LEPROT and LEPROTL1 cooperatively decrease hepatic growth hormone action in mice

Abstract

Growth hormone (GH) is a major metabolic regulator that functions by stimulating lipolysis, preventing protein catabolism, and decreasing insulin-dependent glucose disposal. Modulation of hepatic sensitivity to GH and the downstream effects on the GH/IGF1 axis are important events in the regulation of metabolism in response to variations in food availability. For example, during periods of reduced nutrient availability, the liver becomes resistant to GH actions. However, the mechanisms controlling hepatic GH resistance are currently unknown. Here, we investigated the role of 2 tetraspanning membrane proteins, leptin receptor overlapping transcript (LEPROT; also known as OB-RGRP) and LEPROT-like 1 (LEPROTL1), in controlling GH sensitivity. Transgenic mice expressing either human LEPROT or human LEPROTL1 displayed growth retardation, reduced plasma IGF1 levels, and impaired hepatic sensitivity to GH, as measured by STAT5 phosphorylation and Socs2 mRNA expression. These phenotypes were accentuated in transgenic mice expressing both proteins. Moreover, gene silencing of either endogenous Leprot or Leprotl1 in H4IIE hepatocytes increased GH signaling and enhanced cell-surface GH receptor. Importantly, we found that both LEPROT and LEPROTL1 expression were regulated in the mouse liver by physiologic and pathologic changes in glucose homeostasis. Together, these data provide evidence that LEPROT and LEPROTL1 influence liver GH signaling and that regulation of the genes encoding these proteins may constitute a molecular link between nutritional signals and GH actions on body growth and metabolism.

Increased Expression of Fibroblast Growth Factor 21 (FGF21) during Chronic Undernutrition Causes Growth Hormone Insensitivity in Chondrocytes by Inducing Leptin Receptor Overlapping Transcript (LEPROT) and Leptin Receptor Overlapping Transcript-like 1 (LEPROTL1) Expression

“…Silencing of LEPROT increases leptin receptor localization on the plasma membrane (20,22), whereas overexpression of LEPROT decreases cell-surface exposure of leptin receptor (20) and GH receptor (19). The effect of LEPROT on the intracellular trafficking of membrane receptors appear to be specific, as it has been shown that it does not affect EGF receptor trafficking (19,20,22). LEPROT and LEPROTL1 mRNAs are widely expressed in mammalian tissues and organs, including the liver.…”

“…At the end of the incubation, the cultures were washed 5 times with ice-cold PBS containing 0.2% BSA (PBS/BSA), detached with trypsin/ethylenediamine tetraacetate for cell counting, and solubilized in 0.5 mol/liter NaOH and 0.1% Triton X-100 for radioactivity measurement. To determine whether the effects of LEPROT and/or LEPROTL1 on GH binding were specific, we measured EGF binding, as it has been previously shown that the EGF receptor intracellular processing is not affected by LEPROT (19,20,22). Cultured chondrocytes were incubated with 10 5 cpm human 125 I-EGF in the presence or absence of 10 g of unlabeled EGF.…”

“…Previous evidence indicates that fasting-associated increased expression of LEPROT and LEPROTL1 (two transmembrane proteins involved in the regulation of intracellular protein trafficking) reduces GH receptor abundance on the hepatocyte cell membrane (19). Both proteins are also described as endospanin 1 and endospanin 2, respectively (20).…”

Endospanins Regulate a Postinternalization Step of the Leptin Receptor Endocytic Pathway

“…We now provide evidence that endospanin-1 functions by facilitating OB-R degradation. Therefore, we can hypothesize that an increase in endospanin-1 expression levels might be associated with leptin resistance, as it was recently suggested for GH resistance under conditions of fasting and type 1 diabetes mellitus (32). It would be of interest to determine if endospanin-1 is actually overexpressed in the arcuate nucleus of mice fed a high fat diet.…”

“…The silencing of endospanin-1 only resulted in the up-regulation of cell surface expression of leptin receptors OB-Ra and OB-Rb, among the few receptors that have been tested so far. Recently, it has been reported that both endospanins also regulate cell surface expression of the growth hormone receptor (GHR) (32). Although cellular mechanisms underlying this modulation of GHR expression were not investigated, similarities with OB-R can be found.…”