Activation of phosphorylation of plasma membrane insulin-like growth factor-I receptors in the kidney of Syrian hamsters by diethylstilbestrol.
In the present work we have investigated activation of phosphorylation of plasma membrane insulin-like growth factor-I receptors (IGF-IR) by diethylstilbestrol (DES). Insulin-like growth factor-I (IGF-I) stimulated the activity of membrane protein tyrosine kinase(s) (PTK) in both normal and DES-treated hamster kidneys. The level of IGF-I-stimulated PTK(s) almost doubled after 15 days of DES treatment. Autophosphorylation experiments revealed that phosphorylation of a 95 kDa band (presumably the beta subunit of IGF-IR) was 2-fold higher in the membranes of kidney from DES-treated animals compared with controls. To understand the mechanism of activation of IGF-I-dependent PTK by DES, we investigated the relationship between the binding capacity of IGF-I to membrane proteins and the level of IGF-IR. The binding of [125I]IGF-I to membranes from the DES-treated group was 30% higher than that of age-matched normal kidney (P < 0.001). Scatchard analysis of the binding data for both normal and DES-treated hamster kidney revealed a single class binding site for IGF-I with a dissociation constant (Kd) of 4.1 and 4.6 nM and a maximum binding capacity (Bmax) of 1786 and 2086 fmol/200 micrograms protein respectively. Therefore, the difference observed in [125I]IGF-I binding between DES-treated and normal kidney membranes may be partially due to an increase in the number of IGF-I binding sites, with no change in the affinity of the receptors for IGF-I. An enhanced level of IGF-IRs in membranes from DES-treated animals was visualized by autoradiography following affinity labeling of membrane proteins subjected to SDS-PAGE. Under reducing conditions a molecular band of 132 kDa was evident. The 132 kDa band represents the alpha-subunit of IGF-IRs. Northern blot analyses revealed that DES treatment increased the level of IGF-IR mRNA 2-fold compared with that of controls. These findings suggest that an enhanced level of IGF-IR coupled with qualitative changes may be responsible for the activation of IGF-I-dependent PTK on DES exposure. Whether the stimulation of IGF-IR phosphorylation by exposure to a carcinogenic dose of DES may be a factor in the induction of renal cancer in Syrian hamsters is not clear.
Chen
C W
CW
Department of Environmental Health Sciences, University of Alabama, Birmingham 35294-0008, USA.
Roy
D
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D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D006224:Cricetinae; D004054:Diethylstilbestrol / Q000494:pharmacology*; D004789:Enzyme Activation; D015870:Gene Expression; D007334:Insulin-Like Growth Factor I / Q000502:physiology*; D007668:Kidney / Q000378:metabolism*; D008297:Male; D008647:Mesocricetus; D010766:Phosphorylation; D012333:RNA, Messenger / Q000235:genetics; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism*; D017451:Receptors, Somatomedin / Q000378:metabolism*
D012333:RNA, Messenger; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I; D004054:Diethylstilbestrol; D020794:Receptor Protein-Tyrosine Kinases