Regulation of IGFBP-4 and -5 expression in rat granulosa cells

Novel markers of human ovarian granulosa cell differentiation toward osteoblast lineage: A microarray approach

Under physiological conditions, human ovarian granulosa cells (GCs), are responsible for a number of processes associated with folliculogenesis and oogenesis. The primary functions of GCs in the individual phases of follicle growth are: Hormone production in response to follicle stimulating hormone (FSH), induction of ovarian follicle atresia through specific molecular markers and production of nexus cellular connections for communication with the oocyte. In recent years, interest in obtaining stem cells from particular tissues, including the ovary, has increased. Special attention has been paid to the novel properties of GCs during long‑term in vitro culture. It has been demonstrated that the usually recycled material in the form of follicular fluid can be a source of cells with stem‑like properties. The study group consisted of patients enrolled in the in vitro fertilization procedure. Total RNA was isolated from GCs at 4 time points (after 1, 7, 15 and 30 days of culture) and was used for microarray expression analysis (Affymetrix® Human HgU 219 Array). The expression of 22,480 transcripts was examined. The selection of significantly altered genes was based on a P‑value <0.05 and expression higher than two‑fold. The leucine rich repeat containing 17, collagen type I α1 chain, bone morphogenetic protein 4, twist family bHLH transcription factor 1, insulin like growth factor binding protein 5, GLI family zinc finger 2 and collagen triple helix repeat containing genes exhibited the highest changes in expression. Reverse‑transcription‑quantitative PCR was performed to validate the results obtained in the analysis of expression microarrays. The direction of expression changes was validated in the majority of cases. The presented results indicated that GCs have the potential of cells that can differentiate towards osteoblasts in long‑term in vitro culture conditions. Increased expression of genes associated with the osteogenesis process suggests a potential for uninduced change of GC properties towards the osteoblast phenotype. The present study, therefore, suggests that GCs may become an excellent starting material in obtaining stable osteoblast cultures. GCs differentiated towards osteoblasts may be used in regenerative and reconstructive medicine in the future.

Regulation of insulin-like growth factor (IGF) bioactivity by sequential proteolytic cleavage of IGF binding protein-4 and -5

The biological activity of IGF-I and -II is controlled by six binding proteins (IGFBPs), preventing the IGFs from interacting with the IGF receptor. Proteolytic cleavage of IGFBPs is one mechanism by which IGF can be released to bind the receptor. The IGFBPs are usually studied individually, although the presence of more than one of the IGFBPs in most tissues suggests a cooperative function. Thus, the IGFBPs are part of regulatory networks with proteolytic enzymes in one end and the IGF receptor in the other end. We have established a model system that allows analysis of the dynamics between IGF, IGFBP-4 and -5, the IGF receptor, and the proteolytic enzyme PAPP-A, which specifically cleaves both IGFBP-4 and -5. We demonstrate different mechanisms of IGF release from IGFBP-4 and -5: cooperative binding to IGF is observed for the proteolytic fragments of IGFBP-5, but not fragments of IGFBP-4. Furthermore, we find that PAPP-A-mediated IGF-dependent cleavage of IGFBP-4 is inhibited by IGFBP-5, which sequesters IGF from IGFBP-4, and that cleavage of both IGFBP-4 and -5 is required for the release of bioactive IGF. Finally, we show that cell surface-localized proteolysis of IGFBP-4 represents the final regulatory step of efficient IGF delivery to the receptor. Our data define a regulatory system in which molar ratios between the IGFBPs and IGF and between the different IGFBPs, sequential proteolytic cleavage of the IGFBPs, and surface association of the activating proteinase are key elements in the regulation of IGF receptor stimulation.

Insulin-like growth factor-1 (IGF-1) stimulates the IGF binding protein system in rat theca interstitial cells

There has been considerable interest in rat ovarian insulin-like growth factor binding proteins IGFBPs because they are potent inhibitors of FSH action.In situ, IGFBP-2 and -4 and IGFBP-3 mRNAs are expressed in rat theca interstitial (TIC) and theca lutein cells respectively. Although much is known about IGFBPs in rat TIC at the mRNA level, the synthesis and regulation of IGFBP proteins remain poorly understood. The purpose of this study was to identify the species of IGFBPs produced by TIC and to determine the effects of LH and IGF-1 on their expression. This was accomplished by culturing rat TIC for 2 days in serum-free medium with graded doses of LH and/or IGF-I, and measuring IGFBP mRNAs in the cells and IGFBP proteins in the conditioned media by RT-PCR and Western immunoblotting respectively. The RT PCR analysis identified strong bands for IGFBP-2 and -4 mRNAs in TIC. In some treatments, the mRNAs for IGFBP-3 and -6 were also identified, but transcripts for IGFBP-1 and -5 were undetectable. Two species of IGFBPs were detected in the conditioned media of control (untreated) TIC, the 31 kDa IGFBP-2 and the 24 kDa (non-glycosylated) and 28 kDa (glycosylated) forms of IGFBP-4. There was no detectable IGFBP-5 and barely detectable amounts of IGFBP-3 and -6 in the conditioned media. Treatment with LH (0.2-20 μU/ml) caused no significant changes in the levels of the 31 kDa IGFBP-2 and the 24 kDa and 28 kDa IGFBP-4 bands, and there was no detectable IGFBP protease activity. In contrast, IGF-I (100 ng/ml) stimulated the expression of IGFBP-2, IGFBP-4 and a 17.5 kDa IGFBP-4 fragment. The immunoreactive IGFBP-4 fragment suggests the media contained an IGFBP-4 protease. The IGF-I effects were dose dependent (ED(50)=12.4±3.3 ng/ml). Co-treating TIC with LH (0.2-20 μU/ml) caused no significant change in the activity of IGF-I in stimulating the expression of IGFBP-2, IGFBP-4 and IGFBP-4 protease. We have demonstrated that IGF-I acts directly on rat TIC to stimulate the expression of the intrinsic IGFBP system. LH, either alone or together with IGF-I, did not significantly change the expression of TIC IGFBP proteins. Therefore, we hypothesize that IGF-I, but not LH, may be a physiologically important regulator of the IGFBP system in rat TIC. Because IGF-I is a potent stimulator of theca function, changes in the expression of this intrinsic IGFBP system could have new implications for ovarian androgen production, both at the physiologic and pathophysiologic levels.

Gonadotropin-releasing hormone overcomes follicle-stimulating hormone's inhibition of insulin-like growth factor-5 synthesis and promotion of its degradation in rat granulosa cells

The effect of a gonadotropin-releasing hormone-agonist (GnRH-a) on the synthesis of insulin-like growth factor-binding protein-5 (IGFBP-5), a physiological marker for atresia, was investigated. Granulosa cells obtained from diethylstilbestrol (DES)-treated immature female rats were cultured in serum-free medium for 72 h with GnRH-a and the conditioned media were subjected to immunoblot analysis using rat IGFBP-5 specific antibody. GnRH-a caused a dose-dependent (ED50 = 8.6 x 10(-11) M) accumulation of IGFBP-5, which migrated as 35 (non-glycosylated) and 36 kDa (glycosylated) bands under reducing conditions. A maximally effective dose of GnRH-a (10(-9) M) caused a 4-fold increase in IGFBP-5 accumulation. In contrast, increasing doses of porcine follicle-stimulating hormone (pFSH) caused a biphasic effect on IGFBP-5 accumulation. A low dose of pFSH (0.25 ng/ml) increased and higher doses of pFSH (22.5 ng/ml) decreased the 35 and 36 kDa IGFBP-5 bands. In the presence of high doses of pFSH (20.75 ng/ml), a 22 kDa band corresponding to a cleaved IGFBP-5 fragment appeared in the media. When the granulosa cells were cultured with a saturating dose of pFSH, co-addition of GnRH-a dose dependently inhibited the FSH effects (ED50 = (2.3-3.7) x 10(-10) M). The GnRH-a effects were completely blocked by co-incubation with GnRH-antagonist. IGFBP-5 mRNA accumulation levels were increased by GnRH-a in a dose dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)