Subcellular localization and internalization of the four human leptin receptor isoforms.
There are four known isoforms of the human leptin receptor (HLR) with different C-terminal cytoplasmic domains (designated by the number of unique C-terminal amino acids). In cells expressing HLR-5, -15, or -274, 15-25% of the leptin binding sites were located at the plasma membrane. In contrast, in cells expressing HLR-67, only 5% of the total binding sites were at the plasma membrane. Immunofluorescent microscopy showed that all four isoforms partially co-localized with calnexin and beta-COP, markers of the endoplasmic reticulum and the Golgi, respectively. All isoforms were also detected in an unidentified punctate compartment. All isoforms were internalized via clathrin-mediated endocytosis, but at different rates. After 20 min at 37 degrees C, 45% of a bound cohort of labeled ligand had been internalized by HLR-15, 30% by HLR-67, 25% by HLR-274, and 15% by HLR-5. Degradation of internalized leptin occurred in lysosomes. Overnight exposure to leptin down-regulated all isoforms, but to a variable extent. HLR-274 displayed the greatest down-regulation and also appeared to reach lysosomes more quickly than the other isoforms. The faster degradation of HLR-274 may help to terminate leptin signaling.
Barr
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Diabetes Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA.
Lane
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Taylor
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D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D006801:Humans; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D011956:Receptors, Cell Surface; D054411:Receptors, Leptin; D015398:Signal Transduction
D002352:Carrier Proteins; C515448:LEPR protein, human; D020033:Protein Isoforms; D011956:Receptors, Cell Surface; D054411:Receptors, Leptin