Apoptosis inhibitory activity of cytoplasmic p21Cip1/WAF1 in monocytic differentiation

Abstract

p21 Cip1/WAF1 inhibits cell-cycle progression by binding to G 1 cyclin/CDK complexes and proliferating cell nuclear antigen (PCNA) through its N-and C-terminal domains, respectively. The cell-cycle inhibitory activity of p21 Cip1/WAF1 is correlated with its nuclear localization. Here, we report a novel cytoplasmic localization of p21 Cip1/WAF1 in peripheral blood monocytes (PBMs) and in U937 cells undergoing monocytic differentiation by in vitro treatment with vitamin D3 or ectopic expression of p21 Cip1/WAF1 , and analyze the biological consequences of this cytoplasmic expression. U937 cells which exhibit nuclear p21 Cip1/WAF1 demonstrated G 1 cell-cycle arrest and subsequently differentiated into monocytes. The latter event was associated with a cytoplasmic expression of nuclear p21 Cip1/WAF1 , concomitantly with a resistance to various apoptogenic stimuli. Biochemical analysis showed that cytoplasmic p21 Cip1/WAF1 forms a complex with the apoptosis signalregulating kinase 1 (ASK1) and inhibits stress-activated MAP kinase cascade. Expression of a deletion mutant of p21 Cip1/WAF1 lacking the nuclear localization signal (ΔNLS-p21) did not induce cell cycle arrest nor monocytic differentiation, but led to an apoptosis-resistant phenotype, mediated by binding to and inhibition of the stress-activated ASK1 activity. Thus, cytoplasmic p21 Cip1/WAF1 itself acted as an inhibitor of apoptosis. Our findings highlight the different functional roles of p21 Cip1/WAF1 , which are determined by its intracellular distribution and are dependent on the stage of differentiation.

C-terminal deletion mutant p21WAF1/CIP1 enhances E2F-1–mediated apoptosis in colon adenocarcinoma cells

“…Experiments utilizing N -or C -terminal deletion mutants of p21, or p21 mutants containing point mutations in the CDK2-or PCNA -binding domains provide consistent data that show if p21 has lost the ability to translocate to the nucleus, it can no longer inhibit cell cycle progression. 36,37,51 This observation is true even with the retention of p21's ability to inhibit CDK2 activity, 36,51 which appears to be a requirement for inhibition of cell cycle progression in addition to nuclear localization. In the present study, we used an adenoviral vector expressing a C -terminal deletion mutant p21 gene ( Ad -p21 -PCNA ).…”

Cancer-specific enhancement of cisplatin-induced cytotoxicity with triptolide through an interaction of inactivated glycogen synthase kinase-3β with p53

“…Several recent studies have demonstrated that p53-dependent p21 induction inhibits the apoptotic response, and p21 attenuation may make genotoxic chemotherapeutic agents more effective by subverting the normal repair process or, more directly, by promoting the apoptotic process through inhibition of p21 interaction with apoptosis signal-regulating kinase 1, which is upstream of JNK (Asada et al, 1999;Gartel and Tyner, 2002;Weiss, 2003). In our condition, CDDP/triptolide combination attenuated p53 downstream molecules such as p21, MDM2 and Puma which have short half-lives for mRNA and protein, but had no remarkable influence on the expression of Bax, which has relatively longer half-life, and activated JNK evoked mitochondrial apoptosis using Bax transactivation as a proapoptotic mediator.…”

Role of the translational repressor 4E-BP1 in the regulation of p21Waf1/Cip1 expression by retinoids

“…p21 Waf1/Cip1 plays important roles in cell-cycle progression via effects on CDK (44), while under certain circumstances, it also promotes cell survival (45,46). Although the role of cap-dependent mRNA translation in p21 Waf1/Cip1 -expression is not well-defined, there is some evidence that rapamycin blocks p21 Waf1/Cip1 -expression in response to thrombopoietin (47), EGF (48) and chemotherapeutic agents (49).…”