Posted on behalf of Retread
An incredible article appeared last month in the journal Science. If it can be verified and if it applies generally, our conception of just how genes coding for protein are turned on will be radically changed (yes, there are many other kinds of genes other than those coding for proteins). If DNA compaction, nucleosomes, histones, lysine methylation and demethylation, the histone code, nuclear hormone receptors (particularly the estrogen receptor), DNA glycosylase and topoisomerase aren’t old friends have a look at the first comment on this post for the background you need. Don’t worry, there is plenty of chemistry to follow.
Some histone code modifications are reversible, particularly acetylation of the epsilon amino group of lysine. Enzymes acetylating histone lysines are called histone acetylases, those removing it are called histone deacetylatases (HDACs). However, lysine methylation was thought to be permanent until ’04 when several enzymes able to demethylate lysine were found. One such enzyme is called LSD1 (it has nothing to do with the hallucinogen). It removes the two methyl groups from lysine #9 of histone #3 (H3K9me2). If this modification is present on a nucleosome near a gene, the gene is silenced, so the methyls must be removed so the protein it codes for can be made.
The estrogen receptor + estrogen complex bound to the ERE (the estrogen response element – a 15 nucleotide DNA sequence) triggers H3K9me2 removal. The process of demethylation is oxidative (how else would you split a nitrogen to hydrocarbon bond?). Hydrogen peroxide is produced, a loose cannon which oxidizes the juicy electron-rich bases of DNA nearby, forming in particular 8 oxo-guanine, as guanine is the most easily oxidized DNA base. Since 21% of the DNA bases in our genome are guanine, H2O2 doesn’t have far to look. This calls in some fairly heavy artillery (DNA glycosylase to remove the 8 oxo-guanine, topoisomerase IIbeta to unwind the DNA so it can be repaired, the repair enzymes, etc, etc…). Naturally this opens up the compacted DNA structure around the gene allowing RNA polymerase II to do its work transcribing the estrogen responsive gene into mRNA (once the damage is repaired).
So according to this paper, estrogen turns on gene transcription by damaging DNA. This is fantastic (if true). There’s more. The estrogen receptor is but one member of a group of proteins called nuclear hormone receptors. The name comes from the fact that other hormones (progesterone, androgen, thyroid, glucocorticoids, mineralocorticoids) have their own proteins that turn on (or turn off) genes the same way. Subsequently it was found that some vitamin metabolites (vitamin D3, vitamin A) have similar receptors even though they aren’t hormones. The human genome contains 48 such proteins. Less than half of them have known ligands. Those with known ligands have their finger in just about every metabolic pie in the cell.
One final point. It has been estimated that 8-oxoguanine is formed 100,000 times each day in every cell. Perhaps its formation is physiologic rather than pathologic. Where does that leave antioxidant therapy, which has been touted to do everything but cure hemorrhoids? Well, one such trial was done on 29,000 Finnish men at high risk for lung cancer (they were smokers) [New England J. Med. vol. 330 pp. 1029-1035 (1994)] Alpha tocopherol (one antioxidant used in the study) didn’t decrease the incidence of lung cancer, and there was an 18% higher incidence of lung cancer among the men receiving beta carotene (another antioxidant). In medicine, theory is great but data trumps it every time.