An early S phase checkpoint is regulated by the E2F1 transcription factor

Estrogen receptor regulates E2F1 expression to mediate tamoxifen resistance

Antiestrogen resistance often develops with prolonged exposure to hormone therapies, including tamoxifen, and is a major problem in the treatment of breast cancer. Understanding the mechanisms involved in the development of antiestrogen resistance is an important step in the development of new targeted therapies. Two forms of antiestrogen resistance exist: de novo resistance and acquired resistance. To mimic acquired resistance, we have established a tamoxifen-resistant breast cancer cell line (MCF-7TamR) by treating parental MCF-7 cells with tamoxifen over a period of 6 months to select for cells with the resistant phenotype. Characterization of the MCF-7TamR cells under normal, hormone-deprived, and tamoxifen-treated conditions suggests that these cells continue to grow in the presence of tamoxifen. Additionally, a greater percentage of resistant cells enter the S phase under tamoxifen conditions, compared with parental MCF-7 cells. Consistent with these growth results, molecular analysis indicates that tamoxifen-resistant cells express higher levels of cyclin E1, cdk2, ACTR, and E2F1. Faslodex or ICI 182, 780 (ICI)-mediated degradation of estrogen receptor (ER) reduced the proliferation of these cells, as well as the level of E2F1 expression in tamoxifen-resistant cells, suggesting that tamoxifen resistance and E2F1 expression are in part dependent on ER. We further showed that tamoxifen enhances the ERalpha/Sp-1 interaction and promotes the recruitment of ERalpha and Sp-1 to the proximal promoter of E2F1 in resistant cells. Collectively, our findings suggest that tamoxifen resistance is a result of increased ERalpha/Sp-1 interaction, which enhances the expression of E2F1 to promote tamoxifen resistance.

Microenvironmental regulation of proliferation in multicellular spheroids is mediated through differential expression of cyclin-dependent kinase inhibitors

Multicellular spheroids composed of transformed cells are known to mimic the growth characteristics of tumors and to develop gradients in proliferation with increasing size. This progressive accumulation of quiescent cells is presumably an active process that occurs in response to the microenvironmental stresses that develop within the three-dimensional structure, and, yet, little is known regarding either the signals that induce the cell cycle arrest or the molecular basis for the halt in proliferation. We have previously reported that regulation of cyclin-dependent kinase (CDK) inhibitors (CKIs) differs in monolayer versus spheroid cell culture. In this study, we have examined the expression of three CKIs in EMT6 mouse mammary carcinoma and MEL28 human melanoma spheroids, as a function both of spheroid size and of location within the spheroid. We report that expression of the CKIs p18(INK4c), p21(waf1/cip1), and p27(Kip1) all increase as the spheroid grows and develops a quiescent cell fraction. However, by examining protein expression in discrete regions of the spheroid, we have found that only p18(INK4c) and p27(Kip1) expression positively correlate with growth arrest, whereas p21(waf1/cip1) is expressed predominantly in proliferating cells. Further analysis indicated that, in the quiescent cells, p18(INK4c) is found in increasing association with CDK6, whereas p27(Kip1) associates predominantly with CDK2. In MEL28 cells, CDK2 activity is completely abrogated in the inner regions of the spheroid, whereas in EMT6 cells, CDK2 activity decreases in accordance with a decrease in expression. We also observed a decrease in all cell cycle regulatory proteins in the innermost spheroid fraction, including CDKs, CKIs, and cyclins. Induction of CKIs from separate families, as well as their association with distinct target CDKs, suggests that there may be multiple checkpoints activated to ensure cell cycle arrest in non-growth-conducive environments. Furthermore, because very similar observations were made in both a human melanoma cell line and a mouse mammary carcinoma cell line, our results indicate that these checkpoints, as well as the signal transduction pathways that activate them, are highly conserved.

Apoptosis and brain ischaemia

There is increasing evidence that some neuronal death after brain ischaemia is mediated by the action of cysteine-requiring aspartate-directed proteases (caspases), the proteases responsible for apoptosis in mammals, although this form of neuronal death is not always accompanied by the morphological changes that are typical of apoptosis in other tissues. Caspase-mediated neuronal death is more extensive after transient than permanent focal brain ischaemia and may contribute to delayed loss of neurons from the penumbral region of infarcts. The activation of caspases after brain ischaemia is largely consequent on the translocation of Bax, Bak, and other BH3-only members of the Bcl-2 family to the mitochondrial outer membrane and the release of cytochrome c, procaspase-9, and apoptosis activating factor-1 (Apaf-1) from the mitochondrial intermembrane space. How exactly ischaemia induces this translocation is still poorly understood. NF-kappaB, the c-jun N-terminal kinase-c-Jun pathway, p53, E2F1, and other transcription factors are probably all involved in regulating the expression of BH3-only proteins after brain ischaemia, and mitochondrial translocation of Bad from sequestering cytosolic proteins is promoted by inactivation of the serine-threonine kinase, Akt. Other processes that are probably involved in the activation of caspases after brain ischaemia include the mitochondrial release of the second mitochondrial activator of caspases (Smac) or direct inhibitor-of-apoptosis-binding protein with low pI (DIABLO), the accumulation of products of lipid peroxidation, a marked reduction in protein synthesis, and the aberrant reentry of neurons into the cell cycle. Non-caspase-mediated neuronal apoptosis may also occur, but there is little evidence to date that this makes a significant contribution to brain damage after ischaemia. The intracellular processes that contribute to caspase-mediated neuronal death after ischaemia are all potential targets for therapy. However, anti-apoptotic interventions in stroke patients will require detailed evaluation using a range of outcome measures, as some such interventions seem simply to delay neuronal death and others to preserve neurons but not neuronal function.

The E2F-1 transcription factor is negatively regulated by its interaction with the MDMX protein

Several proteins with important roles in oncogenesis have been shown to regulate the function of the E2F-1 transcription factor, which is known to activate the expression of genes required for proliferation and apoptosis. Here we identify the MDMX oncoprotein as an E2F-1-binding factor, from a yeast-two hybrid screen using a portion of the E2F-1 protein as "bait." We demonstrate that the region within MDMX needed for the E2F-1:MDMX interaction is located in the central part of the protein, C-terminal of the p53-binding domain. The region within E2F-1 needed for this association is adjacent to the DNA binding domain. Further, when expressed in vivo or in vitro the MDMX protein migrates as two isoforms on SDS-PAGE, the faster migrating isoform having the stronger affinity for the E2F-1 proteins. It appears that this interaction reduces the ability of E2F-1 to bind DNA. Expression of MDMX along with E2F-1 and Dp-1 in Saos2 cells reduces the ability of E2F-1 to bind to its consensus DNA sequence, without altering E2F-1 protein levels. These data indicate that the MDMX protein is capable of associating with E2F-1 and negatively regulating its DNA binding ability.

The amino-terminus of the E2F-1 transcription factor inhibits DNA replication of autonomously replicating plasmids in mammalian cells

The E2F1 transcription factor plays a pivotal role in driving cells out of a quiescent state and into the S phase of the cell cycle, in part by transactivating genes needed for DNA replication including DHFR, thymidine kinase, and DNA Polymerase alpha. E2F1 has also been implicated in regulating an S phase checkpoint, however its role in this checkpoint is not well defined. To determine how E2F1 affects such a checkpoint, we utilized an in vivo replication assay employing a plasmid based SV40 origin of replication, transfected into cells expressing SV40 large T antigen. Here we show that expression of full length E2F1, or only its N terminus, represses replication from plasmids containing the SV40 origin, while N terminal deletions of E2F1 do not. E2F1 appears to inhibit the elongation phase of replication and not the initiation phase since it does not affect the replication of other cotransfected plasmids containing only the SV40 origin. Further, inhibition of replication is dependent on both the amino-terminus of the E2F1 protein and on a DNA sequence that is contained within the 3' end of the E2F1 cDNA. Additionally, both full-length E2F1, or just its N-terminus, form protein complexes with two portions of the 3' end of the E2F1 cDNA. These data provide a clue to the mechanism by which E2F1 regulates transit through the S phase checkpoint, by acting on a specific DNA sequence via its amino-terminal region, to inhibit elongation of DNA replication.

Role of EP(1) and EP(4) PGE(2) subtype receptors in serum-induced 3T6 fibroblast cycle progression and proliferation

Recent studies have suggested that prostaglandin E(2) (PGE(2)) subtype receptors (EP) are involved in cellular proliferation and tumor development. We studied the role of EP(1) and EP(4) PGE(2) subtype receptor antagonists AH-6809 and AH-23848B, respectively, in serum-induced 3T6 fibroblast proliferation. This was significantly reduced in a dose-dependent manner (IC(50) approximately 100 and approximately 30 microM, respectively) to an almost complete inhibition, without any cytotoxic effect. However, the effect of each antagonist on 3T6 cell cycle progression clearly differed. Whereas the EP(1) antagonist increased the G(0)/G(1) population, the EP(4) antagonist brought about an accumulation of cells in early S phase. These effects were associated with a decrease in cyclin D and E levels in AH-6809-treated 3T6 cells and lower cyclin A levels in AH-23848B-treated fibroblasts with respect to control cells. The G(0)/G(1) accumulation caused by AH-6809 seems to be intracellular Ca(2+) concentration ([Ca(2+)](i)) dependent, because a 6-h 1 microM thapsigargin treatment allowed G(0)/G(1)-arrested cells to enter S phase. Similarly, treatment with 20 microM forskolin for 6 h allowed S-phase and G(2)/M progression of AH-23848B-treated cells. This study shows that the inhibitory effect of the EP(1) and EP(4) antagonists on serum-induced 3T6 fibroblast growth is due to their effect at various levels of the cell cycle machinery, suggesting that PGE(2) interaction with its different subtype receptors regulates progression through the cell cycle by modulating cAMP and [Ca(2+)](i).

Growth arrest in A549 cells during hyperoxic stress is associated with decreased cyclin B1 and increased p21(Waf1/Cip1/Sdi1) levels

Exposure to high concentrations of oxygen has previously been shown to cause growth arrest in A549 cells, a distal lung epithelial cell line. We found that when A549 cells were exposed to 95% oxygen they underwent substantial growth inhibition. This was associated with induction of p21(Waf1/Cip1/Sdi1) protein and a decrease in cyclin B1 protein. Flow cytometry revealed that A549 cells exposed to hyperoxia had a significant decrease in the percentage of cells in G(1) and a modest but significant increase in the percentage of cells in S phase and G(2)/M, consistent with cells entering S phase. A549 cells in room air and hyperoxia were then treated with nocodazole, a mitotic inhibitor. Room air A549 cells treated with nocodazole showed a marked increase in G(2)/M consistent with mitotic arrest. In contrast, hyperoxic treated cells had a modest but significant decrease in G(1) but only a minimal increase in G(2)/M consistent with partial G(1)/S arrest and growth inhibition in S phase. To further investigate the role of p21(Waf1/Cip1/Sdi1) as a checkpoint regulator during hyperoxic growth inhibition, HCT116 cells with wild-type and null p21(Waf1/Cip1/Sdi1) were exposed to hyperoxia. Both wild-type p21(+/+) cells and null p21(-/-) cells underwent growth inhibition when exposed to hyperoxia. At 48 h the hyperoxic treated HCT116 p21(+/+) had a similar cell cycle distribution as the hyperoxic treated HCT116 p21(-/-) cells, suggesting that p21(Waf1/Cip1/Sdi1) may not be essential for growth arrest during hyperoxia. These findings suggest that hyperoxia causes partial growth arrest at different phases of the cell cycle but primarily in S phase, that hyperoxic growth arrest is associated with a decrease in cyclin B1 protein and that p21 induction may not be essential for hyperoxic growth arrest.