Nuclear exclusion of transcription factors associated with apoptosis in developing nervous tissue

Contrary effects of cytokines on mRNAs of cell cycle- and ECM-related proteins in hRPE cells in vitro

PURPOSE

Retinal pigment epithelial (RPE) cells play a pivotal role in the pathogenesis of proliferative vitreoretinopathy (PVR). As a result of a breakdown of the blood-retina barrier, growth factors obtain access to the subretinal space and stimulate several retinal cell types. The aim of the present study was to analyze the effect of several growth factors on the proliferation of human (h)RPE cells, and on the mRNA expression of transcription factors, cell cycle proteins, and extracellular matrix (ECM) proteins.

METHODS

hRPE cells were incubated in the presence of TGF-beta1, TGF-beta2, PDGF, VEGF, or bFGF for 24-72 h. Cell proliferation was assessed by determinating BrdU incorporation. Changes in mRNA expression of c-fos, c-myc, PCNA, FEN1, Ki67, collagen III, and collagen IV were investigated by ribonuclease protection assay (RPA).

RESULTS

RPE cell proliferation was significantly increased by exposure to PDGF and bFGF for 48 h, and was decreased by application of TGF-beta1 and TGF-beta2 for 48 and 72 h. All the tested growth factors significantly elevated the amounts of c-fos mRNA (after 1 h) and of c-myc mRNA (after 24 h). PDGF and bFGF up-regulated the expression of Ki67 mRNA, and down-regulated that of collagen III and collagen IV mRNA after 24 h. TGF-beta1 and TGF-beta2 decreased the expression of Ki67 mRNA, and increased that of collagen III and collagen IV mRNA.

CONCLUSION

Our results show that distinct cytokines may induce contrary effects with respect to proliferation of, and ECM formation by, hRPE cells in vitro. This knowledge may be useful for the development of improved therapeutic approaches.

Nuclear proteasomal degradation and cytoplasmic retention underlie early nuclear exclusion of transcription factor Max upon axon damage

The behavior of the transcription factor Max in axon-damaged retinal ganglion cells (RGC) was investigated in explants from the rat retina, used as a tissue culture model of the central nervous system (CNS). Axon damage leads to an apparent rapid shift in the localization of Max from the nucleus to the cytoplasm, in advance of markers of apoptosis. This nuclear exclusion resisted treatments with calpeptin or the CRM1 exportin inhibitor leptomycin B, but was prevented by low temperature. Inhibition of either transcription or translation prevented RGC death, but only the latter robustly prevented nuclear exclusion. The proteasome inhibitor lactacystin prevented nuclear exclusion, whereas newly synthesized Max still accumulated in the cytoplasm of the axon-damaged RGC. The results show that proteosomal degradation of nuclear Max coupled with continued expression and cytoplasmic accumulation of Max, with blockade of nucleocytoplasmic transport of the newly synthesized protein, is an early event after CNS axonal damage.

Cytoplasmic c-Jun N-terminal immunoreactivity: a hallmark of retinal apoptosis

1. We investigated the association of c-Jun with apoptosis within retinal tissue. Explants of the retina of neonatal rats were subject to a variety of procedures that cause apoptosis of specific classes of retinal cells at distinct stages of differentiation. The expression of c-Jun was detected by Western Blot, and immunohistochemistry was done with antibodies made for either N-terminal or C-terminal domains of c-Jun, and correlated with apoptosis detected either by chromatin condensation or by in situ nick end labeling of fragmented DNA. 2. c-Jun protein content was increased in retinal tissue subject to induction of both photoreceptor and ganglion cell death. 3. c-Jun N-terminal immunoreactivity was found mainly in the cytoplasm of apoptotic cells regardless of cell type, of the stage of differentiation, including proliferating cells, or of the means of induction of apoptosis. 4. The data are consistent with the hypothesis that c-Jun is involved in the control of cell death in retinal tissue, but other proteins that cross-react with c-Jun N-terminal antibodies may also be major markers of retinal apoptosis. 5. Antibodies directed to c-Jun N-terminal (aa 91-105) are useful tools to follow apoptotic changes in retinal tissue.