BRCA1 regulates acetylation and ubiquitination of estrogen receptor-alpha

Inherited mutations of the breast cancer susceptibility gene BRCA1 confer a high risk for breast cancer development. The (300)RXKK and (266)KXK motifs have been identified previously as sites for acetylation of the estrogen receptor-alpha (ER-alpha), and (302)K was also found to be a site for BRCA1-mediated mono-ubiquitination of ER-alpha in vitro. Here we show that ER-alpha proteins with single or double lysine mutations of these motifs (including K303R, a cancer-associated mutant) are resistant to inhibition by BRCA1, even though the mutant ER-alpha proteins retain the ability to bind to BRCA1. We also found that BRCA1 overexpression reduced and knockdown increased the level of acetylated wild-type ER-alpha, without changing the total ER-alpha protein level. Increased acetylation of ER-alpha due to BRCA1 small interfering RNA was dependent upon phosphatidylinositol 3-kinase/Akt signaling and on up-regulation of the coactivator p300. In addition, using an in vitro acetylation assay, we found that in vitro-translated wild-type BRCA1 but not a cancer-associated point mutant (C61G) inhibited p300-mediated acetylation of ER-alpha. Furthermore, BRCA1 overexpression increased the levels of mono-ubiquitinated ER-alpha protein, and a BRCA1 mutant that is defective for ubiquitin ligase activity but retains other BRCA1 functions (I26A) did not ubiquitinate ER-alpha or repress its activity in vivo. Finally, ER-alpha proteins with mutations of the (300)RXKK or (266)KXK motifs showed modest or no BRCA1-induced ubiquitination. We propose a model in which BRCA1 represses ER-alpha activity, in part, by regulating the relative degree of acetylation vs. ubiquitination of ER-alpha.

Hydrophile scanning as a complement to alanine scanning for exploring and manipulating protein-protein recognition: application to the Bim BH3 domain

Alanine scanning has been widely employed as a method of identifying side chains that play important roles in protein-protein and protein-peptide interactions. Here we show how an analogous and complementary technique, hydrophile scanning, can provide additional insight on such interactions. Mutation of a wild-type residue to alanine removes most of the side-chain atoms, and the effect of this removal is typically interpreted to indicate contribution of the deleted side chain to the stability of the complex. Hydrophile scanning involves systematic mutation of wild-type residues to a cationic or anionic residue (lysine or glutamic acid, in this case). We find that the results of these mutations provide insights on interactions between polypeptide surfaces that are complementary to the information obtained via alanine scanning. We have applied this technique to a peptide that corresponds to the BH3 domain of the pro-apoptotic protein Bim. The wild-type Bim BH3 domain binds strongly to the anti-apoptotic proteins Bcl-x(L) and Mcl-1. Combining information from the alanine, lysine, and glutamic acid scans has enabled us to identify Bim BH3 domain mutants containing only two or three sequence changes that bind very selectively either to Bcl-x(L) or Mcl-1. Our findings suggest that hydrophile scanning may prove to be a broadly useful tool for revealing sources of protein-protein recognition and for engineering selectivity into natural sequences.

Altered GABAergic neurotransmission is associated with increased kainate-induced seizure in prostaglandin-endoperoxide synthase-2 deficient mice

Excitotoxicity involves over activation of brain excitatory glutamate receptors and has been implicated in neurological, neurodegenerative and neuropsychiatric diseases. Metabolism of arachidonic acid (AA) through the phospholipase A(2) (PLA(2))/prostaglandin-endoperoxide synthase (PTGS) pathway is increased after excitotoxic stimulation. However, the individual roles of the PTGS isoforms in this process are not well established. We assessed the role of the PTGS isoforms in the process of excitotoxicity by exposing mice deficient in either PTGS-1 (PTGS-1(-/-)) or PTGS-2 (PTGS-2(-/-)) to the prototypic excitotoxin, kainic acid (KA). Seizure intensity and neuronal damage were significantly elevated in KA-exposed PTGS-2(-/-), but not in PTGS-1(-/-), mice. The increased susceptibility was not associated with an alteration in KA receptor binding activity or mediated through the CB1 endocannabinoid receptor. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was decreased in the CA1 pyramidal neurons of PTGS-2(-/-) mice, suggesting an alteration of GABAergic function. In wild-type mice, six weeks treatment with the PTGS-2 selective inhibitor celecoxib recapitulated the increased susceptibility to KA-induced excitotoxicity observed in PTGS-2(-/-) mice, further supporting the role of PTGS-2 in the excitotoxic process. The increased susceptibility to KA was also associated with decreased brain levels of PGE(2), a biomarker of PTGS-2 activity. Our results suggest that PTGS-2 activity and its specific products may modulate neuronal excitability by affecting GABAergic neurotransmission. Further, inhibition of PTGS-2, but not PTGS-1, may increase the susceptibility to seizures.

Chronic carbamazepine selectively downregulates cytosolic phospholipase A2 expression and cyclooxygenase activity in rat brain

BACKGROUND

Carbamazepine is a mood stabilizer used as monotherapy or as an adjunct to lithium in the treatment of acute mania or the prophylaxis of bipolar disorder. Based on evidence that lithium and valproate, other mood stabilizers, reduce brain arachidonic acid turnover and its conversion via cyclooxygenase to prostaglandin E(2) in rat brain, one possibility is that carbamazepine also targets the arachidonic acid cascade.

METHODS

To test this hypothesis, carbamazepine was administered to rats by intraperitoneal injection at a daily dose of 25 mg/kg for 30 days.

RESULTS

Carbamazepine decreased brain phospholipase A(2) activity and cytosolic phospholipase A(2) protein and messenger RNA levels without changing significantly protein and activity levels of calcium-independent phospholipase A(2) or secretory phospholipase A(2). Cyclooxygenase activity was decreased in carbamazepine-treated rats without any change in cyclooxygenase-1 or cyclooxygenase-2 protein levels. Brain prostaglandin E(2) concentration also was reduced. The protein levels of other arachidonic acid metabolizing enzymes, 5-lipoxygenase and cytochrome P450 epoxygenase, were not significantly changed nor was the brain concentration of the 5-lipoxygenase product leukotriene B(4).

CONCLUSIONS

Carbamazepine downregulates cytosolic phospholipase A(2)-mediated release of arachidonic acid and its subsequent conversion to prostaglandin E(2) by cyclooxygenase. These effects may contribute to its therapeutic actions in bipolar disorder.