Here is the Insulin Cheat Sheet I put together in September
The following was a nice little concise summation of the effect insulin has on GH, GHRs and intracellular events, put together about 6 months back. It was designed to be a pointer to the primary studies that demonstrate each point.
It might be of use to someone so I post it here today:
INSULIN CODEX - I
Many factors are known to regulate the responsiveness of the growth hormone receptor (GHR) to growth hormone (GH). The most important are insulin, thyroid [SEE: THYROID HORMONES CODEX] and sex hormones [SEE: ESTROGEN CODEX & TESTOSTEONE CODEX].
The growth-promoting action of GH is mediated by IGF-I which is produced mainly in the liver, but also in extrahepatic tissues. There is strong evidence that the anabolic action of GH requires the presence of insulin and adequate nutrition. This is exemplified in type 1 diabetes where IGF-I levels are low and longitudinal growth is impaired despite high serum levels of GH [1, 2]. These abnormalities are corrected by insulin treatment [3, 4].
Insulin's effect on GHR expression
The effects of insulin on GHR expression and function are tissue specific.
In cultures of rat hepatoma cells, insulin increases GHRs [5]. In animal studies, insulin deficiency results in a decrease of GH binding and GHR expression in liver [6, 7], which can be reversed by insulin administration [6, 8]. In extra-hepatic tissues such as bone and kidney, there is evidence that insulin down-regulates GHRs [9, 6–8].
It is well established that surface membrane receptors are dynamically regulated, with cell surface abundance representing the net balance of "recycling of internalised receptors" and translocation of newly synthesized receptors to the cell membrane.
There is recent evidence that the surface translocation of GH receptors is inhibited by insulin.
Insulin dose-dependently stimulates liver GHR synthesis and GH binding, however increasing insulin concentrations reduce GHR surface translocation, which overcomes the effect on receptor synthesis [5].
These findings show that the mechanism by which insulin regulates tissue responsiveness to GH is complex and in part mediated by effects on GHR expression and surface translocation.
Decrease in receptor surface availability with high dose insulin may represent rapid mechanism for insulin regulation of the GHR function.
In human studies, there is also evidence that insulin modulates the expression of GHRs. This is based on measurement of circulatory levels of GHBP. As GHBP is derived from proteolytic cleavage of the extracellular domain of the GH receptor, change in GHBP levels may reflect GH receptor status [10].
Thus when insulin levels are low, high levels of GH does not translate into a rise in circulating IGF-I [11-17]. In type I diabetes, GHBP levels are low and associates with low IGF-I levels [18]. These investigations have also observed a significant positive correlation between levels of GHBP and total insulin dose, suggesting that GHR status in humans is dependent on adequate insulinisation [18].
Insulin's effect on GH receptor signaling
There is strong evidence that insulin modulates GHR signaling in addition to the effects on receptor expression and surface translocation.
In rat hepatoma cells, low dose insulin administration results in GH-induced stimulation of JAK2 phosphorylation however high dose insulin treatment results in inhibitory effect [5, 19].
The effect of insulin on GHR function appears to be mediated by the PI-3 kinase and MAPK/ERK pathways [5, 20, 21]. It has been shown that insulin increases GH signaling by enhancing GH-induced activation of MAPK/ERK pathway through post signalling cross-talk [21].
In human muscle, in vivo, ERK1/2 phosphorylation was increased by insulin, but insulin per se did not induce phosphorylation of Stat5. [22]
Overall Summation
Insulin regulates GHR expression, translocation and GHR function. The regulation of GH receptor expression is complex and tissue dependent. Insulin stimulates hepatic GHR synthesis and GH binding but down-regulates GHR expression in kidney and bone tissue.
In liver, high concentrations of insulin reduce GHR surface translocation, in such a way as to regulate receptor surface availability.
The effects of insulin on GHR function are mediated by stimulation of GH-induced JAK2 phosphorylation, PI-3 kinase and MAPK/ERK pathways.
SOURCES:
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12 - Hartman ML, Veldhuis JD, Johnson ML, Lee MM, Alberti KG, Samojlik E, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. 1992;74:757–65.
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16 - Ohashi S, Kaji H, Abe H, Chihara K. Effect of fasting and growth hormone (GH) administration on GH receptor (GHR) messenger ribonucleic acid (mRNA) and GH-binding protein (GHBP) mRNA levels in male rats. Life Sci. 1995;57:1655–66.
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22 - Growth Hormone Signaling in Vivo in Human Muscle and Adipose Tissue: Impact of Insulin, Substrate Background, and Growth Hormone Receptor Blockade, Charlotte Nielsen, et al., The Journal of Clinical Endocrinology & Metabolism July 2008 Vol. 93, No. 7 2842-2850