Synthesis and binding of insulin-like growth factor I by human glomerular mesangial cells.
Insulin-like growth factor I (IGF-I) has been found in the kidney, but its precise cellular localization is not known. Since there is evidence that IGF-I is an autocrine factor in many tissues and since murine mesangial cells have IGF-I receptors, we examined whether human mesangial cells produce IGF-I. Culture medium conditioned by mesangial cells was concentrated by reverse phase chromatography and applied to a Sephadex G-100 column equilibrated in a denaturing buffer. Two major species with apparent mol wt (MW) of 7,500 and 25,000 daltons were identified by IGF-I RIA. To determine whether the high MW species possessed IGF-I binding activity, appropriate fractions were desalted, incubated with [125I]Thr59-IGF-I for 2 h at 30 C, and applied to a Sephadex G-100 column equilibrated in a nondissociating buffer. The major peak of radioactivity was confined to a high MW region; there was no radioactivity in the fractions corresponding to 7,500 daltons. Further characterization of 7,500 dalton IGF-I immunoreactive species by reverse phase high performance liquid chromatography showed that it coeluted with synthetic human IGF-I. Isoelectric focusing revealed it to have a pI between 8.1 and 8.5, corresponding to the pI of human IGF-I of 8.25. Northern blot analyses of poly(A)+ RNA from human mesangial cells and human liver using a cDNA probe for human IGF-I showed that a 2.0-kilobase transcript predominated in the mesangial cells, whereas the liver contained 1.1- and 2.0-kilobase species. Specific binding of IGF-I to mesangial cells was demonstrated, and competition curves indicated a rank order of potency (IGF-I greater than IGF-II greater than insulin) consistent with type I IGF receptors. We conclude that human mesangial cells 1) express IGF-I mRNA transcripts, 2) secrete IGF-I and IGF-I-binding activity, and 3) possess specific IGF-I receptors. These data suggest that IGF-I may act as an autocrine or paracrine factor that regulates glomerular cell functions.
Aron
D C
DC
Department of Medicine, Veterans Administration Medical Center, Case Western Reserve University, Cleveland, Ohio 44106.
Rosenzweig
J L
JL
Abboud
H E
HE
D000818:Animals; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D005920:Glomerular Mesangium / Q000378:metabolism*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D007525:Isoelectric Focusing; D051379:Mice; D011956:Receptors, Cell Surface / Q000302:isolation & purification; D017451:Receptors, Somatomedin; D013002:Somatomedins / Q000096:biosynthesis*
D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I