IGF-I Receptor Messenger Research Articles

Up-regulation of pituitary [125I]insulin-like growth factor-I (IGF-I) binding and IGF binding protein-2 and IGF-I gene expression by estrogen.

To evaluate the effects of estrogen on the rat pituitary insulin-like growth factor (IGF) system, binding of [125I]IGF-I and in situ hybridization for IGF-I, the IGF-I receptor and IGF binding protein-2 (IGFBP-2) were coupled with quantitative autoradiography. The groups included intact cycling females, intact males, and gonadectomized males and females with or without estrogen pellet implants. Binding of [125I]IGF-I in the anterior lobe of the pituitary occurred in dense clusters over a diffuse lower density background. [125I]IGF-I binding was significantly increased in the estrogen-treated groups and was highest at proestrus compared to the rest of the estrous cycle. IGF-I receptor messenger RNA (mRNA) was distributed diffusely through the anterior pituitary and was not different between the respective gonadectomized and estrogen-treated groups. IGFBP-2 mRNA was clustered throughout the anterior pituitary and was significantly higher in the estrogen-treated groups as noted above for [125I]IGF-I binding. IGF-I mRNA was diffuse throughout the anterior pituitary and was also significantly higher in the estrogen-treated groups. In the neural lobe, [125I]IGF-I binding, IGFBP-2 mRNA, IGF-I receptor mRNA, and IGF-I mRNA were all uniformly distributed and did not differ between groups. The results show that circulating estrogen differentially regulates components of the pituitary IGF-I system in a region-specific manner and suggest that a portion of IGF binding in the anterior pituitary may be to IGFBP-2.

Up-regulation of pituitary [125I]insulin-like growth factor-I (IGF-I) binding and IGF binding protein-2 and IGF-I gene expression by estrogen

To evaluate the effects of estrogen on the rat pituitary insulin-like growth factor (IGF) system, binding of [125I]IGF-I and in situ hybridization for IGF-I, the IGF-I receptor and IGF binding protein-2 (IGFBP-2) were coupled with quantitative autoradiography. The groups included intact cycling females, intact males, and gonadectomized males and females with or without estrogen pellet implants. Binding of [125I]IGF-I in the anterior lobe of the pituitary occurred in dense clusters over a diffuse lower density background. [125I]IGF-I binding was significantly increased in the estrogen-treated groups and was highest at proestrus compared to the rest of the estrous cycle. IGF-I receptor messenger RNA (mRNA) was distributed diffusely through the anterior pituitary and was not different between the respective gonadectomized and estrogen-treated groups. IGFBP-2 mRNA was clustered throughout the anterior pituitary and was significantly higher in the estrogen-treated groups as noted above for [125I]IGF-I binding. IGF-I mRNA was diffuse throughout the anterior pituitary and was also significantly higher in the estrogen-treated groups. In the neural lobe, [125I]IGF-I binding, IGFBP-2 mRNA, IGF-I receptor mRNA, and IGF-I mRNA were all uniformly distributed and did not differ between groups. The results show that circulating estrogen differentially regulates components of the pituitary IGF-I system in a region-specific manner and suggest that a portion of IGF binding in the anterior pituitary may be to IGFBP-2.

Anatomical relationships in the patterns of insulin-like growth factor (IGF)-I, IGF binding protein-1, and IGF-I receptor gene expression in the rat kidney.

The rat kidney is both a target of circulating insulin-like growth factor-I (IGF-I) and a site of local IGF-I production. In order to identify which renal structures produce IGF-I and the functionally related IGF binding protein 1 (IGFBP-1), and which structures are potential sites of circulating or endogenous renal IGF action, we have employed in situ hybridization to localize IGF-I, IGFBP-1, and IGF-I receptor messenger RNAs (mRNAs) in the rat kidney. The effects of hypophysectomy (Hx) and GH replacement on renal IGF-I, IGFBP-1, and IGF-I receptor gene expression have also been evaluated. IGF-I and IGFBP-1 mRNAs are both localized in the epithelial cells of medullary thick ascending limbs (TALs) of Henle's loops in the normal rat kidney. IGF-I receptor mRNA is also abundant in TALs, but, in addition, is distributed throughout the distal nephron and collecting duct, and in the glomerulus, with lowest levels found in proximal tubules. Hx and GH treatment had complex effects on patterns of renal IGF-I and IGFBP-1 gene expression. In general, Hx resulted in decreased IGF-I and increased IGFBP-1 mRNA levels, and GH treatment produced the opposite effects, while IGF-I receptor mRNA levels were not significantly effected by either treatment. However, the most dramatic effect produced by the interruption of the pituitary-renal axis was the demonstration of reciprocal changes in IGF-I vs. IGFBP-1 gene expression in individual kidneys and even in individual nephrons, suggesting a local interaction between IGF-I and IGFBP-1 in the regulation of their respective mRNA levels. Functional implications issuing from these anatomical relationships in renal patterns of IGF-I, IGFBP-1, and IGF-I receptor gene expression are that IGF-I, if secreted into the tubular lumen, possibly carried or modulated by IGFBP-1, may act on luminal TAL and downstream receptor sites. The specific physiological role of IGF-I produced in TALs is open to speculation. Glomerular IGF-I receptor sites, based on their localization upstream and distant from local sources of IGF-I production, are predicted to be targets for circulating IGFs.

Cellular pattern of insulin-like growth factor-I (IGF-I) and IGF-I receptor gene expression in the developing and mature ovarian follicle.

We have employed in situ hybridization histochemistry to map the cellular pattern of insulin-like growth factor-I (IGF-I) and IGF-I receptor gene expression in developing rat ovaries from the time of birth through adulthood, and in response to hypophysectomy and gonadotropin replacement. From the early postnatal period, both IGF-I and IGF-I receptor messenger RNAs (mRNAs) were highly abundant and evenly distributed in granulosa cells of small, growing follicles. In large follicles, however, IGF-I gene expression was heterogeneous. IGF-I mRNA was most abundant in granulosa cells lining the antrum and surrounding the oocyte, but was low or undetectable in mural granulosa cells of Graafian follicles, and was also undetectable in luteinized granulosa cells of corpora lutea. IGF-I receptor mRNA was evenly distributed in developing and mature follicles and was highly abundant in the luteinized granulosa cells of corpora lutea. IGF-I receptor but not IGF-I mRNA was detected in growing oocytes. Hypophysectomy resulted in a decrease and treatment with PMSG resulted in an increase in follicular IGF-I receptor mRNA levels, whereas there was no change in IGF-I mRNA levels in the same protocol. In summary, high levels of both IGF-I and IGF-I receptor gene expression occur in the granulosa cells of actively growing follicles, suggesting that granulosa cell IGF-I may have a role in follicular or oocyte growth. IGF-I gene expression is lost concomitant with follicular enlargement and granulosa cell differentiation, whereas IGF-I receptor gene expression continues at high levels in luteinized granulosa cells, suggesting that IGF effects on differentiated granulosa cell function are due to circulating, not local, hormone. Finally, granulosa cell IGF-I receptor gene expression appears to be regulated by the gonadotropin present in pregnant mare serum.

Studies on Rat Luteal Cell Response to Insulin-Like Growth Factor I (IGF-I): Identification of a Specific Cell Membrane Receptor for IGF-I in the Luteinized Rat Ovary*

Insulin-like growth factor-I (IGF-I) has been shown to stimulate biosynthesis of progesterone and other differentiated functions of granulosa cells. The presence of the IGF-I receptor messenger RNA and IGF-I receptor as well as the role of IGF-I in the amplification of gonadotropin action in luteinized rat ovarian cells were assessed in the present study. Rat ovarian luteal tissue were obtained on day 6 of pseudopregnancy. After collagenase dispersion, luteal cells were cultured with or without IGF-I for 72 h in a serum-free medium in the absence or presence of human CG (100 ng/ml), 8-bromo-cAMP (1 mM), and 25-OH cholesterol (30 micrograms/ml). IGF-I alone increased (P less than 0.01) progesterone secretion 47% over controls. When combined with hCG the increase in progesterone secretion was enhanced 6-fold (P less than 0.001) over controls. The effects of 8 bromo-cAMP on progesterone secretion also was amplified (P less than 0.001) when IGF-I was present in the medium. When 25-OH cholesterol, a diffusable substrate for steroidogenesis, was included in the incubation medium along with IGF-I, progesterone secretion was enhanced significantly (P less than 0.01) over controls and either treatment alone. Binding studies performed with ovarian luteal cell membranes in the ovary revealed a dissociation constant of 0.94 nM for the IGF-I receptor. Autoradiograms from affinity labeling studies revealed a band corresponding to 132 Kd which is characteristic for the alpha-subunit of the type I IGF receptor. Northern blot analysis showed the presence of a major transcript at approximately 11 kilobases, which agrees with the size of the IGF-I receptor messenger RNA. In conclusion, we provide evidence for the fact that IGF-I regulates differentiated functions of the corpus luteum through interaction with specific high affinity IGF-I receptors.

Insulin-like growth factor I (IGF-I) receptors and IGF-I action in oligodendrocytes from rat brains

Oligodendrocyte progenitor cells were prepared by mechanical dissociation of 1-day-old rat brain cultures. These cells undergo proliferation and differentiation into oligodendrocytes as demonstrated by the expression of proliferation and differentiation-related specific antigens. We have used this unique culture system to characterize insulin-like growth factor I (IGF-I) receptors and their action in the central nervous system (CNS). 125I-IGF-I specifically binds to these cultures with high affinity. Competition-inhibition data suggest that IGF-I is most potent in competing for 125I-IGF-I binding, followed by IGF-II and insulin. Scatchard analyses of the binding data indicate a curvilinear plot with a K d for high affinity of 0.2 nM, and a B max of 247 fmol/mg, and a K d for low affinity of 3.2 nM and B max of 1213 fmol/mg protein. Covalent cross-linking followed by SDS-PAGE analysis demonstrated a radioactive band of M r 135,000 which corresponds to the α subunit of the IGF-I receptor. Solution hybridization/RNase protection assay produced a single protected band corresponding to IGF-I receptor messenger RNA, further confirming the presence of these receptors. Incubation of progenitor cells with IGF-I resulted in a time- and concentration-dependent increase in [ 3H]thymidine incorporation and cell numbers. This effect appears to be mediated by IGF-I receptors since IGF-II and insulin were proportionately less potent. In addition to its effect on proliferation, IGF-I also increased the number of 4E7- and GC-antigen positive cells. These observations indicate that oligodendrocytes in primary culture express specific IGF-I receptors and that the interaction of IGF-I with these receptors results in the proliferation as well as differentiation of oligodendrocytes.