Erratum: Myc and Ras collaborate in inducing accumulation of active cyclin E/Cdk2 and E2F

Molecular mechanism concerning conformational changes of CDK2 mediated by binding of inhibitors using molecular dynamics simulations and principal component analysis

Cyclin-dependent kinase 2 (CDK2) has been regarded as a promising drug target for anti-tumour agents. In this study, molecular dynamics (MD) simulations and principal component (PC) analysis were used to explore binding mechanism of three inhibitors 1PU, CDK, 50Z to CDK2 and influences of their bindings on conformational changes of CDK2. The results show that bindings of inhibitors yield obvious impacts on internal dynamics, movement patterns and conformational changes of CDK2. In addition, molecular mechanics generalized Born surface area (MM-GBSA) was applied to calculate binding free energies between three inhibitors and CDK2 and evaluate their binding ability to CDK2. The results show that CDK has the strongest binding to CDK2 among the current three inhibitors. Residue-based free energy decomposition method was further utilized to decode the contributions of a single residue to binding of inhibitors, and it was found that three inhibitors not only produce hydrogen bonding interactions and hydrophobic interactions with key residues of CDK2, which promotes binding of three inhibitors to CDK2, but also share similar binding modes. This work is expected to be helpful for design of efficient drugs targeting CDK2.

Activation of extracellular signal-regulated kinase (ERK) in G2 phase delays mitotic entry through p21CIP1

Extracellular signal-regulated kinase activity is essential for mediating cell cycle progression from G(1) phase to S phase (DNA synthesis). In contrast, the role of extracellular signal-regulated kinase during G(2) phase and mitosis (M phase) is largely undefined. Previous studies have suggested that inhibition of basal extracellular signal-regulated kinase activity delays G(2)- and M-phase progression. In the current investigation, we have examined the consequence of activating the extracellular signal-regulated kinase pathway during G(2) phase on subsequent progression through mitosis. Using synchronized HeLa cells, we show that activation of the extracellular signal-regulated kinase pathway with phorbol 12-myristate 13-acetate or epidermal growth factor during G(2) phase causes a rapid cell cycle arrest in G(2) as measured by flow cytometry, mitotic indices and cyclin B1 expression. This G(2)-phase arrest was reversed by pre-treatment with bisindolylmaleimide or U0126, which are selective inhibitors of protein kinase C proteins or the extracellular signal-regulated kinase activators, MEK1/2, respectively. The extracellular signal-regulated kinase-mediated delay in M-phase entry appeared to involve de novo synthesis of the cyclin-dependent kinase inhibitor, p21(CIP1), during G(2) through a p53-independent mechanism. To establish a function for the increased expression of p21(CIP1) and delayed cell cycle progression, we show that extracellular signal-regulated kinase activation in G(2)-phase cells results in an increased number of cells containing chromosome aberrations characteristic of genomic instability. The presence of chromosome aberrations following extracellular signal-regulated kinase activation during G(2)-phase was further augmented in cells lacking p21(CIP1). These findings suggest that p21(CIP1) mediated inhibition of cell cycle progression during G(2)/M phase protects against inappropriate activation of signalling pathways, which may cause excessive chromosome damage and be detrimental to cell survival.

Molecular events associated with accelerated proliferative response in rat livers when partial hepatectomy is preceded by a sham operation

BACKGROUND

When a sham operation is performed 6 h before partial hepatectomy (PH), the regenerative response is accelerated suggesting that sham operation itself contributes to cellular events leading to proliferation.

MATERIALS AND METHODS

In order to examine the mechanisms implicated in this acceleration, we compared the activation of several factors associated with the progression through the cell cycle at various times after PH and after PH preceded by sham operation (S6 h + PH). The effect of a single sham (S) and two combined sham operations (S6 h + S) was also examined. Nonoperated rats were used as controls (C).

RESULTS

The early factors NF-kappaB and Stat3 were activated after S6 h + PH and S6 h + S. C-jun expression was increased 0.5 h and 2 h after PH and 6 h after sham. There was no further increase in S6 h + PH and S6 h + S. In contrast, c-myc expression returned to baseline levels after S6 h and a new increase was observed 2 h after S6 h + PH but not after S6 h + S. P53 mRNA was significantly expressed 6 h after S6 h + PH, but at a level similar than that observed 6 and 12 h after PH alone. An earlier increase in c-Ha-ras mRNA and cyclin E protein was found in S6 h + PH, in comparison with PH alone.

CONCLUSIONS

The first divergent response between the two combined models involved c-myc expression. However, major differences related to the accelerated liver regenerative response observed after S6 h + PH were found at late time points associating an earlier expression of c-Ha-ras and nuclear cyclin E.

Adaptive hypersensitivity to estradiol: potential mechanism for secondary hormonal responses in breast cancer patients

Women with hormone dependent breast cancer initially respond to hormone deprivation therapy with tamoxifen or oophorectomy for 12-18 months but later relapse. Upon secondary therapy with aromatase inhibitors, patients often experience further tumor regression. The mechanisms responsible for secondary responses are unknown. We postulated that hormone deprivation induces hypersensitivity to estradiol. Evidence of this phenomenon was provided in a model system involving MCF-7 cells grown in vitro and in xenografts. To determine if the ER transcriptional process is involved in hypersensitivity, we examined the effect of estradiol on ER reporter activity, PgR, PS2, and c-myc as markers and found no alterations in hypersensitive cells. Next, we examined whether MAP kinase may be upregulated in the hypersensitive cells as a reflection of increased growth factor secretion or action. Basal MAP kinase activity was increased both in vitro and in vivo in hypersensitive cells. Proof of principle studies indicated that an increase in MAP kinase activity induced by TGFalpha administration caused a two- to three-fold shift to the left in estradiol dose response curves in wild type cells. Blockade of MAP kinase with PD98059 returned the shifted curve back to baseline. These data suggested that MAP kinase overexpression could induce hypersensitivity. To determine why MAP kinase was increased, we excluded constitutive receptor activity and growth factor secretion by the demonstration that the pure anti-estrogen, ICI 182780, could inhibit MAP kinase activation. We also excluded hypersensitivity to estradiol induced growth factor secretion, and thus MAP kinase activation, since estradiol stimulated MAP kinase at 24, 48, and 72 h at the same concentrations in hypersensitive as in wild type cells. Surprisingly, a series of experiments suggested that MAP kinase increased in hypersensitive cells as a result of estrogen activation via a non-genomic pathway. We examined the classical signal pathway in which SHC is phosphorylated and binds to SOS and GRB-2 to activate Ras, Raf, and MAP kinase. With 5-20 min of exposure, estradiol caused binding of SHC to the estrogen receptor, phosphorylation of SHC, binding of GRB-2 to SOS, and activation of MAP kinase. All of these affects could be blocked by ICI 182780. Taken together, these observations suggest that the cell membrane ER pathway may be responsible for upregulation of MAP kinase and hypersensitivity in cells adapted to estradiol deprivation.

The Myc/Max/Mad network and the transcriptional control of cell behavior

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.