Characterization of a novel quiescence responsive element downregulated by v-Src in the promoter of the neuroretina specific QR1 gene

Role for transcription factor TFII-I in the suppression of SSeCKS/Gravin/Akap12 transcription by Src

The SSeCKS/Gravin/AKAP12 gene, encoding a kinase scaffolding protein with metastasis-suppressing activity, is transcriptionally downregulated in Src-transformed cells through the recruitment of HDAC1 to a Src-responsive proximal promoter site charged with Sp1, Sp3 and USF1. However, the ectopic expression of these proteins formed a suppressive complex in Src-transformed but not in parental NIH3T3 cells, suggesting the involvement of additional repressor factors. Transcription factor II-I (TFII-I) [general transcription factor 2i (Gtf2i)] was identified by mass spectrometry as being associated with the SSeCKS promoter complex in NIH3T3/Src cells, and moreover, the Src-induced tyrosine phosphorylation of TFII-I significantly increased its binding to the SSeCKS proximal promoter. siRNA-mediated knockdown of TFII-I or the expression of TFII-I(Y248/249F) caused the derepression of SSeCKS in NIH3T3/Src cells. Taken with previous data showing that the tyrosine phosphorylation of TFII-I facilitates its nuclear translocation, these data suggest that Src-family kinase-mediated phosphorylation converts a portion of TFII-I into a transcriptional repressor.

v-Src-mediated Down-regulation of SSeCKS Metastasis Suppressor Gene Promoter by the Recruitment of HDAC1 into a USF1-Sp1-Sp3 Complex

SSeCKS (Src-suppressed C kinase substrate), also called gravin/AKAP12, is a large scaffolding protein with metastasis suppressor activity. Two major isoforms of SSeCKS are expressed in most cell and tissue types under the control of two independent promoters, designated alpha and beta, separated by 68 kb. SSeCKS transcript and protein levels are severely decreased in Src- and Ras-transformed fibroblasts and in many epithelial tumors. By dissecting its promoters with progressive deletion analysis, we identified the sequence between -106 and -49 in the alpha proximal promoter as the minimal v-Src-responsive element, which contains E- and GC-boxes bound by USF1 and Sp1/Sp3, respectively. Both E- and GC-boxes are crucial for v-Src-responsive and basal promoter activities. v-Src does not alter USF1 binding levels at the E-box, but it increases Sp1/Sp3 binding to the GC-box despite no change in their cellular protein abundance. SSeCKS alpha and beta transcript levels in v-Src/3T3 cells can be restored by treatment with the histone deacetylase inhibitor, trichostatin A, but not with the DNA demethylation agent, 5-azacytidine. Chromatin changes are found only on the alpha promoter even though the beta proximal promoter contains a similar E- and GC-box arrangement. Recruitment of HDAC1 is necessary and sufficient to cause repression of alpha proximal promoter activity, and the addition of Sp1 and/or Sp3 potentiates the repression. Our data suggest that suppression of the beta promoter is facilitated by Src-induced changes in the alpha promoter chromatinization mediated by a USF1-Sp1-Sp3 complex.

SSeCKS/Gravin/AKAP12 attenuates expression of proliferative and angiogenic genes during suppression of v-Src-induced oncogenesis

Background

SSeCKS is a major protein kinase C substrate with kinase scaffolding and metastasis-suppressor activity whose expression is severely downregulated in Src- and Ras-transformed fibroblast and epithelial cells and in human prostate, breast, and gastric cancers. We previously used NIH3T3 cells with tetracycline-regulated SSeCKS expression plus a temperature-sensitive v-Src allele to show that SSeCKS re-expression inhibited parameters of v-Src-induced oncogenic growth without attenuating in vivo Src kinase activity.

Methods

We use cDNA microarrays and semi-quantitative RT-PCR analysis to identify changes in gene expression correlating with i) SSeCKS expression in the absence of v-Src activity, ii) activation of v-Src activity alone, and iii) SSeCKS re-expression in the presence of active v-Src.

Results

SSeCKS re-expression resulted in the attenuation of critical Src-induced proliferative and pro-angiogenic gene expression including Afp, Hif-1α, Cdc20a and Pdgfr-β, and conversely, SSeCKS induced several cell cycle regulatory genes such as Ptpn11, Gadd45a, Ptplad1, Cdkn2d (p19), and Rbbp7.

Conclusion

Our data provide further evidence that SSeCKS can suppress Src-induced oncogenesis by modulating gene expression downstream of Src kinase activity.

Rhodopsin, Violet and Blue Opsin Expressions in the Chick Are Highly Dependent on Tissue and Serum Conditions

The molecular, cellular or tissue environment can influence the expression of genes and thereby regulate processes of tissue formation. Here we determined the tissue and serum dependence of the expression of all photopigments in the chick by a series of distinct retinal cell cultures, analyzed by RT-PCR using specific primers for all four opsins and rhodopsin followed by quantitative scanning of the respective gel bands. For comparison, we first determined expression of all opsins during normal chick retinogenesis, which began with red and violet opsins at E12, shortly followed by blue and green opsins and finally rhodopsin at E14. This period corresponds to the time of synaptogenesis in the inner retina. All cultures were started with 6-day-old dissociated retinal cells. Cells were kept at low or high cell density (called LoDens or HiDens), or they were reaggregated as retinal spheres, whereby all of them were raised at low (2%) or high serum (12%) levels (called LoSer or HiSer). In LoDens/HiSer cultures, expression of all opsins was weak. At HiDens/LoSer red and green opsin expression was strong, while rhodopsin and violet/blue remained low. In HiDens/HiSer cultures the expression of red and green was strong; rhodopsin was almost normal, while violet and green were low. In reaggregates at high serum the expression came closest to a normal retina, but violet and blue opsins were still below normal. At low serum, however, violet and blue were negligible and rhodopsin was low. This in vitro study shows that rhodopsin, followed by violet and blue opsin expressions is highly dependent on serum, cell density and tissue conditions, while red and green opsins are more autonomous.