Insulin-like growth factor binding protein expression in human retinal pigment epithelial cells

MAP kinase pathway is involved in IGF-1-stimulated proliferation of human retinal pigment epithelial cells (hRPE)

PURPOSE

To investigate the mitogenic activity of insulin-like growth factor-1 (IGF-1) on the proliferation of human retinal pigment epithelial cells (hRPE) and to elucidate the role of vascular endothelial growth factor (VEGF) and MAP kinase (MAPK) in the IGF-1 signaling cascade.

METHODS

Human RPE specimens were obtained from postmortem non-pathological eyes and cultured in vitro through several passages. Cellular proliferation in the presence of increasing concentrations of IGF-1 and IGF-1 + PD98059 (a known MAPK inhibitor) was measured by [(3)H]thymidine incorporation; trypan blue exclusion studies (T) verified cell viability. Under the same experimental conditions, synthesis of VEGF was measured utilizing [(14)C]methionine immunoprecipitation and immunocytochemical methods as well as Western blot analysis.

RESULTS

IGF-1 stimulated hRPE proliferation, as demonstrated by [(3)H]thymidine incorporation. There was also an IGF-1-induced increase in VEGF synthesis as measured quantitatively by [(14)C]methionine-VEGF immunoprecipitation. This was qualitatively confirmed by immunocytochemistry and Western blotting. PD98059 suppressed both IGF-1-induced cell proliferation as well as IGF-1-stimulated VEGF production.

CONCLUSIONS

These studies suggest that IGF-1 is a mitogen for hRPE cells and also stimulates production of the angiogenic factor, VEGF. Additionally, PD98059 inhibits the production of VEGF, suggesting that the MAP kinase pathway is involved in IGF-1-mediated angiogenesis.

Autocrine effects of IGF-I-induced VEGF and IGFBP-3 secretion in retinal pigment epithelial cell line ARPE-19

Hypoxia-induced physiological stress plays a central role in various neovascular diseases of the eye. Increased expression of hypoxia-inducible factor 1α (HIF-1α) and subsequent formation of HIF-1 dimers active at the vascular endothelial growth factor (VEGF) promoter lead to expression of this potent angiogenic factor in the retina, including retinal pigment epithelial (RPE) cells. We previously demonstrated that insulin-like growth factor I (IGF-I) stimulates VEGF and IGF binding protein (IGFBP)-3 secretion in RPE cells. In this study we examined IGF-I-induced HIF-1α expression, VEGF and IGFBP-3 secretion, and the autocrine actions of VEGF and IGFBP-3 on these processes in the spontaneously transformed RPE cell line ARPE-19. Cells were treated with CoCl2, IGF-I, recombinant human (rh)IGFBP-3, and rhVEGF. Immunoblot analysis revealed IGF-I-induced upregulation of total HIF-1α protein, whereas luciferase reporter assays of HIF-1 transcriptional activity demonstrated accumulation of HIF-1α correlated with the formation of functional HIF-1 heterodimers. Western and ligand blot analyses of RPE cell conditioned medium confirmed that IGF-I stimulated VEGF and IGFBP-3 secretion. rhVEGF stimulated IGFBP-3 secretion in an IGF-I- and HIF-1α-independent manner, whereas rhIGFBP-3 attenuated IGF-I-induced VEGF secretion. These findings demonstrate the multifaceted autocrine regulation of IGF-I-induced VEGF secretion by IGFBP-3 secreted in response to both IGF-I and, to a lesser extent, VEGF. These results provide evidence for HIF-1-dependent and -independent mechanisms by which IGF-I regulates VEGF and IGFBP-3 secretion.

A reassessment of insulin-like growth factor binding protein gene expression in the human retinal pigment epithelium

The role of insulin-like growth factors (IGF) in regulating cell differentiation and proliferation is in part modulated by the IGF binding protein (IGFBP) family of genes. Previous studies of the human retinal pigment epithelium (RPE) have detected expression of IGFBP-2, -3, and -6. However, recent experiments in our lab have suggested a broader pattern of IGFBP gene family expression in the RPE cell than has previously been recognized. We have examined the gene expression profile of IGFBP-1 to -6 and the related protein, IGFBP-rP1, in RPE cell lines derived from ten donors eyes using RT-PCR, ELISA, and Western methods. Transcripts of IGFBP-1 to -6 and -rP1 were consistently detected in human RPE cells. IGFBP-3, -5, -6, and -rP-1, appear to be constitutively expressed in the RPE, whereas IGFBP-1, -2, and -4, were expressed at variable levels in the cell lines examined. IGFBP secretion by the RPE in vitro was confirmed by ELISA (IGFBP-1, -2, -3, -4, and -6) and Western blot analysis (IGFBP-5 and -rP1). There was, in general, a strong correlation between gene-specific transcription levels and protein secretion by the RPE. Our studies demonstrate that the major IGFBP family genes are ubiquitously expressed in explanted human RPE cells in vitro. This broad expression profile and the recent evidence that IGFBPs have IGF-independent biological activity suggest that the IGFBP family genes may constitute a previously unrecognized and complex regulatory system in the human retina and RPE.

Retinal pigment epithelial cells: autocrine and paracrine stimulation of extracellular matrix contraction

BACKGROUND

This study was carried out to examine the biological activity of contraction promoters produced by dedifferentiating retinal pigment epithelial cells (RPE) and to evaluate the importance of autocrine and paracrine effects within a semi-closed environment like the vitreal cavity.

METHODS

RPE at different stages of dedifferentiation in culture were examined for their ability (a) to generate tractional forces in vitro, with and without serum stimulation, and (b) to produce and release contraction-stimulating proteins. Autocrine versus paracrine effects of cell-secreted promoters were tested by using RPE or human dermal fibroblasts (HDF) as target cells. The contraction-stimulating activity of the cell-secreted promoters was partially characterized and compared to the activity of defined promoters.

RESULTS

Our study confirmed that RPE can synthesize and secrete cell-contraction-promoting factor(s) active in stimulating the development of tractional forces by RPE as well as HDF. The quantity of biological activity secreted per cell decreases with progressive dedifferentiation, yet the responsiveness of the cell to contraction promoters increases. The contraction promoter(s) synthesized by RPE is partially distinct from the promoters in serum, TGF-beta 1 and beta 2, IGF-1, ET-1 and PDGF. The contraction-promoting effects of the RPE product(s) can be completely blocked by staurosporine.

CONCLUSION

De-differentiation of RPE is characterized by increasing capacity to generate tractional forces and decreasing synthetic capacity. RPE within a semi-closed system like the vitreal cavity can, theoretically, act both as promoting and active component of traction-related events (tractional retinal detachment).