Alterations in nucleoli, including increased numbers, increased size, altered architecture and increased function are hallmarks of
prostate cancer cells. The mechanisms that result in increased nucleolar size, number and function in
prostate cancer have not been fully elucidated. The nucleolus is formed around repeats of a transcriptional unit encoding a
45S ribosomal RNA (
rRNA) precursor that is then processed to yield the mature 18S, 5.8S and 28S
RNA species. Although it has been generally accepted that
tumor cells overexpress rRNA species, this has not been examined in clinical
prostate cancer. We find that indeed levels of the
45S rRNA, 28S, 18S and 5.8S are overexpressed in the majority of human primary
prostate cancer specimens as compared with matched benign tissues. One mechanism that can alter nucleolar function and structure in
cancer cells is hypomethylation of CpG dinucleotides of the upstream
rDNA promoter region. However, this mechanism has not been examined in
prostate cancer. To determine whether rRNA overexpression could be explained by hypomethylation of these CpG sites, we also evaluated the DNA methylation status of the
rDNA promoter in
prostate cancer cell lines and the clinical specimens.
Bisulfite sequencing of genomic
DNA revealed two roughly equal populations of loci in cell lines consisting of those that contained densely methylated
deoxycytidine residues within CpGs and those that were largely unmethylated. All clinical specimens also contained two populations with no marked changes in methylation of this region in
cancer as compared with normal. We recently reported that MYC can regulate rRNA levels in human
prostate cancer; here we show that MYC
mRNA levels are correlated with 45S, 18S and
5.8S rRNA levels. Further, as a surrogate for nucleolar size and number, we examined the expression of
fibrillarin, which did not correlate with rRNA levels. We conclude that rRNA levels are increased in human
prostate cancer, but that hypomethylation of the
rDNA promoter does not explain this increase, nor does hypomethylation explain alterations in nucleolar number and structure in
prostate cancer cells. Rather, rRNA levels and nucleolar size and number relate more closely to MYC overexpression.