Arnold B. Barton

Stable expression of constitutively-activated STAT3 in benign prostatic epithelial cells changes their phenotype to that resembling malignant cells

BackgroundSignal transducers and activators of transcription (STATs) are involved in growth regulation of cells. They are usually activated by phosphorylation at specific tyrosine residues. In neoplastic cells, constitutive activation of STATs accompanies growth dysregulation and resistance to apoptosis through changes in gene expression, such as enhanced anti-apoptotic gene expression or reduced pro-apoptotic gene expression. Activated STAT3 is thought to play an important role in prostate cancer (PCA) progression. Because we are interested in how persistently-activated STAT3 changes the cellular phenotype to a malignant one in prostate cancer, we used expression vectors containing a gene for constitutively-activated STAT3, called S3c, into NRP-152 rat and BPH-1 human benign prostatic epithelial cells.ResultsWe observed that prostatic cell lines stably expressing S3c required STAT3 expression for survival, because they became sensitive to antisense oligonucleotide for STAT3. However, S3c-transfected cells were not sensitive to the effects of JAK inhibitors, meaning that STAT3 was constitutively-activated in these transfected cell lines. NRP-152 prostatic epithelial cells lost the requirement for exogenous growth factors. Furthermore, we observed that NRP-152 expressing S3c had enhanced mRNA levels of retinoic acid receptor (RAR)-α, reduced mRNA levels of RAR-β and -γ, while BPH-1 cells transfected with S3c became insensitive to the effects of androgen, and also to the effects of a testosterone antagonist. Both S3c-transfected cell lines grew in soft agar after stable transfection with S3c, however neither S3c-transfected cell line was tumorigenic in severe-combined immunodeficient mice.ConclusionsWe conclude, based on our findings, that persistently-activated STAT3 is an important molecular marker of prostate cancer, which develops in formerly benign prostate cells and changes their phenotype to one more closely resembling transformed prostate cells. That the S3c-transfected cell lines require the continued expression of S3c demonstrates that a significant phenotypic change occurred in the cells. These conclusions are based on our data with respect to loss of growth factor requirement, loss of androgen response, gain of growth in soft agar, and changes in RAR subunit expression, all of which are consistent with a malignant phenotype in prostate cancer. However, an additional genetic change may be required for S3c-transfected prostate cells to become tumorigenic.

Patterns of meiotic double-strand breakage on native and artificial yeast chromosomes

The preferred positions for meiotic double-strand breakage were mapped onSaccharomyces cerevisiae chromosomes I and VI, and on a number of yeast artificial chromosomes carrying human DNA inserts. Each chromosome had strong and weak double-strand break (DSB) sites. On average one DSB-prone region was detected by pulsed-field gel electrophoresis per 25 kb of DNA, but each chromosome had a unique distribution of DSB sites. There were no preferred meiotic DSB sites near the telomeres. DSB-prone regions were associated with all of the known “hot spots” for meiotic recombination on chromosomes I, III and VI.

Meiotic Recombination at the Ends of Chromosomes in Saccharomyces Cerevisiae

Meiotic reciprocal recombination (crossing over) was examined in the outermost 60–80 kb of almost all Saccharomyces cerevisiae chromosomes. These sequences included both repetitive gene-poor subtelomeric heterochromatin-like regions and their adjacent unique gene-rich euchromatin-like regions. Subtelomeric sequences underwent very little crossing over, exhibiting approximately two- to threefold fewer crossovers per kilobase of DNA than the genomic average. Surprisingly, the adjacent euchromatic regions underwent crossing over at twice the average genomic rate and contained at least nine new recombination “hot spots.” These results prompted an analysis of existing genetic mapping data, which showed that meiotic reciprocal recombination rates were on average greater near chromosome ends exclusive of the subtelomeres. Thus, the distribution of crossovers in S. cerevisiae appears to resemble that found in several higher eukaryotes where the outermost chromosomal regions show increased crossing over.

Molecular Cloning of Chromosome I DNA from Saccharomyces cerevisiae: Characterization of the 54 kb Right Terminal CDC15‐FLO1‐PHO11 Region

Gene density near the ends of Saccharomyces cerevisiae chromosomes is much lower than on the rest of the chromosome. Non‐functional gene‐fragments are common and a high proportion of the sequences are repeated elsewhere in the genome. This sequence arrangement suggests that the ends of chromosomes play a structural rather than a coding role and may be analogous to the highly repeated heterochromatic DNA of higher organisms. In order to evaluate the function of the ends of S. cerevisiae chromosomes, the rightmost 54‐kb of DNA from chromosome I was investigated. The region contains 16 open reading frames (ORFs) and two tRNA genes. Gene‐disruption studies indicated that none of these genes are essential for growth on rich or minimal medium, mating or sporulation. In contrast to the central region where 80% of the genes are transcribed when cells are grown on rich medium, only seven ORFs and the two tRNA genes appeared to produce transcripts. Six of the transcribed ORFs were from the centromere‐proximal part of the region, leaving the rightmost 35‐kb with only a single sequence that is transcribed during vegetative growth. Two genes located 3 and 10‐kb from the chromosome I telomere are almost identical to two genes located somewhat further from the chromosome VIII telomere. Surprisingly, the chromosome VIII copies were transcribed while the chromosome I genes were not. These results suggest that the chromosome I genes may be repressed by a natural telomere position effect. The low level of transcription, absence of essential genes as well as the repetitive nature of these sequences are consistent with their having a structural role in chromosome function.

Telomeric silencing of an open reading frame in Saccharomyces cerevisiae.

The endmost chromosome I ORF is silenced by a natural telomere position effect. YAR073W/IMD1 was found to be transcribed at much higher levels in sir3 mutants and when its adjacent telomere was removed from it. These results suggest that telomeres play a role in silencing actual genes.