Expression of PRV-1 Decreases Dependency of Cells on Growth Factors for Proliferation.

Polycythemia rubra vera-1 (PRV-1) is a GPI-linked protein expressed on surface of neutrophils. Polycythemia vera and essential thrombocythemia were found to be associated with an increase in the level of PRV-1 mRNA; however, the function of PRV-1 remains unknown. We have studied the functional effect of expression of PRV-1, both in heterologous cell line and in neutrophils. cDNA encoding PRV-1 was subcloned from a leukocyte cDNA library and expressed in Chinese hamster ovary cells (CHO). After seeding equal numbers of CHO cells stably transfected with PRV-1 or empty plasmid, cell count was conducted at different time intervals and under different concentration of fetal bovine serum (FBS) in growth media. The numbers of PRV-1 transfected cells were higher than sham-transfected cells at 24, 48, 96 and 144 hrs after seeding, and this difference was enhanced with cells growing in a medium with a reduced concentration of serum with the maximal effect seen in serum-free medium. The percentage of apoptotic cells was similar between PRV-1 and sham-transfected cells. These results are suggestive of that CHO cells expressing PRV-1 proliferate faster than sham-transfected cells. The difference in proliferation rate was more significant at lower concentrations of serum and was the highest in serum-free medium. An immunoblot analysis with anti-phosphotyrosine antibody on the whole cell lysate demonstrated that the presence of PRV-1 alters cell signaling. A 60kDa protein band that disappeared after removal of serum in growth medium of sham-transfected cells continued to be present in the PRV-1 transfected cell lysate regardless of the presence or absence of FBS in the media. In order to understand the function of PRV-1 in neutrophils, we used the fact that in 85% of individuals PRV-1 is expressed only by a subgroup of neutrophils. We separated PRV-1 expressing neutrophils of an individual from those lacking this protein in the same donor, using anti-PRV-1 monoclonal antibody. We then compared the gene expression profile of the two groups of neutrophil using DNA microarray technique. Expression of PRV-1 was associated with several folds increase in expression of growth factors (fracture callus-1, X 14.9 times; Insulin-like 3, X 6.7; neural proliferation differentiation, and control-1 or NPDC1, X 3.4 times) and post-translational modifying enzyme of cell surface protein (N-acetylase N sulfotransferase-1, X 18.9 times). Under physiologic conditions, transcriptional regulation of the PRV-1 gene limits its expression to only a certain percentage of neutrophils. In order to investigate the role of epigenetic factors in regulation of transcription of the PRV-1 gene, we analyzed methylation of CpG dinucleotide in promoter of the PRV-1 gene in the genomic DNA obtained from neutrophils expressing PRV-1 to those that don’t express this protein in the same individual. Expression of PRV-1 was associated with a decrease in methylation of the CpG dinucleotide located 2937 bp before initiation codon (45% methylated in PRV-1 negative neutrophils compared to 30% methylated in PRV positive neutrophils). We are currently studying the methylation pattern of CpG islands inside the PRV-1 gene. Studying the effect of DNA methylation on expression of PRV-1 in physiologic conditions, may shed light to the mechanism of overexpression of PRV-1 in MPD.