To do so, we transfected p53/MDMX double knock-out MEF cells with MDMX expression vectors together with an empty vector or a plasmid encoding 14-3-3 or 14-3-3 K50E that was previously shown to be defective in MDMX-binding (29,38). assays, leading to the induction of p21 in p53-null cells. Knockdown of 14-3-3 inversely alleviated the induction of p21 levels by DNA damage. Hence, our study as presented here unravels a new role for 14-3-3 in protecting p21 from MDMX-mediated proteasomal turnover, which may partially account for DNA damage-induced elevation of p21 levels MK-5172 sodium salt independent of p53. Keywords:Cell Cycle, DNA Damage, p53, Proteasome, Ubiquitination, 14-3-3, MDMX, p21 == Introduction == The cell cycle (defined into G1, S, G2, and M phases) essential for stem cell proliferation, embryogenesis, animal development, and tumor growth is highly regulated by a number of cellular proteins (14). Post-completion of their functions during the cell cycle, these proteins are often disposed through ubiquitin-dependent or independent proteolysis executed by either 26 S or 20 S proteasome (5,6). One of these short living cell cycle regulatory proteins is p21 (p21cip1/waf1) which acts to pause cycling cells mostly at G1 and sometime at G2 by directly binding to cyclin-dependent kinases and inhibiting their activity (710). The protein level of p21 is oscillated during the cell cycle (1113). Both ubiquitin-dependent and independent mechanisms have been shown to mediate p21 proteasomal turnover during the cell cycle or in response to specific signals (1417). On one hand, several E3 ubiquitin ligases, such as Skp2, MKRN1, or CRL4Cdt2 were shown to ubiquitinate p21 and mediate its degradation (14,1820). On the other hand, it has been also shown that p21 can be discarded through ubiquitin-independent proteasomal pathways. For example, it was previously shown that p21 is degraded directly by 20 S proteasome without involvement of ubiquitination (2123). Also, two p53s suppressors, MDM2 and MDMX (24,25), can directly bind to p21 and mediate its ubiquitin-independent proteolysis by 26 S proteasome during the cell cycle (15,17,26). Their actions can be independent of, though also cooperative with each other (26). Furthermore, REG, a proteasome activator, has been reported to facilitate ubiquitin- and ATP-independent proteasomal degradation of p21 (27,28). Therefore, it becomes clear that the stability of this critical cell cycle regulator p21 is subjected to the regulation by multiple cellular proteins. Whether the process of its terminal fate requires ubiquitination or not is probably signal-, cell-, or even microenvironment-dependent, or depends upon the balanced expression of p21 and one or more of its aforementioned destroyers. Perhaps, this is also modulated by upstream regulators of the above p21 destroyers. When studying the regulation of MK-5172 sodium salt MDMX in response to DNA damage, we and others previously found that MDMX is phosphorylated by Chk1 or Chk2 and this phosphorylation enhances the binding of MDMX to 14-3-3 or other isoforms of 14-3-3, except 14-3-3, leading to inactivation of MDMX and subsequent activation of p53 (2931). This pathway was later confirmed by an elegant knock-in study in mice (32). Because both p21 and 14-3-3 bind to the extended central region of MDMX (26,29,33), we speculated that 14-3-3 might suppress MDMX-mediated p21 degradation by influencing the p21 binding to MDMX. Our initial test indeed confirmed this idea. As detailed below, our further analyses demonstrate that 14-3-3 can inhibit MDMX-mediated p21 degradation and thus induce p21 level by inhibiting their interaction independent of p53 and in response to DNA damage. == MATERIALS AND METHODS == == == == == == Cell Lines, Plasmids, and Antibodies TNFRSF17 == Human embryonic kidney (HEK) epithelial 293 cells, human non-small lung carcinoma H1299 cells (p53-deficicent), mouse embryonic fibroblast (MEFp53/) single knock-out (SK)2and (MEFp53//MDMX/) double knock-out (DX) cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) MK-5172 sodium salt supplemented with 10% fetal bovine serum (FBS), 100 units/ml penicillin and 0.1 mg/ml streptomycin at 37 C in a 5% CO2humidified atmosphere. The GST-14-3-3 (K50E) mutant plasmid was generated by mutating Lys-50 to Glu-50 from wild type 14-3-3 (wt)-pGEX4T-1 plasmid by site-directed mutagenesis. Myc-14-3-3 was subcloned from Flag-14-3-3 plasmid using BamHI and XhoI sites. The other plasmids utilized here were previously described (16,26,29,34). Monoclonal anti-Flag, anti–tubulin, and -actin (Sigma), polyclonal anti-GST (Sigma), polyclonal anti-p21 (M19, Santa Cruz Biotechnology), monoclonal anti-c-Myc (9E10, Santa Cruz Biotechnology), polyclonal anti- MDMX (Bethyl Laboratories and H130, Santa Cruz Biotechnology) or monoclonal anti-MDMX (d-4, Santa Cruz Biotechnology and 8C6), GFP (FL, Santa Cruz Biotechnology), polyclonal anti-14-3-3 (C16, Santa Cruz Biotechnology) were purchased and utilized. == Transfection, Western Blot (WB), and Co-immunoprecipitation (co-IP) Analyses == Cells were transfected with plasmids as indicated in figure legends using TransFectin reagents (Bio-Rad), following the manufacturer’s protocol. The cells were harvested.