Reduction of adiposity with prolonged growth hormone treatment in old obese rats: effects on glucose handling and early insulin signaling.
OBJECTIVE
Growth Hormone (GH) promotes loss of body fat and causes insulin resistance. It is debated whether reduction of body fat mass during long term growth hormone (GH) administration improves carbohydrate metabolism. To answer this question we assessed carbohydrate handling and tissue specific function of the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1) during prolonged GH treatment of obese rats.


METHODS
Body fat % estimated by DEXA scanning, plasma IGF-I, glucose and insulin were studied in 17 months old dietary induced obese rats treated for 4, 21 or 41 days (GH: 4 mg/kg/d or saline total n=90). Adipose tissue, muscle and liver samples were obtained after 21 days and expression and tyrosine phosphorylation of IR and IRS-1 proteins and the degree of IRS-1-Janus Kinase-2 (JAK2) interaction were analyzed by immunoprecipitation and immunoblotting.


RESULTS
Forty-one days GH treatment caused the body fat to decline significantly to 20+/-3% (Mean+/-SEM), whereas it remained steady on 51+/-4% in the pair fed group. Insulin levels in response to OGTT were significantly elevated throughout the experiment. IR amount was elevated in adipose tissue but decreased in liver after GH treatment while IR phosphorylation was increased in muscle only. IRS-1 amount was elevated in adipose tissue and muscle while IRS-1 phosphorylation was increased only in liver. The association of IRS-1 with JAK-2 was increased in liver and muscle.


CONCLUSIONS
An extensive reduction of fat mass did not improved signs of insulin resistance in GH treated old obese rats. The molecular events associated with GH treatment included tissue specific changes in the function of IR and IRS-1 suggesting the liver to be the primary site of insulin resistance. Furthermore, the association of IRS-1with JAK-2 in the course of GH signaling could present a mechanism for GH to directly induce insulin resistance.
Johansen
T
T
Department of Pharmacological Research, Discovery, Novo Nordisk, DK-2760 Måløv, Denmark. thuj@novonordisk.com
Laurino
C
C
Barreca
A
A
Malmlöf
K
K
D000273:Adipose Tissue / Q000378:metabolism*; D000375:Aging; D000818:Animals; D019540:Area Under Curve; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D005947:Glucose / Q000378:metabolism; D013006:Growth Hormone / Q000627:therapeutic use*; D006801:Humans; D047468:Immunoprecipitation; D007328:Insulin / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance; D053614:Janus Kinase 2; D050356:Lipid Metabolism; D008099:Liver / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D009765:Obesity / Q000188:drug therapy*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D013997:Time Factors; D014443:Tyrosine / Q000378:metabolism
C526292:IRS1 protein, human; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526296:Irs1 protein, rat; D010750:Phosphoproteins; D011518:Proto-Oncogene Proteins; D014443:Tyrosine; D012313:RNA; D013006:Growth Hormone; D011505:Protein-Tyrosine Kinases; C507924:JAK2 protein, human; C507927:Jak2 protein, rat; D053614:Janus Kinase 2; D005947:Glucose