Centre for Cancer Research and Cell Biology, Queen's University Belfast, Belfast, BT9 7BL, UK.
J Cell Sci. 2010 Nov 1;123(Pt 21):3718-26. doi: 10.1242/jcs.068924. Epub 2010 Oct 12.
Although the retinoblastoma protein (Rb) functions as a checkpoint in the cell cycle, it also regulates differentiation. It has recently been shown that Rb is acetylated during differentiation; however, the role of this modification has not been identified. Depletion of Rb levels with short hairpin RNA resulted in inhibition of human keratinocyte differentiation, delayed cell cycle exit and allowed cell cycle re-entry. Restoration of Rb levels rescued defects in differentiation and cell cycle exit and re-entry; however, re-expression of Rb with the major acetylation sites mutated did not. During keratinocyte differentiation, acetylation of Rb is mediated by PCAF and it is further shown that PCAF acetyltransferase activity is also required for normal differentiation. The major acetylation sites in Rb are located within the nuclear localization sequence and, although mutation did not alter Rb localization in cycling cells, the mutant is mislocalized to the cytoplasm during differentiation. Studies indicate that acetylation is a mechanism for controlling Rb localization in human keratinocytes, with either reduction of the PCAF or exogenous expression of the deacetylase SIRT1, resulting in mislocalization of Rb. These findings identify PCAF-mediated acetylation of Rb as an event required to retain Rb within the nucleus during keratinocyte differentiation.
尽管视网膜母细胞瘤蛋白（Rb）在细胞周期中作为检查点起作用，但它也调节分化。最近已经表明，Rb 在分化过程中被乙酰化；然而，这种修饰的作用尚未确定。用短发夹 RNA 耗尽 Rb 水平会导致人角质形成细胞分化抑制、细胞周期退出延迟并允许细胞周期重新进入。Rb 水平的恢复挽救了分化和细胞周期退出和重新进入的缺陷；然而，具有主要乙酰化位点突变的 Rb 的重新表达并没有。在角质形成细胞分化过程中，Rb 的乙酰化由 PCAF 介导，并且进一步表明 PCAF 乙酰转移酶活性对于正常分化也是必需的。Rb 中的主要乙酰化位点位于核定位序列内，尽管突变没有改变循环细胞中 Rb 的定位，但突变体在分化过程中错误定位到细胞质。研究表明，乙酰化是控制人角质形成细胞中 Rb 定位的一种机制，要么减少 PCAF，要么外源表达去乙酰化酶 SIRT1，导致 Rb 定位错误。这些发现确定了 PCAF 介导的 Rb 乙酰化是在角质形成细胞分化过程中使 Rb 保留在核内的一个必需事件。
