Abstract
Growth hormone (GH) plays a pivotal role in growth and metabolism, with growth promotion mostly attributed to generation of insulin-like growth factor I (IGF-I) in liver or at local sites of GH action, whereas the metabolic effects of GH are considered to be intrinsic to GH itself. To distinguish the effects of GH from those of IGF-I, we developed a Cre-lox-mediated model of tissue-specific deletion of the growth hormone receptor (GHR). Near total deletion of the GHR in liver (GHRLD) had no effect on total body or bone linear growth despite a >90% suppression of circulating IGF-I; however, total bone density was significantly reduced. Circulating GH was increased 4-fold, and GHRLD displayed insulin resistance, glucose intolerance, and increased circulating free fatty acids. Livers displayed marked steatosis, the result of increased triglyceride synthesis and decreased efflux; reconstitution of hepatic GHR signaling via adenoviral expression of GHR restored triglyceride output to normal, whereas IGF-I infusion did not correct steatosis despite restoration of circulating GH to normal. Thus, with near total absence of circulating IGF-I, GH action at the growth plate, directly and via locally generated IGF-I, can regulate bone growth, but at the expense of diabetogenic, lipolytic, and hepatosteatotic consequences. Our results indicate that IGF-I is essential for bone mineral density, whereas hepatic GH signaling is essential to regulate intrahepatic lipid metabolism. We propose that circulating IGF-I serves to amplify the growth-promoting effects of GH, while simultaneously dampening the catabolic effects of GH.
Highlights
The metabolic effects of Growth hormone (GH) include antagonism to insulin action, thereby predisposing to diabetes [2,3,4, 14], promotion of lipolysis [2], and enhancement of protein synthesis with factor I; rhIGF-I, recombinant human insulin-like growth factor I (IGF-I); ALS, acid labile subunit; Stat, signal transducers and activators of transcription; micro-CT, micro-computed tomography; ELISA, enzyme-linked immunosorbent assay; ANOVA, analysis of variance
Global deletion of Ghr obtained after crossing Ghrfl/fl to E2a-Cre mice resulted in progeny with severe postnatal growth failure, consistent with the phenotype of Laron mice [19] (Fig. 1, C and D), documenting that Cre-mediated removal of exon 4 of the Ghr gene is sufficient to abolish growth hormone receptor (GHR) action
By demonstrating virtual total suppression of circulating IGF-I (ϳ95%), our study leads to the conclusion that circulating growth hormone alone is sufficient to promote normal growth, providing that GHR, along with its intact post-receptor signaling cascade, is expressed in the remaining tissues outside of the liver, at the growth plate [23]
Summary
Mice—Ghr exon 4 floxed mice were generated through standard gene-targeting methods as described [16]. Blood insulin levels were measured with the Mercodia ultrasensitive mouse insulin ELISA kit (Mercodia) following the manufacturer’s protocol. Growth hormone levels were measured with the mouse growth hormone ELISA kit (Diagnostic Systems Laboratories, Inc.) following the manufacturer’s protocol. To analyze the growth plate, tibia bones were harvested from 12-week-old male mice and fixed in 10% formalin followed by decalcification in 100 mM EDTA for 10 days. Adenovirus-mediated GHR Reconstitution and IGF-I Infusion—Adenoviruses expressing either full-length mouse Ghr or full-length mouse LacZ were administrated to 12-week-old female Ghr liver-specific deletion (GHRLD) mice through the tail vein (109 infectious particles/mouse). For the IGF-I infusion experiment, osmotic minipumps (DURECT Co., Cupertino, CA) loaded with either rhIGF-I or placebo (Tercica, Inc., Brisbane, CA) were implanted subcutaneously into the mid-back region of 16-week-old male GHRLD mice. Statistical Analysis—In all experiments, unless specified, we used two-tail Student’s t tests to assess statistical significance. p value Ͻ 0.05 was considered as significant
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