Degradation of Transcription Repressor ZBRK1 through the Ubiquitin-Proteasome Pathway Relieves Repression of Gadd45a upon DNA Damage

ABSTRACT Induction of gene expression in response to DNA damage is important for repairing damaged DNA for cell survival. Previously, we identified a novel zinc finger protein, ZBRK1, which contains a KRAB domain at the N terminus, eight zinc fingers at the center, and a BRCA1-binding region at the C terminus. In a BRCA1-dependent manner, ZBRK1 represses Gadd45a transcription through binding to a specific sequence in intron 3. In addition, ZBRK1-binding sequences are located at the regulatory region of many DNA damage-inducible genes, suggesting that ZBRK1 may have a role in DNA damage response. However, it is unclear how transcription repression by ZBRK1 is relieved subsequent to DNA damage. Here we report that ZBRK1 is rapidly degraded upon treatment with the DNA-damaging agents UV and methyl methanesulfonate. Specific proteasome inhibitors block DNA damage-induced degradation of ZBRK1, and the polyubiquitinated form of ZBRK1 is detectable, suggesting that the ubiquitin-proteasome pathway mediates the degradation of ZBRK1. In both BRCA1-proficient and -deficient cells, ZBRK1 is degraded with similar efficiencies independent of BRCA1 E3 ligase activity. By analysis of a series of ZBRK1 mutants, a 44-amino-acid element located between the N-terminal KRAB domain and the eight zinc fingers was found to be sufficient for the DNA damage-induced degradation of ZBRK1. Cells expressing a ZBRK1 mutant lacking the 44-amino-acid element are hypersensitive to DNA damage and are compromised for Gadd45a derepression. These results indicate that ZBRK1 is a novel target for DNA damage-induced degradation and provide a mechanistic explanation of how ZBRK1 is regulated in response to DNA damage.

On the thermal changes accompanying basic substitutions

The author gives an account of a series of experiments which he made on the heat evolved during the mutual reaction of acids and bases upon one another, from which he draws the general conclusion that when the influence of all extraneous circumstances is eliminated from the result, the change of temperature is determined by the na­ture of the base, and not by the acid element of the combination. Hence he deduces the general law that, when one base displaces another from any of its neutral combinations with an acid, the heat evolved or abstracted is always the same, whatever the acid element may be, provided the bases are the same. The base employed in the first set of experiments for displacing others was the hydrate of potash in a state of dilute solution of known strength; this was rapidly mixed, in a suitable apparatus, with an equivalent solution of the salt to be decomposed; the change of temperature which re­sulted was accurately determined, and the due corrections for the influence of the vessels and the specific heats of the solutions and of the precipitates produced, were applied. The experimental results are stated in various tables, from which it appears that the changes of temperature, referred to 1000 parts of water, were, with salts of The differences in the results of experiments with different acids, the author observes, are not greater than usually occur in chemical reactions, in consequence of the uncertainty that exists with regard to the accurate proportions of chemical equivalents. He points out various circumstances in experiments of this nature, which tend to affect the results and lead to inaccurate conclusions, if care be not taken to guard against these sources of error. One of the principal of these is the heat which is generally evolved by the separation of a base, or new compound, in a solid form: and the author discusses the influence of this change on the results deduced from his experi­ments. He considers that these experiments sufficiently establish the general principle announced in the beginning of his paper.

On the heat disengaged during metallic substitutions

In a paper which was published in the Philosophical Transactions for 1844, the author deduced from the experimental inquiry there recorded the general law, that when one base displaces another from any of its neutral combinations with an acid, the heat evolved or abstracted is always the same, whatever that acid element may be, provided the bases are the same. Extending a similar inquiry to salts with metallic bases, he establishes, as the result of the investigation of which an account is given in the present paper, the general principle that when an equivalent of one and the same metal replaces another in a solution of any of its salts of the same order, the heat developed is, with the same metals, constantly the same, the expression “of a solution of the same order” being understood to mean a solution in which the same precipitate is produced by the addition of an alkali, or, on one view of the composition of such salts, in which the metal exists in the same state of oxidation. The metallic salts, in the precipitation of which by other metals the evolved heat was ascertained, were those of copper precipitated by zinc, iron or lead; of silver, precipitated by zinc or copper; and of lead, mercury, and platinum precipitated by zinc: and the acid elements were either the sulphuric, hydrochloric, acetic or formic acids. From the last series of experiments the author deduces, that if three metals A, B, and C, be so related that it is capable of displacing B and C from their combinations, and also B capable of displacing C, then the heat developed in the substitution of A for C will be equal to that developed in the substitution of B for C; and a similar rule may be applied to any number of metals similarly related.

II. On the thermal changes accompanying basic substitutions

In a communication made to the Royal Irish Academy, nearly three years ago, I described a series of experiments on the heat evolved during the mutual reaction of acids and bases upon one another, from which the general conclusion was deduced, that when the influence of all extraneous circumstances is eliminated, the heat is determined by the basic and not by the acid element of the combination. Nearly at the same time an important memoir was published by M. Hess on thermo-che­mistry, in which an opposite result was arrived at, deduced however from a very limited number of experiments, and merely announced by its author, as a probable generalization, the accuracy of which could only be determined by further researches. The principle, as stated by M. Hess, is this, that different bases disengage the same quantity of heat in combining with the same acid. In the present state of chemical knowledge we cannot attempt the resolution of this problem by direct experiments on the anhydrous acids and bases, even if we adopt the hypothesis, no longer universally admitted by chemists, that the proximate constituents of neutral salts are the ordinary acids and bases. Experiments per­formed with the concentrated acids are not adapted to yield simple results, since the mere circumstance of dilution with water produces the evolution of large quantities of heat in the case of some acids, and none, or a very slight variation of temperature in the case of others. It is for this reason that when an alkaline solution is neutral­ized by the addition of an equivalent of nitric acid, the heat disengaged is very dif­ferent, according to the state of concentration of the acid; while the same circum­stance produces little or no effect, when the tartaric acid is employed. If we insti­tute a further comparison between the results, it will be found that while no simple relation exists between the temperatures obtained with different acids in a concen­trated state, there is a very close approximation to an equal development of heat when the same base is neutralized by any dilute acid.