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Expression, Action, and Steroidal Regulation of Insulin-Like Growth Factor-I (IGF-I) and IGF-I Receptor in the Rat Corpus Luteum: Their Differential Role in the Two Cell Populations Forming the Corpus Luteum^*

T. G. PARMER, C. T. ROBERTS, J.R., D. LEROITH, E. Y. ADASHI, I. KHAN, N. SOLAN, S. NELSON, M. ZILBERSTEIN and G. GIBORI

Department of Physiology and Biophysics, University of Illinois College of Medicine Chicago, Illinois 60612;
the Diabetes Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health Bethesda, Maryland 20892;
The Departments of Obstetrics/Gynecology and Physiology, University of Maryland School of Medicine Baltimore, Maryland 21201

Address all correspondence and requests for reprints to: Dr. Geula Gibori, Department of Physiology and Biophysics, University of Illinois, 835 South Wolcott, Chicago, Illinois 60612.

Abstract

The overall aim of this investigation was to examine the expression and steroidal regulation of insulin-like growth factor-I (IGF-I) and the IGF-I receptor in the rat corpus luteum and to examine the specificity of IGF-I action in the two luteal cell populations. We first examined whether the corpus luteum expresses the IGF-I and IGF-I receptor genes. Using a solution hybridization/RNase protection assay, IGF-I and IGFI receptor mRNAs were represented by protected bands 224 and 265 bases in length, respectively. In addition, Northern blot analysis showed that, as in liver, rat IGF-I and IGF-I receptor cDNAs hybridized with 7.5-, 1.8-, and 0.8- to 1.2-kilobase transcripts and an 11-kilobase transcript, respectively. Both IGF-I and IGF-I receptor mRNAs were detected on all days of pregnancy tested (days 5β21). Since the rat corpus luteum increases remarkably in size and steroidogenic capacity at midpregnancy due to estradiol stimulation, we determined whether these developmental changes are accompanied by an increased expression of the IGF-I and/or IGF-I receptor genes. Total RNA was isolated from corpora lutea of day 12 hypophysectomized-hysterectomized rats treated with or without estradiol for 3 days. Estradiol caused a clear and marked reduction in IGF-I and IGF-I receptor mRNA. [¹²⁵I]IGF-I bound with high specificity and affinity to luteal cell membranes. Large and small cell populations forming corpora lutea of day 3 and 14 pregnant rats were separated by elutriation and used for the determination of binding activity and for cell culture, respectively. IGF-I receptors were found to be localized principally in the large luteal cell population. The small luteal cells had approximately 6.5-fold less IGF-I-binding activity. The difference in binding activity in both cell populations was reflected in the ability of both cell types to respond to IGF-I. IGF-I (25 ng/ml) had a profound effect on the production of progesterone by the large luteal cells. No stimulatory effect of IGF-I on the small luteal cells was observed. Addition of estradiol (10 ng/ml) to the cell culture remarkably enhanced IGF-I stimulation of progesterone biosynthesis by the large luteal cells. In summary, the results of this investigation have revealed that the corpus luteum of the pregnant rat is a major site of expression of both the IGF-I and IGFI receptor genes. It is the large luteal cells forming the corpus luteum that contain the majority of IGF-I receptors and respond to IGF-I with an increase in steroidogenic output. These results also suggest that whereas IGF-I may play a role in luteal cell function, it does not appear to be responsible for the increase in size and steroidogenic capacity that occurs in the corpus luteum at midpregnancy and which is induced by estradiol. (Endocrinology 129: 2924β2932,1991)

Footnotes

^* This work was supported by NIH Grants HD-11119 and HD-12356 (to G.G.) and National Research Service Award HD-7336 (to T.G.P.).

Present address: Department of Obstetrics/Gynecology, Medical College of Georgia, Augusta, Georgia 30912.

Present address: Department of Pharmacology, University of Iowa College of Medicine, Iowa City, Iowa 52242.

NIH Merit Awardee (HD-11119).

Received June 18, 1991.

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