Endogenous inhibins regulate steroidogenesis in mouse TM3 Leydig cells by altering SMAD2 signalling

This study tested the hypothesis that inhibins act in an autocrine manner on Leydig cells using a pre-pubertal Leydig cell line, TM3, as a model of immature Leydig cells. The expression of Inha, Inhba, and Inhbb in TM3 cells was determined by RT-PCR and the production of the inhibin-alpha subunit was confirmed by western blot. Knockdown of Inha expression resulted in significant decreases in the expression of Leydig cell markers Cyp17a1, Cyp11a1, Nr5a1, and Insl3. Western blot showed that activin A, TGFβ1 and TGFβ2 activated SMAD2, and that knockdown of Inha expression in TM3 cells enhanced both activin A- and TGFβ-induced SMAD2 activation. SB431542, a chemical inhibitor of the TGFβ/activin type I receptors, blocked ligand-induced SMAD2 activation and the downregulation of Cyp17a1 expression. Our findings demonstrate that TGFβs and activin A negatively regulate steroidogenic gene expression in TM3 cells via ALK4/5 and SMAD2 and endogenous inhibins can counter this regulation.

Abstract POSTER-TECH-1105: Assessment of ovarian cancer spheroid attachment and invasion of mesothelial cells in real time to profile the molecular signature of the invasive interface

Background and Aims: Ovarian cancer is the most common cause of death from gynaecological cancer worldwide (1). Often called the “silent killer”, the high mortality rate is associated with the late stage in which the disease is diagnosed (2). Unlike other solid cancers, ovarian cancer cells rarely spread via the vasculature, instead shed from the primary tumor and disseminate within the peritoneal fluid or ascites (2). The formation of spheroids within the non-adherent peritoneal environment is a barrier to effective treatment, due to the enhanced ability of spheroids to survive chemotherapies and seed distal metastases (3-5).

Invading ovarian cancer cells interact with mesothelial cells lining the peritoneal surface, attach to, and invade the underlying basement membrane to establish secondary lesions (6). The molecular events at the interface between invading ovarian cancer and peritoneal cells at the onset of invasion have been poorly studied, existing methods capture events after invasion is well-underway. Therefore, the regulatory signals which govern the initiation of a metastatic lesion are poorly understood. We aimed: i) to develop an innovative method to co-culture ascites-derived primary ovarian cancer cells to measure invasion through peritoneal mesothelial cell layers in real time (7); ii) identify the unique protein expression profile at the ovarian cancer spheroid-peritoneal interface during early invasion using proteomics.

Methods: Real Time Cell Analysis (RTCA) technology (xCELLigence, ACEA) was adapted to establish a three-dimensional co-culture model of the tumor microenvironment of the peritoneum (7). RTCA wells were coated with Matrigel (basement membrane matrix) and a confluent monolayer of human mesothelial cells. Multicellular spheroids were generated from primary ovarian cancer lines (high-grade serous carcinoma, leiomysarcoma, adenocarcinoma, carcinosarcoma) or benign ascites derived cells cultured under non-adherent conditions. Spheroids were added to the RTCA plate and real time measurements were conducted to determine the precise period when invasion commenced. Parallel cultures were histologically-preserved at invasion initiation and processed for assessment by Matrix-Assisted Laser Desorption/Ionization (MALDI) imaging.

Results: We assessed invasion in a panel of ovarian cancer samples, benign ascites cells and the mesothelial cell line. Continuous real time measurements revealed that individual ovarian cancer samples exhibited differences in the timing of the onset of invasion. High-grade serous carcinoma samples initiated invasion rapidly after addition, with some samples detected in under 30 min, a time period not captured by traditional Transwell assays. The benign and mesothelial cells themselves did not invade. MALDI imaging allowed for the identification of peptides expressed at the tumor-mesothelial interface during early invasion. These molecules represent novel molecules involved in the initiation of the metastatic process and are the subject of further characterisation.

Conclusion: This method represents a high-throughput quantitative analysis of ovarian cancer spheroid invasion of mesothelial and ECM barriers. The study has provided an unprecedented insight into the molecular mechanisms and molecules involved in the initiation of ovarian cancer invasion of the mesothelium, identifying new therapeutic targets.

References: 1. Marsha Rivkin Center for Ovarain Cancer Research - Facts and Figures Survival Rates. 2014. 2. Ahmed N et al., Front Oncol. 2013;3(256). 3. Aguilar-Gallardo C et al., Stem Cell Rev. 2012 Sep;8(3):994-1010. 4. Burleson KM et al., Gynecol Oncol. 2004 Apr;93(1):170-81. 5. Liao J et al., PLoS One. 2014;9(1):e84941 6. Kenny HA et al., Int J Cancer. 2007 Oct 1;121(7):1463-72. 7. Bilandzic M et al., Journal of Visual Experimentation. 2014 20/05/2014 (87).

Citation Format: Maree Bilandzic, Adam Rainczuk, Andrew Stephens, Nicole Fairweather, Thomas Jobling, Kaye L Stenvers. Assessment of ovarian cancer spheroid attachment and invasion of mesothelial cells in real time to profile the molecular signature of the invasive interface [abstract]. In: Proceedings of the 10th Biennial Ovarian Cancer Research Symposium; Sep 8-9, 2014; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(16 Suppl):Abstract nr POSTER-TECH-1105.

Assessment of ovarian cancer spheroid attachment and invasion of mesothelial cells in real time

Ovarian cancers metastasize by shedding into the peritoneal fluid and dispersing to distal sites within the peritoneum. Monolayer cultures do not accurately model the behaviors of cancer cells within a nonadherent environment, as cancer cells inherently aggregate into multicellular structures which contribute to the metastatic process by attaching to and invading the peritoneal lining to form secondary tumors. To model this important stage of ovarian cancer metastasis, multicellular aggregates, or spheroids, can be generated from established ovarian cancer cell lines maintained under nonadherent conditions. To mimic the peritoneal microenvironment encountered by tumor cells in vivo, a spheroid-mesothelial co-culture model was established in which preformed spheroids are plated on top of a human mesothelial cell monolayer, formed over an extracellular matrix barrier. Methods were then developed using a real-time cell analyzer to conduct quantitative real time measurements of the invasive capacity of different ovarian cancer cell lines grown as spheroids. This approach allows for the continuous measurement of invasion over long periods of time, which has several advantages over traditional endpoint assays and more laborious real time microscopy image analyses. In short, this method enables a rapid, determination of factors which regulate the interactions between ovarian cancer spheroid cells invading through mesothelial and matrix barriers over time.

Getting to know ovarian cancer ascites: opportunities for targeted therapy-based translational research

More than one third of ovarian cancer patients present with ascites at diagnosis, and almost all have ascites at recurrence. The presence of ascites correlates with the peritoneal spread of ovarian cancer and is associated with poor disease prognosis. Malignant ascites acts as a reservoir of a complex mixture of soluble factors and cellular components which provide a pro-inflammatory and tumor-promoting microenvironment for the tumor cells. Subpopulations of these tumor cells exhibit cancer stem-like phenotypes, possess enhanced resistance to therapies and the capacity for distal metastatic spread and recurrent disease. Thus, ascites-derived malignant cells and the ascites microenvironment represent a major source of morbidity and mortality for ovarian cancer patients. This review focuses on recent advances in our understanding of the molecular, cellular, and functional characteristics of the cellular populations within ascites and discusses their contributions to ovarian cancer metastasis, chemoresistance, and recurrence. We highlight in particular recent translational findings which have used primary ascites-derived tumor cells as a tool to understand the pathogenesis of the disease, yielding new insights and targets for therapeutic manipulation.

Betaglycan alters NFκB-TGFβ2 cross talk to reduce survival of human granulosa tumor cells

The molecular pathways controlling granulosa cell tumor (GCT) survival are poorly understood. In many cell types, nuclear factor-κB (NFκB) and TGFβ coordinately regulate cell survival to maintain tissue homeostasis. Because GCT cell lines exhibit constitutively activated NFκB, we hypothesized that NFκB blocks TGFβ-mediated cell death in GCT cells. To test this hypothesis, we used the human GCT cell line KGN, which exhibits loss of betaglycan, a TGFβ co-receptor. After inhibition of NFκB in KGN cells, re-expression of betaglycan resulted in a decrease in cell viability, which was further decreased by TGFβ2. Intriguingly, TGFβ2 increased NFκB reporter activity in control cells, but betaglycan expression suppressed both basal and TGFβ2-stimulated NFκB activity. Chemical inhibition of Mothers against decapentaplegic homolog 2/3 (SMAD2/3) signaling or SMAD2/3 gene silencing revealed that both SMADs contributed to cell survival. Furthermore, inhibiting NFκB activity resulted in a specific reduction in SMAD3 expression. Conversely, overexpression of SMAD3 increased basal NFκB activity and countered betaglycan-mediated suppression of NFκB activity. Finally, ERK1/2 activation emerged as the point of convergence of NFκB, SMAD3, and TGFβ2/betaglycan governance of GCT cell viability. Key findings in KGN cells were reproduced in a second GCT cell line, COV434. Collectively, our data establish that both SMAD2/3 and NFκB signaling pathways support GCT cell viability and suggest the existence of a positive feedback loop between NFκB and SMAD3 signaling in late-stage GCT. Furthermore, our data suggest that loss of betaglycan during tumor progression in GCT alters the functional outcomes generated by NFκB and TGFβ pathway cross talk.

Reprint of: Betaglycan: a multifunctional accessory

Betaglycan is a co-receptor for the TGFβ superfamily, particularly important in establishing the potency of its ligands on their target cells. In recent years, new insights have been gained into the structure and function of betaglycan, expanding its role from that of a simple co-receptor to include additional ligand-dependent and ligand-independent roles. This review focuses on recent advances in the betaglycan field, with a particular emphasis on its newly discovered actions in mediating the trafficking of TGFβ superfamily receptors and as a determinant of the functional output of TGFβ superfamily signalling. In addition, this review encompasses a discussion of the emerging roles of the betaglycan/inhibin pathway in reproductive cancers and disease.

Betaglycan: a multifunctional accessory

Betaglycan is a co-receptor for the TGFβ superfamily, particularly important in establishing the potency of its ligands on their target cells. In recent years, new insights have been gained into the structure and function of betaglycan, expanding its role from that of a simple co-receptor to include additional ligand-dependent and ligand-independent roles. This review focuses on recent advances in the betaglycan field, with a particular emphasis on its newly discovered actions in mediating the trafficking of TGFβ superfamily receptors and as a determinant of the functional output of TGFβ superfamily signalling. In addition, this review encompasses a discussion of the emerging roles of the betaglycan/inhibin pathway in reproductive cancers and disease.

Inhibins: from reproductive hormones to tumor suppressors

Inhibins are peptide hormones shown originally to be produced by the gonads to regulate the secretion of follicle stimulating hormone by pituitary gonadotropes. Although gonadotropes have been regarded as the canonical inhibin target cells, in recent years extrapituitary actions of inhibins have come into the spotlight. In particular, disruptions to the local actions of inhibins in peripheral tissues might underlie certain diseases, especially cancers of the reproductive tract. This review focuses on recent advances in the inhibin field, with a particular emphasis on the determinants of inhibin availability, mechanisms of inhibin action, and the physiological relevancy of local inhibin actions in the development and progression of reproductive cancers.

Fetal testis dysgenesis and compromised Leydig cell function in Tgfbr3 (beta glycan) knockout mice

Betaglycan (Tgfbr3) is a coreceptor for transforming growth factor-beta (TGFB) superfamily ligands. In the current study, a defect in seminiferous cord formation was detected in 12.5-13.5 days postcoitum (dpc) beta glycan null murine testis. Immunohistochemistry with antibodies against cell-specific markers revealed defects in somatic cell populations. To confirm these data, quantitative real-time PCR was performed to determine changes in the expression levels of genes involved in fetal testis cell differentiation and function. The expression levels of the Leydig cell markers Insl3, Cyp17a1, Cyp11a1, Star, and Hsd3b1 were reduced in knockout testis compared to wild-type testis, beginning at 12.5 dpc. Whole mount in situ hybridization confirmed that Cyp11a1 expression was reduced in the null testis, but its distribution pattern was unchanged. Apoptosis was not affected by the loss of beta glycan, but proliferation within the interstitium was reduced at 14.5 dpc. However, morphometric analysis showed no changes in Leydig cell counts between the wild-type and the knockout testes at 14.5 dpc, indicating that fetal Leydig function, rather than number, was affected by the loss of beta glycan. The expression levels of Sertoli cell markers Dhh, Sox9, and Amh were also reduced in the knockout testis at 14.5 dpc. However, the expression of fetal germ cell markers Pou5f1 and DDX4 were not changed across the genotypes at any age examined. Our data show that the presence of beta glycan is required for normal cord formation, normal fetal Leydig cell development, and the establishment of fetal testis endocrine function, thus implicating TGFB superfamily members as regulators of early fetal testis structure and function.

Reducing betaglycan expression by RNA interference (RNAi) attenuates inhibin bioactivity in LbetaT2 gonadotropes

Betaglycan is an inhibin-binding protein co-receptor, the forced expression of which confers inhibin responsiveness on cells previously non-responsive to inhibin. The present study determines whether removal of betaglycan expression in otherwise inhibin-responsive cells will render the cells insensitive to inhibin. Small interfering RNAs (siRNAs) designed to the betaglycan gene were transfected into LbetaT2 gonadotrope cells to 'knock-down' betaglycan expression. To control for non-specific effects, siRNAs corresponding to an unrelated sequence (BF-1) were used. Two activin-responsive promoter constructs were used to assess inhibin bioactivity; an ovine FSHbeta promoter (oFSHbeta-lux), and a construct containing three copies of the activin-responsive sequence from the GnRHR promoter (3XpGRAS-PRL-lux). Activin stimulated the activity of both promoters 5-8-fold. Inhibin suppressed these activin-stimulated promoter activities by 52+/-11% and 51+/-7%, respectively. Similar inhibin suppression was also seen for cells co-transfected with the control BF-1 siRNAs. In contrast, inhibin's ability to suppress activin-stimulated activity was significantly reduced (33+/-3%, p<0.005 and 24+/-4%, p<0.045, respectively) in cells co-transfected with betaglycan siRNAs. These results demonstrated that endocrine effects of inhibin as a negative feedback controller of FSH production in gonadotropes are dependent on betaglycan expression.

Loss of betaglycan contributes to the malignant properties of human granulosa tumor cells

Betaglycan is a type III TGFbeta receptor that modulates cellular sensitivity to inhibins and TGFbeta. Previous studies have suggested that betaglycan acts as a tumor suppressor in certain human epithelial cancers. However, the roles of betaglycan in ovarian granulosa cell tumors (GCTs) are poorly understood. The objective of this study was to determine whether human GCTs exhibit betaglycan expression and, if so, what impact this receptor has on tumor biology. Real-time PCR was used to quantify betaglycan transcripts in human GCTs (n = 17) and normal premenopausal ovaries (n = 11). This analysis established that GCTs exhibited a significant 2-fold lower mean betaglycan mRNA level as compared with the normal ovary (P < 0.05). Similarly, two human GCT cell lines, KGN and COV434, exhibited low betaglycan expression and poor responsiveness to TGFbeta and inhibin A in luciferase reporter assays, which was restored by stable transfection of wild-type betaglycan. Betaglycan significantly increased the adhesion of COV434 (P < 0.05) and KGN (P < 0.0001) cells, decreased cellular invasion through Matrigel, and inhibited wound healing. Expression of mutant forms of betaglycan that are defective in TGFbeta and/or inhibin binding in each GCT cell line revealed that the inhibitory effects of betaglycan on wound healing were most strongly linked to the inhibin-binding region of betaglycan. Furthermore, knockdown of INHA mRNA expression abrogated the betaglycan-mediated inhibition of wound healing and invasion, whereas both INHA silencing and TGFbeta neutralization abolished the betaglycan-mediated increase in adhesion to substrate. These data suggest that loss of betaglycan contributes to the pathogenesis of GCTs.

Differential expression of TGFBR3 (betaglycan) in mouse ovary and testis during gonadogenesis

TGFBR3 is an accessory receptor that binds to and modulates the activities of both transforming growth factor-beta (TGFbeta) and inhibin, two members of the TGFbeta superfamily of growth factors that regulate many aspects of reproductive biology. Tgfbr3 is known to be expressed in adult testis and ovary, but little is known about this receptor during gonadogenesis. Herein, we describe Tgfbr3 expression in the male and female fetal and neonatal murine gonad. Real-time PCR analysis revealed that Tgfbr3 mRNA was expressed at higher levels in the developing testis compared to ovary. TGFBR3 was expressed within the fetal testis interstitium, predominantly by Leydig cells, but expression shifted inside the seminiferous cords at birth. In contrast, TGFBR3 was detected in both the somatic and germ cell lineages in the fetal and neonatal ovary. This differential expression pattern suggests divergent roles for this TGFBR3 in developing testis and ovary.

Heart and liver defects and reduced transforming growth factor beta2 sensitivity in transforming growth factor beta type III receptor-deficient embryos

The type III transforming growth factor beta (TGFbeta) receptor (TbetaRIII) binds both TGFbeta and inhibin with high affinity and modulates the association of these ligands with their signaling receptors. However, the significance of TbetaRIII signaling in vivo is not known. In this study, we have sought to determine the role of TbetaRIII during development. We identified the predominant expression sites of TbetaRIII mRNA as liver and heart during midgestation and have disrupted the murine TbetaRIII gene by homologous recombination. Beginning at embryonic day 13.5, mice with mutations in TbetaRIII developed lethal proliferative defects in heart and apoptosis in liver, indicating that TbetaRIII is required during murine somatic development. To assess the effects of the absence of TbetaRIII on the function of its ligands, primary fibroblasts were generated from TbetaRIII-null and wild-type embryos. Our results indicate that TbetaRIII deficiency differentially affects the activities of TGFbeta ligands. Notably, TbetaRIII-null cells exhibited significantly reduced sensitivity to TGFbeta2 in terms of growth inhibition, reporter gene activation, and Smad2 nuclear localization, effects not observed with other ligands. These data indicate that TbetaRIII is an important modulator of TGFbeta2 function in embryonic fibroblasts and that reduced sensitivity to TGFbeta2 may underlie aspects of the TbetaRIII mutant phenotype.

The specificity of receptor binding by vascular endothelial growth factor-d is different in mouse and man

Human vascular endothelial growth factor-D (VEGF-D) binds and activates VEGFR-2 and VEGFR-3, receptors expressed on vascular and lymphatic endothelial cells. As VEGFR-2 signals for angiogenesis and VEGFR-3 is thought to signal for lymphangiogenesis, it was proposed that VEGF-D stimulates growth of blood vessels and lymphatic vessels into regions of embryos and tumors. Here we report the unexpected finding that mouse VEGF-D fails to bind mouse VEGFR-2 but binds and cross-links VEGFR-3 as demonstrated by biosensor analysis with immobilized receptor domains and bioassays of VEGFR-2 and VEGFR-3 cross-linking. Mutation of amino acids in mouse VEGF-D to those in the human homologue indicated that residues important for the VEGFR-2 interaction are clustered at, or are near, the predicted receptor-binding surface. Coordinated expression of VEGF-D and VEGFR-3 in mouse embryos was detected in the developing skin where the VEGF-D gene was expressed in a layer of cells beneath the developing epidermis and VEGFR-3 was localized on a network of vessels immediately beneath the VEGF-D-positive cells. This suggests that VEGF-D and VEGFR-3 may play a role in establishing vessels of the skin by a paracrine mechanism. Our study of receptor specificity suggests that VEGF-D may have different biological functions in mouse and man.

Characterization of mouse A33 antigen, a definitive marker for basolateral surfaces of intestinal epithelial cells

The murine A33 antigen is emerging as a definitive marker of intestinal epithelial cells. Cloning and sequence determination of cDNAs encoding mA33 antigen predict a novel type 1 transmembrane protein of 298 amino acids, comprising an extracellular domain with two immunoglobulin-like domains, a single-span transmembrane domain, and a highly acidic cytoplasmic domain. On the basis of conservation of amino acid sequence and genomic structure, the mA33 antigen is a member of a growing subfamily within the immunoglobulin superfamily, which includes transmembrane proteins CTX/ChT1, CTM/CTH, and CAR. During embryonic development, mA33 antigen expression is first observed in the inner cell mass of blastocysts before implantation. Intestinal expression of mA33 antigen is initiated in the hindgut at E14.5 and increases steadily throughout late embryonic and postnatal life into adulthood. The protein is specifically expressed on the basolateral surfaces of intestinal epithelial cells of all lineages, independent of their position along the rostrocaudal and crypt-villus axes. Thus the mA33 antigen appears to be a novel marker for both proliferating and differentiating intestinal epithelial cells.

Increased expression of type 1 insulin-like growth factor receptor messenger RNA in rat hippocampal formation is associated with aging and behavioral impairment

Insulin-like growth factor messenger RNAs are expressed in adult rat brain. However, little is known about the effects of aging on the expression of the insulin-like growth factors, their receptors, and their binding proteins in different regions of rat brain. The goal of the current study was to assess whether there is altered expression of the insulin-like growth factor system during normal aging in the hippocampal formation, a region particularly vulnerable to the aging process. A spatial learning task in the Morris water maze was used to assess the cognitive status of young (7-8-month-old) and aged (28-29-month-old) male Long-Evans rats. Sites of expression and abundance of insulin-like growth factor-I, type 1 insulin-like growth factor receptor, and insulin-like growth factor binding protein-4 messenger RNAs were then examined by in situ hybridization histochemistry and solution or northern blot hybridization assays. In situ hybridization histochemistry revealed no qualitative differences in the regional distribution of insulin-like growth factor-I, type 1 receptor, and insulin-like growth factor binding protein-4 messenger RNAs within the hippocampal formation of young and aged rats. However, quantitative analysis of messenger RNA abundance in hippocampal tissue homogenates showed a significant age-related increase in type 1 receptor messenger RNA (n = 25; t = -2.5; P < 0.02). Furthermore, linear regression analysis indicated that type 1 receptor messenger RNA abundance was significantly correlated with spatial learning impairment in the water maze (r = 0.44; P < 0.03) such that greater behavioral impairment was associated with higher type 1 receptor messenger RNA levels in the hippocampal formation. Neither insulin-like growth factor-I nor insulin-like growth factor binding protein-4 messenger RNA abundance was related to age or behavior. However, linear regression revealed a negative correlation between insulin-like growth factor-I messenger RNA abundance and type 1 receptor messenger RNA abundance in aged hippocampus (r = -0.72, P < 0.01). These data indicate that increased hippocampal expression of type 1 receptor messenger RNA is associated with aging and cognitive decline. The correlation between type 1 receptor and insulin-like growth factor-I messenger RNA abundance in the hippocampal formation of aged rats suggests that insulin-like growth factor availability may influence type 1 receptor expression. However, because no overall age difference was found in the amount of insulin-like growth factor-I messenger RNA in the hippocampal formation, decreased insulin-like growth factor from other sources such as the cerebrospinal fluid and the peripheral circulation may be involved in up-regulating type 1 receptor messenger RNA. Alternatively, type 1 receptor messenger RNA regulation may be part of a trophic response to the degenerative and regenerative events that occur within the hippocampal formation during aging.

Cloning and characterization of a cDNA encoding a novel fatty acid binding protein from rat brain

We have adopted a polymerase chain reaction approach to identify and clone a cDNA that contains the complete coding sequence of a novel fatty acid binding protein (FABP) from a rat brain lambda gt10 library. Sequencing of the brain FABP (B-FABP) cDNA revealed an open reading frame coding for a protein with 132 amino acids and a predicted size of approximately 15,000 Da. This putative protein shares extensive sequence homology with other members of the FABP family. Northern blot analysis using the B-FABP cDNA as a probe established the presence of an abundant mRNA approximately 0.8 kb long in rat brain and in the MOCH-1 oligodendrocyte cell line. This transcript was also present in rat liver but not in other tissues examined. A developmental profile of this mRNA in rat brain demonstrated detectable expression in 15-day-old embryos with levels peaking in 1-day postnatal neonates and declining thereafter, reaching a low steady-state level at 3 weeks of age. In situ hybridization histochemistry revealed B-FABP mRNA in various brain regions, with the highest levels in fiber tracts. The B-FABP message was also detected at a lower level in several gray matter regions. The cloning approach used in this study would likely be useful in the identification and isolation of FABP-encoding genes from other tissues and species.

Expression of insulin-like growth factor binding protein-4 and -5 mRNAs in adult rat forebrain

Accumulating evidence indicates that the insulin-like growth factors (IGFs) can act as neurotrophic factors. A family of at least six IGF binding proteins (IGFBPs) has been characterized. The IGFBPs prolong the half-life of IGFs in plasma and may modulate IGF action in a cell- or tissue-specific fashion. Two recently characterized IGFBPs, IGFBP-4 and -5, have been shown by northern blot hybridization to be expressed in rat brain, but their cellular sites of synthesis are poorly characterized. Because IGFBP-4 and IGFBP-5 could potentially modulate IGF actions in the brain, we used in situ hybridization histochemistry and 35S-labeled IGFBP-4 and IGFBP-5 riboprobes to localize sites of IGFBP-4 and -5 mRNA expression in adult rat brain. The two IGFBP mRNAs are abundantly expressed within discrete regions of brain. The expression patterns of the two genes are largely nonoverlapping. Notably, IGFBP-4 mRNA is highly expressed within hippocampal and cortical areas, whereas IGFBP-5 mRNA is not detected above background in these areas. Within the hippocampus, abundant IGFBP-4 mRNA expression is detected in pyramidal neurons of the subfields of Ammon's horn and the subiculum and in the granule cell layer of the anterior hippocampal continuation. In the cortex, IGFBP-4 mRNA is widely expressed in most areas and layers. In contrast, IGFBP-5, but not IGFBP-4, mRNA is detected within thalamic nuclei, leptomeninges, and perivascular sheaths. The distinct expression patterns of IGFBP-4 and -5 mRNAs within the brain suggest that these IGFBPs may modulate paracrine/autocrine actions of the IGFs in discrete brain regions or compartmentalization of the IGFs within the brain.