N-Myc and GCN5 regulate significantly overlapping transcriptional programs in neural stem cells.

Here we examine the functions of the Myc cofactor and histone acetyltransferase, GCN5/KAT2A, in neural stem and precursor cells (NSC) using a conditional knockout approach driven by nestin-cre. Mice with GCN5-deficient NSC exhibit a 25% reduction in brain mass with a microcephaly phenotype similar to that observed in nestin-cre driven knockouts of c- or N-myc. In addition, the loss of GCN5 inhibits precursor cell proliferation and reduces their populations in vivo, as does loss of N-myc. Gene expression analysis indicates that about one-sixth of genes whose expression is affected by loss of GCN5 are also affected in the same manner by loss of N-myc. These findings strongly support the notion that GCN5 protein is a key N-Myc transcriptional cofactor in NSC, but are also consistent with recruitment of GCN5 by other transcription factors and the use by N-Myc of other histone acetyltransferases. Putative N-Myc/GCN5 coregulated transcriptional pathways include cell metabolism, cell cycle, chromatin, and neuron projection morphogenesis genes. GCN5 is also required for maintenance of histone acetylation both at its putative specific target genes and at Myc targets. Thus, we have defined an important role for GCN5 in NSC and provided evidence that GCN5 is an important Myc transcriptional cofactor in vivo.

HDAC6 modulates cell motility by altering the acetylation level of cortactin.

Histone deacetylase 6 (HDAC6) is a tubulin-specific deacetylase that regulates microtubule-dependent cell movement. In this study, we identify the F-actin-binding protein cortactin as a HDAC6 substrate. We demonstrate that HDAC6 binds cortactin and that overexpression of HDAC6 leads to hypoacetylation of cortactin, whereas inhibition of HDAC6 activity leads to cortactin hyperacetylation. HDAC6 alters the ability of cortactin to bind F-actin by modulating a "charge patch" in its repeat region. Introduction of charge-preserving or charge-neutralizing mutations in this cortactin repeat region correlates with the gain or loss of F-actin binding ability, respectively. Cells expressing a charge-neutralizing cortactin mutant were less motile than control cells or cells expressing a charge-preserving mutant. These findings suggest that, in addition to its role in microtubule-dependent cell motility, HDAC6 influences actin-dependent cell motility by altering the acetylation status of cortactin, which, in turn, changes the F-actin binding activity of cortactin.

Reactivation of the silenced and imprinted alleles of ARHI is associated with increased histone H3 acetylation and decreased histone H3 lysine 9 methylation.

ARHI has been identified as a maternally imprinted tumor suppressor gene that maps to chromosome 1p31 and whose expression is markedly down-regulated in breast cancer. To explore possible mechanisms that could silence ARHI expression, we have tested the importance of DNA methylation, histone acetylation and histone methylation in regulating ARHI expression. We found that treatment with CpG demethylating agents and/or histone deacetylase inhibitors could reactivate both the silenced and the imprinted alleles of this tumor suppressor gene. Reactivation of ARHI expression by these reagents is related to the methylation status of the CpG islands in the ARHI promoter, especially CpG island II. Chromatin immunoprecipitation assays revealed that histone H3 lysine 9/18 acetylation levels associated with ARHI in normal cells were significantly higher than those in breast cancer cell lines that lacked ARHI expression. Treatment with a CpG demethylating agent and/or histone deacetylase inhibitor could increase ARHI expression in breast cancer cells, with a corresponding increase in histone H3 lysine 9/18 acetylation and decrease in histone H3 lysine 9 methylation.

Epigenetic regulation of ARHI in breast and ovarian cancer cells.

ARHI (Ras homologue member I) encodes a 26-kDa GTPase with 50-60% amino acid homology to Ras and Rap. ARHI and Ras share similar GTP/GDP binding domains, but exert opposite functions. ARHI is one of the first reported tumor suppressors in the ras superfamily. ARHI is expressed consistently in normal breast and ovarian epithelial cells, but not in breast or ovarian cancers. The loss of ARHI can be related to tumor progression. Reexpression of ARHI induces apoptosis of breast and ovarian cancer cells by a caspase-independent, calpain-dependent pathway. ARHI is consistently expressed in normal breast and ovarian epithelial cells but is dramatically downregulated in more then 70% of breast and ovarian cancers. ARHI is maternally imprinted with methylation of the three CpG islands in the maternal allele of normal cells. ARHI is expressed only from the paternal allele whose three CpG islands are not methylated. Loss of ARHI expression can occur through a genetic event, with loss of heterozygosity observed in 40% of breast, ovarian, and pancreatic cancers; but it can also occur through epigenetic mechanisms, including DNA methylation, histone deacetylation, histone methylation, and transcriptional regulation. Our data suggest that acetylation and methylation of chromatin associated with the ARHI promoter leads to loss of both ARHI expression and the ability to suppress tumor growth. Changes in chromatin that silence ARHI may be driven by methylation-dependent and -independent pathways. Reactivation of both the silenced paternal and imprinted maternal alleles can be achieved by demethylation and inhibition of histone deacetylation.