre-diabetic state.
Skeletal muscle is the main site of insulin-stimulated glucose disposal and aging is
characterized by muscle insulin resistance. It has been suggested that the insulin
resistance of aging results from an age-related accumulation of intramyocellular lipids
which impair insulin action. However, the molecular basis for the accumulation of
intramyocellular fat and insulin resistance in the elderly remains unknown. AMP-activated
protein kinase (AMPK) is an energy-sensing enzyme whose activation results in increased
fatty acid oxidation. Purpose: In the proposed study, we will test the hypotheses that the
decrease in fat oxidation that occurs in muscle from older human subjects is secondary to an
age-mediated reduction in AMPK signaling, in vivo, and that upregulating AMPK signaling
through exercise training will result in (and correlate with) increased fat oxidation,
reduced intramyocellular lipids, and improved insulin action. Using a primary human muscle
cell culture system, also we will test that hypotheses that reduced AMPK signaling in old
myotubes leads to lower fat oxidation (in vitro) and that chemical activation of AMPK in old
myotubes to the same level as young muscle cells will restore insulin action and help
prevent fat-induced insulin resistance. To test these hypotheses the following specific aims
(objectives) are proposed:

Specific Aim 1) To determine whether reduced AMPK signaling in muscle from older subjects,
in vivo, is associated with lower fat oxidation rates and insulin resistance, and whether
physical activity improves glucose homeostasis in older subjects by upregulating AMPK
signaling in muscle. We will test the hypotheses that (i) reductions in AMPK signaling in
muscle from older subjects will be associated with (predict) lower fat oxidation rates and
insulin resistance, in vivo; and (ii) training-induced increases in AMPK signaling in older
subjects will be associated with (predict) increases in fat oxidation, reductions in
intramyocellular lipids, and improvements in insulin action/sensitivity.

Specific Aim 2) To determine whether age-related declines in AMPK signaling are involved in
the reductions in fat oxidation and insulin resistance that occur in aging. Using an in
vitro primary muscle cell culture system, we will test the hypotheses that (i) reduced AMPK
signaling in myotubes from older subjects leads to decreased mitochondrial fatty acid
oxidation; and (ii) reduced AMPK signaling and fat oxidation in myotubes from older subjects
will result in increased susceptibility to fat-induced insulin resistance.

Specific Aim 3) To examine whether the age-related reductions in fat oxidation and insulin
sensitivity in old muscle cells can be reversed by upregulating AMPK signaling. We will test
the hypothesis that chemical activation of AMPK in old myotubes (in vitro) to the same level
as young muscle cells will restore insulin signaling and help prevent fat-induced insulin
resistance.

Inclusion Criteria:

1. healthy, normally active, younger (18-30 y), normal glucose tolerant subjects,
without a family history of T2DM (neither parent nor siblings), and BMI of 23-26
kg/m2.

2. healthy, normally active, older (≥65 y), normal glucose tolerant subjects without a
family history of T2DM, and BMI of 23-26 kg/m2.

3. Women must be non-lactating. Female patients are eligible only if they have a
negative pregnancy test throughout the study period (or postmenopausal).
Postmenopausal women taking hormone replacement will be included if they have been on
a stable dose for ≥6 months. In younger menstruating woman, all metabolic studies
will be performed on the follicular phase of the menstrual cycle.

4. Subjects must have the following laboratory values: Hematocrit ≥ 35%, serum
creatinine ≤ 1.5 mg/dl, AST < 2 X upper limit of normal, ALT < 2 X upper limit of
normal, alkaline phosphatase < 2 X upper limit of normal, normal urinalysis, and
normal platelets, PT and PTT.

Exclusion Criteria:

1. Subjects with diabetes or impaired glucose tolerance based on ADA criteria.

2. Subjects taking drugs known to affect glucose and lipid homeostasis will be excluded.

3. Patients with a history of heart disease (New York Heart Classification greater than
grade II; more than non-specific ST-T wave changes on the ECG), peripheral vascular
disease, or pulmonary disease.

4. Recent pulmonary embolus, poorly controlled blood pressure (systolic BP>170,
diastolic BP>95), resting heart rate >100, electrolyte abnormalities, neuromuscular
or musculoskeletal disease.

5. Subjects who smoke.