Histone Deacetylation of RB-Responsive Promoters: Requisite for Specific Gene Repression but Dispensable for Cell Cycle Inhibition

Abstract

The retinoblastoma tumor suppressor protein (RB) is targeted for inactivation in the majority of human tumors, underscoring its critical role in attenuating cellular proliferation. RB inhibits proliferation by repressing the transcription of genes that are essential for cell cycle progression. To repress transcription, RB assembles multiprotein complexes containing chromatin-modifying enzymes, including histone deacetylases (HDACs). However, the extent to which HDACs participate in transcriptional repression and are required for RB-mediated repression has not been established. Here, we investigated the role of HDACs in RB-dependent cell cycle inhibition and transcriptional repression. We find that active RB mediates histone deacetylation on cyclin A, Cdc2, topoisomerase II␣, and thymidylate synthase promoters. We also demonstrate that this deacetylation is HDAC dependent, since the HDAC inhibitor trichostatin A (TSA) prevented histone deacetylation at each promoter. However, TSA treatment blocked RB repression of only a specific subset of genes, thereby demonstrating that the requirement of HDACs for RB-mediated transcriptional repression is promoter specific. The HDAC-independent repression was not associated with DNA methylation or gene silencing but was readily reversible. We show that this form of repression resulted in altered chromatin structure and was dependent on SWI/SNF chromatin remodeling activity. Importantly, we find that cell cycle inhibitory action of RB is not intrinsically dependent on the ability to recruit HDAC activity. Thus, while HDACs do play a major role in RB-mediated repression, they are dispensable for the repression of critical targets leading to cell cycle arrest.The retinoblastoma tumor suppressor, RB, functions as a negative regulator of cell cycle progression that is frequently inactivated in human cancers (10,22,75). In G 0 and early G 1 cells, RB is hypophosphorylated and inhibits the transition into the S phase of the cell cycle. Mitogenic signaling cascades activate CDK4/cyclin D1 complexes that initiate the phosphorylation of RB on a subset of serine and threonine residues (65). Subsequent phosphorylation catalyzed by CDK2/cyclin E leads to RB hyperphosphorylation (23). These combined events serve to functionally inactivate RB and thereby facilitate progression through the S phase (2, 23). In contrast with mitogenic signaling pathways, antimitogens (e.g., transforming growth factor ␤ or DNA damage) serve to inhibit RB phosphorylation and prevent progression through the cell cycle (28). Thus, RB integrates multiple signaling cascades to modify proliferation. In cancer, RB is inactivated through the activity of several disparate mechanisms. These modes of inactivation include the biallelic inactivation of the RB gene, binding by oncoproteins of DNA tumor viruses, and aberrant phosphorylation (2,3,30,59,62,76). Through these distinct mechanisms of RB inactivation, tumors are able to evade cell cycle regulation and proliferate uncontrollably.RB inhibits cellular proliferation by...

The role of histone acetylation versus DNA damage in drug-induced senescence and apoptosis

“…Chromatin acetylation is known to result in its decondensation, thus facilitating access of transcription machinery to the promoter regions of tumor suppressor genes. 27 Chromatin immunoprecipitation results (Figure 7b) demonstrated that depending on the stress level, DNA damaging drugs can induce histone acetylation and induction of the tumor suppressor p21/WAF1 without significant induction of DNA damage.…”

Sodium butyrate and tributyrin induce in vivo growth inhibition and apoptosis in human prostate cancer

“…It cannot be ruled out that the observed changes could be a parallel issue as for example growth inhibition, induced by sodium butyrate, has been described in a cell line that has abrogated Rb function (Rashid et al, 2001). There might be other and potentially even more important structures such as cyclin A and D, p27 and protein kinase C, which are involved in the process of butyrate-induced growth inhibition (Chen et al, 2003;Kim et al, 2003;Siddiqui et al, 2003). Nevertheless, exploring the activation of p-21, including the role of Sp1 transcription factor, is a very promising approach (Wang et al, 2000;Rashid et al, 2001;Margueron et al, 2003), as this gene appears to be rarely mutated in common human malignancies in contrast to p53 (Terao et al, 2001).…”

Histone deacetylase inhibitors and the promise of epigenetic (and more) treatments for cancer

“…HDACi should therefore lead to activation of RB targets, effectively enhancing a key proliferative pathway. Despite these reservations, HDAC recruitment seems crucial only for a subset of RB targets, and cell-cycle regulation by RB proceeds largely through HDACindependent mechanisms 80 .…”