Developmental defects and p53 hyperacetylation in Sir2 homolog (SIRT1)-deficient mice

Acetylation of p53 augments its site-specific DNA binding both in vitro and in vivo

p53 promotes tumor suppression through its ability to function as a transcriptional factor and is activated by posttranslational modifications that include acetylation. Our earlier study demonstrated that p53 acetylation can enhance its sequence-specific DNA binding in vitro, and this notion was later confirmed in several other studies. However, a recent study has reported that in vitro acetylation of p53 fails to stimulate its DNA binding to large DNA fragments, raising an important issue that requires further investigation. Here, we show that unacetylated p53 is able to bind weakly to its consensus site within the context of large DNA fragments, although it completely fails to bind the same site within short oligonucleotide probes. Strikingly, by using highly purified and fully acetylated p53 proteins obtained from cells, we show that acetylation of the C-terminal domain can dramatically enhance site-specific DNA binding on both short oligonucleotide probes and long DNA fragments. Moreover, endogenous p53 apparently can be fully acetylated in response to DNA damage when both histone deacetylase complex 1 (HDAC1)- and Sir2-mediated deacetylation are inhibited, indicating dynamic p53 acetylation and deacetylation events during the DNA damage response. Finally, we also show that acetylation of endogenous p53 indeed significantly augments its ability to bind an endogenous target gene and that p53 acetylation levels correlate well with p53-mediated transcriptional activation in vivo. Thus, our results clarify some of the confusion surrounding acetylation-mediated effects on p53 binding to DNA and suggest that acetylation of p53 in vivo may contribute, at least in part, to its transcriptional activation functions.

Diversity in the Sir2 family of protein deacetylases

The silent information regulator (Sir2) family of protein deacetylases (Sirtuins) are nicotinamide adenine dinucleotide (NAD)(+)-dependent enzymes that hydrolyze one molecule of NAD(+) for every lysine residue that is deacetylated. The Sirtuins are phylogenetically conserved in eukaryotes, prokaryotes, and Archeal species. Prokaryotic and Archeal species usually have one or two Sirtuin homologs, whereas eukaryotes typically have multiple versions. The founding member of this protein family is the Sir2 histone deacetylase of Saccharomyces cerevisiae, which is absolutely required for transcriptional silencing in this organism. Sirtuins in other organisms often have nonhistone substrates and in eukaryotes, are not always localized in the nucleus. The diversity of substrates is reflected in the various biological activities that Sirtuins function, including development, metabolism, apoptosis, and heterochromatin formation. This review emphasizes the great diversity in Sirtuin function and highlights its unusual catalytic properties.

Effects of diethylstilboestrol on testicular function and luteinizing hormone receptors

Adult male Fisher-344 rats were implanted with DES-filled or empty Silastic capsules. After 14 weeks, capsules were removed and a second group of rats received DES capsules. Seven weeks later, all the rats were sacrificed. DES treatment decreased body, testes and seminal vesicle weights, and removal of the capsules partially restored the weight of these organs. The concentration of testicular LH receptors was increased by DES treatment. Circulating PRL levels were increased and gonadotropin levels were reduced in all animals having received DES at anytime. Plasma testosterone (T) levels were similar in all groups, but testicular T levels were reversibly decreased by DES. Similarly, whereas basal incubation media T levels were unchanged by DES treatment, the steroidogenic response in vitro to hCG was abolished by the presence of DES, and removal of the capsules restored this response. It appears that in this animal model DES and PRL exert opposing effects on testicular LH receptor.