Overexpression of p53 causes G2 arrest, attributable in part to the loss of CDC2 activity. Transcription of cdc2 and cyclin B1, determined using reporter constructs driven by the two promoters, was suppressed in response to the induction of p53. Suppression requires the regions Ϫ287 to Ϫ123 of the cyclin B1 promoter and Ϫ104 to Ϫ74 of the cdc2 promoter. p53 did not affect the inhibitory phosphorylations of CDC2 at threonine 14 or tyrosine 15 or the activity of the cyclin-dependent kinase that activates CDC2 by phosphorylating it at threonine 161. Overexpression of p53 may also interfere with the accumulation of CDC2/cyclin B1 in the nucleus, required for cells to enter mitosis. Constitutive expression of cyclin B1, alone or in combination with the constitutively active CDC2 protein T14A Y15F, did not reverse p53-dependent G2 arrest. However, targeting cyclin B1 to the nucleus in cells also expressing CDC2 T14A Y15F did overcome this arrest. It is likely that several distinct pathways contribute to p53-dependent G2 arrest. INTRODUCTIONThe p53 tumor suppressor helps to protect mammals from developing neoplasia by blocking cell cycle progression or by inducing cell death in response to stress Levine, 1997;Agarwal et al., 1998b). p53-mediated arrest at the G1-S boundary prevents the replication of DNA damaged by ionizing or UV radiation or by chemical mutagens (Kastan et al., 1991;Gujuluva et al., 1994;Ceraline et al., 1998). G1 arrest depends on the ability of p53 to activate the transcription of specific genes (Dulic et al., 1994;Pietenpol et al., 1994). An important target, p21/waf1, inhibits cyclin-dependent kinases (CDKs) 1 2, 4, and 6, which are required to enter S phase (El-Deiry et al., 1993;Harper et al., 1993;Xiong et al., 1993). p53 also inhibits S-phase entry in response to damage to the mitotic spindle, preventing the rereplication of DNA (Cross et al., 1995), possibly by a transcription-independent mechanism (Notterman et al., 1998). p53 mediates G1 arrest in response to nucleotide deprivation, preventing DNA synthesis under conditions that would generate damaged DNA (Chernova et al., 1995;Linke et al., 1996). p53 also protects cells arrested within S phase by a lack of pyrimidine nucleotides, preventing the replication from unbalanced pools of deoxynucleoside triphosphates and consequent DNA damage (Agarwal et al., 1998a). A variety of stimuli that trigger p53-dependent cellular responses increase the ability of p53 to bind to DNA and induce the accumulation of the protein, attributable in large part to an increase in its stability (Maltzman and Czyzyk, 1984;Fritsche et al., 1993;Hupp et al., 1995). Responses to p53 vary in different types of cells and depend on the level of p53 expression. For example, thymocytes undergo p53-dependent apoptosis more readily than do fibroblasts (Lowe et al., 1993;Di Leonardo et al., 1994). In some cells, high levels of p53 induce apoptosis, whereas lower levels induce cell cycle arrest (Chen et al., 1996).p53 plays an important role in regulating the G2-M transition. ...

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Wnt5a is an extracellular glycoprotein that activates Wnt signaling pathways, important in development and tissue homeostasis. Wnt5a expression is often misregulated during cancer progression. In this study, we analyzed the transcriptional regulation of two of the Wnt5a alternative promoters, termed A and B. Transient transfection of promoter A and B luciferase reporter constructs in to NIH3T3 and Caco-2 cells indicated that the separated promoters are both functional and that 300-450 base pair (bp) of upstream sequence is sufficient for activity. Promoter B constructs displayed distinct patterns of expression in the two cell types. The endogenous levels of promoter A-derived transcripts were found to be greater than the promoter B transcripts by four- to sixfold in fibroblast cells. Treatment of NIH3T3 cells with tumor necrosis factor (TNF)-alpha leads to an increase in both promoter A and B activities, but promoter B was more responsive. Using inhibitors of TNF-alpha effector proteins, we provide evidence that the transcription factor nuclear factor-kappaB and the MEK1/2 and p38 kinases have distinct roles in determining the activity levels of promoters, A and B. These results support the conclusion that Wnt5a promoters, A and B, are differentially regulated and provide a model for complex transcriptional regulation of Wnt5a.

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