Exercise Biochemistry Laboratory
Overview of Recent Investigations
We have found that a high fat diet reduces insulin-stimulated glucose uptake and 3-O-methylglucose (3-MG) transport in rodent skeletal muscle and that a portion of the defect can be attributed to a reduced skeletal muscle GLUT4 protein concentration and decreased insulin-stimulated GLUT4 translocation to the plasma membrane. However, skeletal muscle insulin resistance is not always associated with deficiencies in GLUT4 protein concentration. Humans with non-insulin dependent diabetes mellitus and some animal models of genetic obesity possess a normal skeletal muscle GLUT4 protein concentration, but exhibit reduced insulin-stimulated translocation of glucose transporters to the plasma membrane. While we have observed that skeletal muscle insulin resistance appears to be related to decreased insulin-stimulated IRS-1-associated PI3-K activity, this may not be the only component of the insulin signaling cascade that is impaired. We have also evaluated if resistance training exercise reversed skeletal muscle insulin resistance by altering components of the insulin-signaling cascade. The abstracts from our most recent work are presented below:
Singh, M.K., A.D. Krisan,
A.M. Crain, D.E. Collins and B.B. Yaspelkis III. High-fat diet and leptin treatment
alter skeletal muscle insulin-stimulated phosphatidylinositol 3-kinase activity
and glucose transport. Metabolism 52 (9): 1196-1205, 2003.
The aim of this investigation was to evaluate if leptin treatment enhances insulin-stimulated
glucose transport in normal (Experimental Group 1, EXP-1) and insulin-resistant
skeletal muscle (Experimental Group 2, EXP-2) by altering components of the
insulin-signaling cascade and/or glucose transport pathway. In EXP-1, Sprague
Dawley rats were assigned to control-chow fed (CON-CF) or leptin treated-chow
fed (LEP-CF) groups. Animals were implanted with mini-osmotic pumps, which delivered
0.5 mg leptin/kg/d to the LEP-CF animals and vehicle to CON-CF animals for 14
days. For EXP-2, Sprague-Dawley rats consumed normal (CON) or high fat diets
for three months. After the dietary lead in, the high fat diet group was further
subdivided into high fat (HF) and high fat, leptin treated (HF-LEP) animals.
HF-LEP animals were injected with leptin (0.5 mg leptin/kg/d) for 12 days, while
the CON and HF animals were injected with vehicle. Following the treatment periods,
all animals were prepared for and subjected to hind-limb perfusion. In EXP-1,
leptin treatment increased insulin-stimulated skeletal muscle GLUT4 translocation,
which appeared to be due to increased PI3-Kinase activation and Akt phosphorylation.
In EXP-2, the high-fat diet reduced insulin-stimulated glucose transport, in
part, by impairing insulin-stimulated PI3-Kinase activation and glucose transporter
translocation. Leptin treatment reversed high-fat-diet-induced insulin resistance
in skeletal muscle by restoring IRS-1-associated PI3-Kinase activity, total
GLUT4 protein concentration and glucose transporter translocation. Collectively,
these findings suggest that leptin treatment will enhance components of both
the insulin-signaling cascade and glucose transport effector system in normal
and insulin-resistant skeletal muscle.
Yaspelkis III, B.B., M. K. Singh, A. D. Krisan, D.E. Collins, C.C. Kwong
and A.M. Crain. Chronic leptin treatment enhances insulin-stimulated glucose
disposal in skeletal muscle of high-fat fed rats. Life Sciences 74: 1801-1816,
2004.
The aim of this investigation was to evaluate if chronic leptin administration
corrects high fat diet-induced skeletal muscle insulin resistance, in part,
by enhancing rates of glucose disposal and if the improvements are accounted
for by alterations in components of the insulin-signaling cascade. Sprague-Dawley
rats consumed normal (CON) or high fat diets for three months. After the dietary
lead in, the high fat diet group was further subdivided into high fat (HF) and
high fat, leptin treated (HF-LEP) animals. HF-LEP animals were injected twice
daily with leptin (5 mg/100 g body weight) for 10 days, while the CON and HF
animals were injected with vehicle. Following the treatment periods, all animals
were prepared for and subjected to hind limb perfusion. The high fat diet decreased
rates of insulin-stimulated skeletal muscle glucose uptake and glycogen synthesis
in the red gastrocnemius (RG), but did not affect glycogen synthase activity,
rates of glucose oxidation or nonoxidative disposal of glucose. Of interest,
IRS-1-associated PI3-kinase activity and total GLUT4 protein concentration were
reduced in the RG of the high fat-fed animals. Leptin treatment increased rates
of insulin-stimulated glucose uptake and glucose oxidation, and normalized rates
of glycogen synthesis. Leptin appeared to mediate these effects by normalizing
insulin-stimulated PI3-kinase activation and GLUT4 protein concentration in
the RG. Collectively, these data suggest that chronic leptin treatment reverses
the effects of a high fat diet thereby allowing the insulin signaling cascade
and glucose transport effector system to be fully activated which in turn affects
the amount of glucose that is transported across the plasma membrane and made
available for glycogen synthesis.
Krisan, A.D., D.E. Collins,
A.M. Crain, C.C. Kwong, M. K. Singh, J.R. Bernard and B.B. Yaspelkis III. Resistance
training enhances components of the insulin-signaling cascade in normal and
insulin-resistant skeletal muscle. J. Appl. Physiol. (January 5, 2004).
10.1152/japplphysiol.01054.2003.
We recently reported that chronic resistance training (RT) improved insulin-stimulated glucose transport in normal rodent skeletal muscle, in part, to an increased GLUT4 protein concentration (Acta Physiol. Scand. 175: 315-23, 2002). However, it remained to be determined if these improvements resulted from alterations in the insulin-signaling cascade as well. In addition, the possibility existed that RT might also improve skeletal muscle insulin resistance. Thirty two male Sprague Dawley rats were assigned to one of four groups: control diet, sedentary (CON-Sed); control diet, resistance trained (CON-RT); high-fat diet, sedentary (HF-Sed); or high-fat diet, resistance trained (HF-RT). Animals consumed their respective diets for 9 wk and then RT animals performed 12 wk of training (3 sets, 10 repetitions at 75% 1-RM, 3X/wk). All animals remained on their dietary treatments over the 12 wk period. Following the training period, animals were subjected to hind limb perfusions. Insulin-stimulated IRS-1 associated PI-3 kinase activity was enhanced in the red gastrocnemius (RG) and quadriceps (RQ) of the CON-RT and HF-RT animals. aPKC-z/l and Akt activities were reduced in the HF-Sed and normalized in the HF-RT animals. RT increased GLUT4 protein concentration in the RG and RQ of the CON-RT and HF-RT animals. No differences were observed in the total protein concentrations of IRS-1, Akt, aPKC-z/l, or phosphorylation of Akt. Collectively, these findings suggest that RT increases insulin-stimulated carbohydrate metabolism in normal skeletal muscle, and reverses high-fat diet-induced skeletal muscle insulin resistance by altering components of both the insulin-signaling cascade and glucose transporter effector system.
Significance of Investigations
The alterations in the leptin treated muscle resulted in the GLUT4 protein being more effectively translocated to the plasma membrane in response to insulin stimulation thereby normalizing rates of glucose transport. The results of these investigations provide insight to the mechanisms which contribute to the development of skeletal muscle insulin resistance and potential treatment avenues that might be persued. Chronic resistance training improved high-fat diet induced skeletal muscle insulin resistance in rodents. The improvements appear to be the product of increased rates of insulin-stimulated glucose uptake and transport, which are mediated by an increased GLUT4 protein concentration and enhanced insulin signaling. These results suggest that resistance training can be used as an effective treatment modality for insulin resistant individuals.
Planned Investigations
We plan to perform a comprehensive evaluation of the insulin signaling cascade and glucose transport effector system to detail possible mechanism(s) by which chronic consumption of a high fat diet induces skeletal muscle insulin resistance. The proximal insulin signaling cascade will be thoroughly evaluated paying particular attention to IRS-1 and the functionality of this protein during insulin stimulation. The activation of downstream components of the insulin signaling cascade will be determined using kinase activity assays including isoform specific activation of Akt. Additionally, components of the TNFa/NF-kb/Ikba pathway will be evaluated to determine if chronic high fat feeding induces alterations that may contribute to impairment of the classical insulin signaling pathway.