Human chorionic gonadotropin decreases insulin-like growth factor-I gene transcription in rat Leydig cells

Insulin-like growth factors and their binding proteins: Potential relevance to reproductive physiology

Cyclic ovarian follicular development is a complex process that involves proliferation, differentiation, and death of follicle cells. Gonadotropins produced by the pituitary gland have a central role in the regulation of these processes. In addition, a wide range of paracrine and autocrine factors produced in the reproductive organs have been proposed as regulators of reproductive functions. Components of the insulin-like growth factors (IGF) system are widely expressed in the female reproductive tract. The IGFs and their binding proteins play a significant role in several processes of reproductive physiology, including ovarian follicular development, oogenesis and oocyte maturation, ovulation, luteal function, follicular atresia, and testicular function. The majority of these physiological actions of the IGFs are believed to occur via activation of the IGF-I receptor, although the IGF-I effects are modulated by IGF binding proteins (IGFBPs). As much of the data obtained to date have been in the rodent reproductive organs, it may not be possible to directly extrapolate the results to the primate organs. There is a distinct species-difference in the gene expression and functional roles of the IGF-IGFBP system in reproductive organs. However, the disturbance of the IGF-IGFBP system in human reproductive physiology may lead to anovulation, disorders of androgen excess, infertility associated with implantation failure, and male infertility. Further research is needed in domestic animals to determine if manipulation of the IGF-IGFBP system may result in improved reproductive efficiency. As our understanding of the IGF-IGFBP system increases, the uses of human recombinant IGF peptides and IGFBPs as clinical therapy for disease states is becoming a reality. (Reprod Med Biol 2003; 2: 1-24).

Stability of alternatively spliced IGF-I mRNA in growth plate chondrocytes

Insulin-like growth factor-I (IGF-I) enhances the proliferation and hypertrophy of growth plate chondrocytes leading to increased growth plate width thus promoting bone elongation. Differing quantities of the multiple IGF-I transcripts within the growth plate suggest differential regulation of IGF-I. To assess this, the relative stabilities of the 1Ea, 1Eb, and 2Ea IGF-I mRNA classes in 2-6 week old rat growth plates were evaluated. The mean estimated half-life of class 1Ea was 3.7+/-0.05 h, while classes 1Eb and 2Ea decayed gradually over the course of the experiment. Estimated half-lives for each IGF-I mRNA species were unchanged at all ages examined. Incubation with Act D enhanced the transcription of class 1Ea, 1Eb, and 2Ea mRNAs to varying degrees. This implies that the differential stability of alternative IGF-I mRNA classes may be an inherent regulatory component that is not influenced by an animal's developmental state, and the differential abundance of alternative IGF-I mRNA species in the growth plate throughout development is due to transcriptional differences.

Regulation of luteinizing hormone receptor gene expression by insulin-like growth factor-I in an immortalized murine Leydig tumor cell line (BLT-1)

It is postulated that insulin-like growth factor-I (IGF-I), a 70-amino acid mitogenic polypeptide, regulates Leydig cell steroidogenesis. In the present study, we assessed the effect of IGF-I on LH receptor (LHR) gene expression in an immortalized murine Leydig tumor cell line (BLT-1). Culture of BLT-1 cells in the presence of IGF-I (0.1-100 ng/ml) for 24 or 48 h increased their [125I]iodo-hCG binding in a dose-dependent manner up to 275% of the control level. Northern hybridization analysis revealed four major transcripts of LHR mRNA in BLT-1 cells (6.9, 2.6, 1.7, and 1.2 kilobases), and treatment at 10-100 ng/ml of IGF-I increased steady-state levels of LHR mRNAs in coordinate fashion up to 2. 2-fold. IGF-I (30 ng/ml) induced a time-dependent increase in [125I]hCG binding after a lag period of 2-6 h when studied up to 48 h, with a subsequent decrease. A similar response with steady increase up to 72 h was observed in total LHR mRNA. To elucidate the molecular mechanism of IGF-I action on LHR mRNA expression, we measured the transcription rate of the LHR gene by nuclear run-off assay and assessed transcript stability by the actinomycin D blocking method. The results showed that IGF-I treatment had no effect on the transcription rate of the LHR gene, whereas the half-life (t1/2) of LHR mRNA was significantly prolonged (IGF-I-treated cells, 30 +/- 3.8 h; controls, 17 +/- 2.5 h). Furthermore, IGF-I at 30 ng/ml and 100 ng/ml increased the expression of LHR promoter-driven luciferase and cytomegalovirus-promoter driven ss-galactosidase activities in BLT-1 cells; however, the former increased only marginally more than the latter. This suggests that the increase of LHR mRNA by IGF-I in Leydig cells is mainly due to increased mRNA stability.

Expression and effect of insulin-like growth factor I on rat fetal Leydig cell function and differentiation

Insulin like growth factor I (IGF-I) is believed to be a potent para/autocrine stimulator of Leydig cell function in adult testis. We investigated whether IGF-I is also an intratesticular regulator of fetal Leydig cell function by measuring its production in the fetal testis and its ability to affect testicular steroidogenesis during fetal development. Northern blot analysis revealed one major IGF-I transcript of 7-7.5 kb and two minor transcripts of 3.8 and 1.8 kb in 20.5 day fetal testis. IGF-I was detected by RIA in 16.5 fetal day testes, and the amounts of IGF-I secreted by 16.5 and 20.5 fetal day testes in vitro were much greater than the amounts contained in the testes, indicating active synthesis in culture. The secretion of IGF-I by the fetal testis in vitro was increased with testicular age and time in culture. It was not modified by gonadotropins or (Bu)2cAMP. Testosterone secretion by fetal testes explanted 13.5, 16.5, 18.5, and 20.5 days after conception and cultured in the presence or absence of 100 ng/ml LH for 3 days was not affected by the addition of 50 ng/ml IGF-I to the medium. In contrast, the addition of IGF-I to dispersed fetal testicular cells cultured for 3 days in the presence or absence of LH increased the number of Leydig cells identified by a positive cytochemical reaction for 3beta-hydroxysteroid dehydrogenase (3betaHSD). This was more pronounced with cells from 16.5- day-old fetuses (stage when the fetal Leydig cells are differentiating in vivo) than with 20.5-day-old fetuses cells (stage when the number and the function of fetal Leydig cells are stable or decreasing). It results from both an increased differentiation of mesenchymal cells in fetal Leydig cells and an increase in the mitotic index of the fetal Leydig cells, as inferred from the small increase in the percentage of bromodeoxyuridine/3betaHSD-positive cells. Both LH and IGF-I increased significantly testosterone production by day 16.5 cells. In the presence of LH, a high amount of testosterone was produced per 3betaHSD-positive cell; IGF-I further increased this production. This effect was not observed with day 20.5 cells. The amounts of testosterone produced per 3betaHSD-positive cell cultured in the presence of both LH and IGF-I were more than additive. Like IGF-I, insulin (50 ng/ml) increased testosterone secretion per 3betaHSD-positive cells in cultures of day 16.5 cells, but not in those of day 20.5, cells. Lastly, IGF-I also increased the steroidogenic activity of each Leydig cell in cultures containing (Bu)2cAMP, but its effects were weaker than those observed in the presence of LH. This suggests that IGF-I has sites of action both upstream and downstream cAMP generation. These results suggest that IGF-I acts as paracrine/autocrine factor in the differentiation and activity of fetal Leydig cells.

CCAAT/enhancer-binding protein delta activates insulin-like growth factor-I gene transcription in osteoblasts. Identification of a novel cyclic AMP signaling pathway in bone

Insulin-like growth factor-I (IGF-I) plays a key role in skeletal growth by stimulating bone cell replication and differentiation. We previously showed that prostaglandin E2 (PGE2) and other cAMP-activating agents enhanced IGF-I gene transcription in cultured primary rat osteoblasts through promoter 1, the major IGF-I promoter, and identified a short segment of the promoter, termed HS3D, that was essential for hormonal regulation of IGF-I gene expression. We now demonstrate that CCAAT/enhancer-binding protein (C/EBP) delta is a major component of a PGE2-stimulated DNA-protein complex involving HS3D and find that C/EBPdelta transactivates IGF-I promoter 1 through this site. Competition gel shift studies first indicated that a core C/EBP half-site (GCAAT) was required for binding of a labeled HS3D oligomer to osteoblast nuclear proteins. Southwestern blotting and UV-cross-linking studies showed that the HS3D probe recognized a approximately 35-kDa nuclear protein, and antibody supershift assays indicated that C/EBPdelta comprised most of the PGE2-activated gel-shifted complex. C/EBPdelta was detected by Western immunoblotting in osteoblast nuclear extracts after treatment of cells with PGE2. An HS3D oligonucleotide competed effectively with a high affinity C/EBP site from the rat albumin gene for binding to osteoblast nuclear proteins. Co-transfection of osteoblast cell cultures with a C/EBPdelta expression plasmid enhanced basal and PGE2-activated IGF-I promoter 1-luciferase activity but did not stimulate a reporter gene lacking an HS3D site. By contrast, an expression plasmid for the related protein, C/EBPbeta, did not alter basal IGF-I gene activity but did increase the response to PGE2. In osteoblasts and in COS-7 cells, C/EBPdelta, but not C/EBPbeta, transactivated a reporter gene containing four tandem copies of HS3D fused to a minimal promoter; neither transcription factor stimulated a gene with four copies of an HS3D mutant that was unable to bind osteoblast nuclear proteins. These results identify C/EBPdelta as a hormonally activated inducer of IGF-I gene transcription in osteoblasts and show that the HS3D element within IGF-I promoter 1 is a high affinity binding site for this protein.

Immunohistochemical pattern of insulin-like growth factor (IGF) I, IGF II and IGF binding proteins 1 to 6 in carcinoma in situ of the testis

AIM

To study the immunohistochemical localisation of insulin-like growth factor (IGF) I, IGF II, and IGF binding proteins 1-6 in intratubular germ cell neoplasia in the vicinity of solid germ cell tumours of the testis.

METHODS

Testes were obtained from 13 patients (20-35 years old) who had undergone orchidectomy for treatment of a solid germ cell tumour. Tumour cells were verified histologically by their distinctive morphology and by visualisation of placental alkaline phosphatase immunoreactivity.

RESULTS

The majority of carcinoma in situ (CIS) cells were immunopositive for IGF I, whereas no CIS cells stained for IGF II. Of all the IGF binding proteins investigated, CIS cells showed intense immunoreactivity for IGF binding protein 5 and lower expression of all other IGF binding proteins.

CONCLUSIONS

These results suggest that the action of IGF binding protein 5 in CIS cells may modulate the activity of IGF I. This may be related to a proliferative advantage that could facilitate tumour development.

Insulin-like growth factor (IGF-I) mRNA and IGF-I receptor in trout testis and in isolated spermatogenic and Sertoli cells

Few data exist concerning the occurrence and potential role of an insulin-like growth factor (IGF) system in fish gonads. Using Northern and slot blot hybridization with a specific salmon IGF-I cDNA, we confirmed that IGF-I transcription occurs in trout testis. Testicular IGF-I mRNA abundance may be increased by long-term GH treatment in juvenile fish, while shorter treatment with growth hormone (GH) or a gonadotropin (GTH-II) in maturing males had no statistically significant effect. Radiolabelled recombinant human IGF-I binds with high affinity to crude trout testis preparation, to cultured isolated testicular cells, and to a membrane fraction of these cells (Ka = 0.2 to 0.7 x 10(10) M-1; Bmax = 10 to 20 fmol/10(7) cells, and 68 fmol/mg protein of membrane). The binding site was identified as type 1 IGF receptor by its binding specificity (IGF-I > IGF-II >>> insulin) and the molecular size of its alpha-subunit labelled with 125I-IGF-I (M(r)125-140 kDa). 125I-IGF-II also bound to the type 1 receptor whereas IGF-II/ mannose 6 phosphate receptors could not be detected. Separation of isolated testicular cells by Percoll gradient and centrifugal elutriation provided populations enriched in different types of intratubular cells. IGF-I mRNA (detected by reverse transcription + polymerase chain reaction [PCR]) and IGF-I receptors (measured by competitive binding) were observed to a greater extent in Sertoli cell-enriched populations and in spermatogonia with primary spermatocytes. Therefore, IGF-I is a potential paracrine/autocrine regulator inside the spermatogenic compartment and appears as a possible mediator of GH action at the gonadal level in fish.