Acetylation control of the retinoblastoma tumour-suppressor protein

Abstract

The retinoblastoma tumour-suppressor protein (pRb) and p300/CBP co-activator proteins are important for control of proliferation and in tumour cells these are sequestered by viral oncoproteins such as E1A. pRb is involved in negatively regulating growth, and p300/CBP proteins have histone acetyltransferase (HAT) activity, which influences gene expression. Although it is known that phosphorylation by G1 cyclin-dependent kinases (CDKs) regulates pRb activity, the nature and role of other post-translational modifications is not understood. Here we identify acetylation as a new type of modification and level of control in pRb function. Adenovirus E1A, which binds p300/CBP through an amino-terminal transformation-sensitive domain, stimulates the acetylation of pRb by recruiting p300 and pRb into a multimeric-protein complex. Furthermore, pRb acetylation is under cell-cycle control, and acetylation hinders the phosphorylation of pRb by cyclin-dependent kinases. pRb binds more strongly when acetylated to the MDM2 oncoprotein, which indicates that acetylation may regulate protein-protein interactions in the pRb pathway. The acetylation of pRb defines a new level of cell-cycle control mediated by HAT. Furthermore, our results establish a relationship between p300, pRb and acetylation in which E1A acts to recruit and target a cellular HAT activity to pRb.

p300 is required for orderly G1/S transition in human cancer cells

“…Association between p300 and RB varies during G1 p300 and RB are believed to associate in vivo, although it is not clear if this interaction is direct or indirectly mediated through factors such as E1A (Chan et al, 2001;Nguyen et al, 2004). To demonstrate p300/RB association in the absence of E1A, we performed coimmunoprecipitation (Co-IP) experiments in two cell lines, HCT116 and HeLa, which express both proteins, with p300 À cells used as negative controls.…”

“…Instead, accelerated G1/S progression was associated with premature RB hyperphosphorylation, and activation of E2F targets, cyclin E and mcm2. p300/CBP-mediated RB acetylation had previously been shown to inhibit RB phosphorylation (Chan et al, 2001). However, as there was no difference in RB acetylation between HCT116 and p300 À cells, this could not account for the results seen.…”

“…This has led to the suggestion that p300 and CBP may function as classical tumor suppressor genes (Gayther et al, 2000;Ozdag et al, 2002;Iyer et al, 2004b;Ward et al, 2005). p300 and CBP function as prototype histone and factor acetyltransferases (HAT and FAT) by catalyzing the addition of an acetyl group to specific lysine residues on histones and other proteins, including p53, retinoblastoma (RB) and E2F (Gu and Roeder, 1997;Liu et al, 1999;Martinez-Balbas et al, 2000;Chan et al, 2001). Early experiments have suggested that the two homologs play interchangeable, apparently redundant roles in cell physiology.…”

“…Several groups have shown that p300/ CBP and another HAT, p/CAF (p300/CBP associated protein), function as coactivators of E2F-mediated transactivation (Martinez-Balbas et al, 2000). Furthermore, direct modulation of RB function through p300/ CBP-mediated acetylation has been reported, leading to the suggestion that RB acetylation regulates G1/S transition (Chan et al, 2001). Others have shown that acetylation of RB by p300 and p/CAF activates differentiation-specific pathways and cell cycle exit but plays no apparent role in G1/S transition (Nguyen et al, 2004).…”

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

“…The acetylation of pRb enhances its interaction with MDM2 and reduces the efficiency of pRb phosphorylation by CDKs. Early-region 1A stimulates pRb acetylation by recruiting pRb in a ternary complex with p300 (Chan et al, 2001). The biological significance of pRb acetylation and of the role of E1A in its stimulation, however, remains unclear.…”