Variations in transcription-repair coupling in mouse cells

Activation of cell death mediated by apoptosis-inducing factor due to the absence of poly(ADP-ribose) glycohydrolase

We previously demonstrated that the absence of poly(ADP-ribose) glycohydrolase (PARG) led to increased cell death following DNA-damaging treatments. Here, we investigated cell death pathways following UV treatment. Decreased amounts of PARG-null embryonic trophoblast stem (TS) cells were observed following doses of 10-100 J/m2 as compared to wild-type cells. In wild-type cells, caspase-cleaved poly(ADP-ribose) polymerase-1 (PARP-1) and activated caspase-3 were detected 12-24 h after UV treatment. Surprisingly, both were detected at decreased levels only after 24 h in PARG-null TS cells, indicating a decreased level and delayed presence of caspase-mediated events. Further, a time- and dose-dependent accumulation of poly(ADP-ribose) (PAR) levels after UV was observed in PARG-null TS cells and not in wild-type cells. Determination of the levels of nicotinamide adenine dinucleotide (NAD+), the substrate for PAR synthesis and a coenzyme in cellular redox reactions, demonstrated a UV dose-dependent decrease in the level of NAD+ in wild-type cells, while NAD+ levels in PARG-null TS cells remained at higher levels. This indicates no depletion of NAD+ in PARG-null TS cells following increased levels of PAR. Lastly, cell death mediated by apoptosis-inducing factor (AIF) was analyzed because of its dependence on increased PAR levels. The results demonstrate nuclear AIF translocation only in PARG-null TS cells, which demonstrates the presence of AIF-mediated cell death. Herein, we provide compelling evidence that the absence of PARG leads to decreased caspase-3 activity and the specific activation of AIF-mediated cell death. Therefore, the absence of PARG may provide a strategy for specifically inducing an alternative apoptotic pathway.

Lack of strand-specific repair of UV-induced DNA lesions in three genes of the archaeon Sulfolobus solfataricus

In all organisms, specialized systems are devoted to repair of DNA lesions induced by exposure to UV light. In both Eucarya and Bacteria, UV-induced pyrimidine dimers in the transcribed strand of active genes are repaired at a faster rate compared to the non-transcribed strand and the rest of the genome. Preferential repair of transcribed strands requires the Transcription-Repair Coupling Factor in Escherichia coli and the CSA and CSB proteins in humans. These factors are needed for coupling of transcription to nucleotide excision repair (NER), a major pathway for repair of UV-induced lesions. Whereas transcription-coupled NER (TC-NER) is an evolutionary conserved process, not all active genes show preferential repair of transcribed strands. The existence of a NER pathway in the Archaea has not been demonstrated directly, yet it is suggested by the presence and properties of homologues of NER nucleases and helicases. However, none of the proteins responsible for the lesion recognition steps or for TC-NER has been found in archaeal genomes. Moreover, the kinetics of gene or strand-specific repair has never been investigated in any organism of this domain. We have analysed the kinetics of repair of UV-induced DNA damage in the transcribed and non-transcribed strands of three genes of the hyperthermophilic archaeon Sulfolobus solfataricus. We found that in all three genes the two strands are repaired with the same efficiency with each other and with the genome in general, thus providing no evidence of strand bias or transcription coupling of the repair process in the genes analysed. Further studies will be required to test the existence of a transcription-coupled repair pathway in other archaeal genes and to elucidate the mechanism of UV lesion recognition and repair in Archaea.

Somatostatin inhibits IGF-1 mediated induction of VEGF in human retinal pigment epithelial cells

Neovascularization stimulated by IGF-1 mediated induction of vascular endothelial growth factor (VEGF) is one of the leading causes of blindness in humans. It plays a central role in the pathogenesis of proliferative diabetic retinopathy (DR), neovascular glaucoma, exudative age-related macular degeneration (AMD) and retinopathy of prematurity. Neovascularization is a multi-step process that involves complex interactions of a variety of mitogenic factors such as VEGF and IGF-I which are produced locally in the human eye by a variety of cells including retinal pigment epithelial (RPE) cells, retinal capillary pericytes, endothelial cells, Mueller cells and ganglion cells. We hypothesized that somatostatin would inhibit the IGF-1 signal transduction pathway in RPE cells, resulting in decreased VEGF production. We have observed expression of somatostatin receptor protein in retinal pigment epithelial (RPE) cells of the human eye using immunohistochemistry and have confirmed expression of somatostatin receptors in cultured human RPE cells using reverse transcriptase-PCR. IGF-1 induced a dose dependent increase in IGF-1R phosphorylation and in VEGF mRNA levels in cultured human RPE cells. Somatostatin and octreotide, a somatostatin analogue, inhibited IGF-1 receptor (IGF-1R) phosphorylation and decreased VEGF production. Both IGF-1R phosphorylation and accumulation of VEGF mRNA were inhibited by physiological levels of somatostatin and octreotide (1 nM). These results demonstrate somatostatin and octreotide mediated attenuation of both IGF-1R signal transduction and VEGF mRNA accumulation via somatostatin receptor type 2 (sst2). Furthermore, these data suggest a rationale for the use of octreotide as a prophylactic and therapeutic option in disease states that cause ocular neovascularization.

Segmental expression of somatostatin receptor subtypes sst(1) and sst(2) in tubules and glomeruli of human kidney

Somatostatin is known to modulate mesangial and tubular cell function and growth, but the somatostatin receptor (sst) subtypes responsible for these effects have not been defined. There are at least five different sst receptor subtypes (sst(1)-sst(5)). We used RT-PCR to demonstrate that normal human kidney consistently expresses mRNA for sst(1) and sst(2) (9 of 9 donors). Some donors expressed sst(4) or sst(5) mRNA, but none expressed sst(3) mRNA. Expression of sst(1) and sst(2) was further assessed by staining serial sections of normal human kidney with sst(1) and sst(2) antisera, Arachis hypogaea (AH) lectin (to define distal tubule/collecting duct cells), Phaseolus vulgaris lectin (proximal tubules), and Tamm-Horsfall protein (THP) antiserum (thick ascending limb of the loop of Henle). Specificity of antisera was demonstrated by transfection and absorption studies. Sst(2), but not sst(1), was expressed in glomeruli. Intense sst(1) and sst(2) staining localized exclusively to AH+ and THP+ tubules. Thus sst(1) and sst(2) subtype-selective analogs may be useful to beneficially modulate renal cell function in pathological conditions.

Somatostatin receptor gene expression in neuroblastoma

Somatostatin receptor expression is a favorable prognostic factor in human neuroblastoma. Somatostatin receptors have been demonstrated in vitro by pharmacologic analysis of tumor tissue and in vivo by diagnostic radioreceptor scintigraphy. However, which receptor subtypes (sst(1), sst(2), sst(3), sst(4), and sst(5)) are expressed in these tumors has not yet been delineated. We used RT-PCR to analyze expression of the five somatostatin receptor genes in 32 neuroblastoma tumor specimens. All 32 tumor specimens expressed mRNA for c-abl and sst(1); sst(2) mRNA was detected in 27/32 samples and somatostatin mRNA was detected in 30/32 tumor specimens. The remaining receptor subtypes, sst(3), sst(4), and sst(5) were variably expressed. Receptor protein for sst(1) and sst(2) was visualized in tumor neuroblasts as well as in endothelial cells of tumor vessels using immunostaining with specific anti-receptor antibodies. The effect of high expression of somatostatin receptors on cell proliferation was examined in SKNSH neuroblastoma cells transfected with sst(1) and sst(2). SS(14) binding to wild-type SKNSH cells was undetectable; but the native peptide bound with high affinity to the SKNSH/sst(1) and SKNSH/sst(2) neuroblastoma cell lines. Pharmacologic analysis of binding with two long-acting analogues, CH275 and octreotide, confirmed selective expression of sst(1) and sst(2) in stably transfected SKNSH cells. Formation of neuroblastoma xenograft tumors in nude mice was significantly delayed for both SKNSH/sst(1) (P<0.001) and SKNSH/sst(2) (P<0.05) cells compared to wild-type SKNSH. We conclude that: (1) Somatostatin receptors, sst(1) and sst(2), are expressed in the majority of neuroblastomas at diagnosis; and (2) upregulation of functional sst(1) or sst(2) in neuroblastoma cell lines suppresses tumorigenicity in a xenograft model. These observations suggest that somatostatin receptors may be a useful therapeutic target in neuroblastoma.

The octadecanoid signalling pathway in plants mediates a response to ultraviolet radiation

Many plant genes that respond to environmental and developmental changes are regulated by jasmonic acid, which is derived from linolenic acid via the octadecanoid pathway. Linolenic acid is an important fatty-acid constituent of membranes in most plant species and its intracellular levels increase in response to certain signals. Here we report that irradiation of tomato leaves with ultraviolet light induces the expression of several plant defensive genes that are normally activated through the octadecanoid pathway after wounding. The response to ultraviolet light is blocked by an inhibitor of the octadecanoid pathway and it does not occur in a tomato mutant defective in this pathway. The ultraviolet irradiation maximally induces the defence genes at levels where cyclobutane pyrimidine dimer formation, an indicator of DNA damage, is less than 0.2 dimers per gene. Our evidence indicates that this plant defence response to certain wavelengths of ultraviolet radiation requires the activation of the octadecanoid defence signalling pathway.

Strand breaks are repaired efficiently in human ribosomal genes

We examined repair of DNA strand breaks induced by the anti-cancer drug bleomycin in both Pol I and Pol II transcribed genes in permeabilized human fibroblasts. The majority of these breaks (>80%) are single strand breaks (SSBs) thought to be repaired by base excision repair enzymes. Repair was examined in each strand of a 7. 2-kilobase fragment, completely within the Pol I transcribed region of ribosomal DNA (rDNA) and an 8.3-kilobase fragment completely within the Pol II transcribed region of the dihydrofolate reductase (DHFR) gene. Bleomycin dose-response studies revealed no bias for SSBs in either strand of the rDNA fragment. Furthermore, repair of SSBs is rapid (approximately 80% resealed in 60 min) in both the transcribed and nontranscribed strands of rDNA. Rapid repair of SSBs is also observed in both strands of the DHFR gene (approximately 60% resealed in 60 min). In contrast, little (or no) repair of UV photodimers occurs in either strand of human rDNA, regardless of whether cells are confluent or actively growing. Thus, DNA lesions in human ribosomal genes may be more accessible to base excision repair enzymes than those involved in nucleotide excision repair.

Rad23 is required for transcription-coupled repair and efficient overrall repair in Saccharomyces cerevisiae

The repair of UV-induced photoproducts (cyclobutane pyrimidine dimers) in a well-characterized minichromosome, genomic DNA, and a transcribed genomic gene (RPB2) of a rad23delta mutant of Saccharomyces care was examined. Isogenic wild-type cells show a strong bias for the repair of the transcribed strands in both the plasmid and genomic genes and efficient overall repair of both DNAs (>80% of the dimers were removed in 6 h). However, the rad23delta mutant shows (i) no strand bias for repair in these genes and decreased repair of both strands, (ii) partial repair of genomic DNA (approximately 45% in 6 h), and (iii) very poor repair of the plasmid overall approximately 15% in 6 h). These features, coupled with the decreased UV survival of rad23delta cells, indicate that Rad23 is required for both transcription-coupled repair and efficient overall repair in S. cerevisiae.