Zigang Dong

Post-translational modification of p53 in tumorigenesis

Interest in the tumour suppressor p53 has generated much information regarding the complexity of its function and regulation in carcinogenesis. However, gaps still exist in our knowledge regarding the role of p53 post-translational modifications in carcinogenesis and cancer prevention. A thorough understanding of p53 will be extremely useful in the development of new strategies for treating and preventing cancer, including restoration of p53 function and selective killing of tumours with mutant TP53.

ERKs and p38 Kinase Phosphorylate p53 Protein at Serine 15 in Response to UV Radiation

Phosphorylation of the p53 tumor suppressor protein is likely to play an important role in regulating its activity. Serine 15 phosphorylation of p53 leads to a stabilization of p53 by reducing its interaction with murine double minute 2, a negative regulatory partner. Recently, p53 was reported to be activated and phosphorylated at serine 15 following UV radiation. However, the signaling pathway that mediates UV-induced phosphorylation is less well characterized. Here, we provide evidence that UVB-induced phosphorylation of p53 at serine 15 is mediated directly by ERKs and p38 kinase. We find that in a mouse JB6 epidermal cell line, ERKs and p38 kinase form a complex with p53 following UVB radiation. Inhibition of ERKs or p38 kinase activity by the use of a dominant negative mutant of ERK2 or p38 kinase or their respective specific inhibitor, PD98059 or SB202190, results in abrogation of UVB-induced phosphorylation of p53 at serine 15. Strikingly, incubation of UVB-activated ERKs or p38 kinase immunoprecipitated complex with exogenous p53 shows serine 15 phosphorylation of both exogenous and co-precipitated endogenous p53 protein. Additionally, active recombinant ERK1/2 and p38 kinase but not JNKs are also able to phosphorylate p53 at serine 15 in vitro. Furthermore, pretreatment of cells with PD98059 or SB202190 blocks p53-dependent transcription activity but increases the level of p53 co-precipitated murine double minute. These results strongly suggest that both ERKs and p38 kinase have a direct role in UVB-induced phosphorylation of p53 at serine 15 in vivo.

p38 Kinase Mediates UV-induced Phosphorylation of p53 Protein at Serine 389

The p53 tumor suppressor protein is a transcription factor that plays a key role in the process of apoptosis and the cell’s defense against tumor development. Activation of p53 occurs, at least in part, by phosphorylation of its protein. Very recently it has been reported that UV induced a functional activation of p53 via phosphorylation at serine 389. Here, we report that the UV-induced phosphorylation of p53 at serine 389 is mediated by p38 kinase. UVC-induced phosphorylation of p53 at serine 389 was markedly impaired by either pretreatment of cells with p38 kinase inhibitor, SB202190, or stable expression of a dominant negative mutant of p38 kinase. In contrast, there was no inhibition observed in cells treated with specific MEK1 inhibitor, PD98059, or with stable expression of a dominant negative mutant of ERK2 or JNK1. Most importantly, p38 kinase could be co-immunoprecipitated with p53 by using antibodies against p53. Incubation of active p38 kinase with p53 protein caused the phosphorylation of p53 protein at serine 389 in vitro, while no phosphorylation of p53 at serine 389 was observed when p53 was incubated with activated JNK2 or ERK2. Furthermore, pretreatment of cells with SB202190 blocked the p53 DNA binding activity and p53-dependent transcription. These results strongly suggest that the p38 kinase is at least one of the most important mediators of p53 phosphorylation at serine 389 induced by UVC radiation.

Molecular targets of phytochemicals for cancer prevention

Although successful for a limited number of tumour types, the efficacy of cancer therapies, especially for late-stage disease, remains poor overall. Many have argued that this could be avoided by focusing on cancer prevention, which has now entered the arena of targeted therapies. During the process of identifying preventive agents, dietary phytochemicals, which are thought to be safe for human use, have emerged as modulators of key cellular signalling pathways. The task now is to understand how these chemicals perturb these pathways by modelling their interactions with their target proteins.

Mitogen-Activated Protein Kinase Activation in UV-Induced Signal Transduction

Experimental evidence supported by epidemiological findings suggests that solar ultraviolet (UV) irradiation is the most important environmental carcinogen leading to the development of skin cancers. Because the ozone layer blocks UVC (wavelength, 180 to 280 nm) exposure, UVA (UVA I, 340 to 400 nm; UVA II, 320 to 340 nm) and UVB (280 to 320 nm) are probably the chief carcinogenic components of sunlight with relevance for human skin cancer. Substantial contributions to the elucidation of the specific signal transduction pathways involved in UV-induced skin carcinogenesis have been made over the past few years, and most evidence suggests that the cellular signaling response is UV wavelength-dependent. The mitogen-activated protein kinase (MAPK) signaling cascades are targets for UV and are important in the regulation of the multitude of UV-induced cellular responses. Experimental studies have used a range of UVA, UVB, UVC, and various combinations in multiple doses, and the observed effects on activation and phosphorylation of MAPKs are varied. This review focuses on the mechanistic data supporting a role for MAPKs in UV-induced skin carcinogenesis. Progress in understanding the mechanisms of UV-induced signal transduction could lead to the use of these protein kinases as specific targets for the prevention and control of skin cancer. Sunlight is most likely the main cause of skin cancer, which is the most common human cancer. Solar radiation is categorized by wavelength into ultraviolet C (UVC, wavelength 180 to 280 nm), ultraviolet B (UVB, 280 to 320 nm), and ultraviolet A (UVA) regions (UVA I, 340 to 400 nm; UVA II, 320 to 340 nm). All UV spectra have been linked to cancer in experimental animal models. However, the physiologic relevance of UVC may be dubious because all of the UVC radiation is absorbed by the ozone layer of Earths atmosphere. UV radiation at Earths surface consists of 1 to 10% UVB and 90 to 99% UVA and can penetrate human skin, making it the primary target for UV-induced damage and cancer. UV exposure can result in direct or indirect DNA damage, depending on the wavelength and exposure time. Even though DNA damage is a primary initiator in UV-induced skin carcinogenesis, the mechanism behind the tumor-promoting ability of UV is unclear. The assumption in the past was that the mechanisms of UV-stimulated tumor promotion could be based on studies of UVC-induced signal transduction, because the effects were assumed to be similar for all wavelengths of UV. However, accumulating evidence suggests that the cellular responses produced by UV irradiation are likely to be wavelength-dependent. UV activates various signaling pathways that are either oncogenic or protective or both. Many of these pathways are mediated primarily through signaling cascades involving mitogen-activated protein kinases (MAPKs), resulting in the modification of transcription factors such as activator protein-1, which can lead to skin cancer. In light of rising public concern over the increased incidence of skin cancer, this review focuses on the mechanistic data supporting a role for MAPKs in UV-stimulated skin carcinogenesis. Progress in understanding the mechanisms of UV-induced signal transduction could lead to the use of these protein kinases as specific targets for the prevention and control of skin cancer.

The paradox of arsenic: molecular mechanisms of cell transformation and chemotherapeutic effects

Arsenic is a well-documented carcinogen that also appears to be a valuable therapeutic tool in cancer treatment. This creates a paradox for which no unified hypothesis has been reached regarding the molecular mechanisms that determine whether arsenic will act as a carcinogen or as an effectual chemotherapeutic agent. Much of our knowledge with respect to the actions of arsenic has been drawn from epidemiological or clinical studies. The actions of arsenic are likely to be related to cell type, arsenic species, and length and dose of exposure. Arsenic unquestionably induces apoptosis and may specifically target certain tumor cells. Research data strongly suggest that arsenic influences distinct signaling pathways involved in mediating proliferation or apoptosis, including mitogen-activated protein kinases, p53, activator protein-1 or nuclear factor kappa B. The primary purpose of this review is to examine recent findings, from this laboratory and others, that focus on the molecular mechanisms of arsenics actions in cell transformation and as a therapeutic agent.

Molecular mechanism of the chemopreventive effect of resveratrol

Chemoprevention is a promising approach to control human cancer. Resveratrol has been shown to have a potent chemopreventive effect in multiple carcinogenesis models. However, the precise mechanism explaining its anti-carcinogenic effect is not clear. This review summarizes recent studies from our laboratory on the mechanisms of resveratrols effects. In JB6 cells, resveratrol was found to induce apoptosis and inhibit tumor promoter-induced cell transformation. We also found that resveratrol-induced activation of p53 and resveratrol-induced apoptosis occurred through a p53-dependent pathway. The MAP kinases, ERKs, JNKs, or p38 kinases, are involved in resveratrol-induced activation of p53 and apoptosis.

(−)−Epigallocatechin Gallate Overcomes Resistance to Etoposide-Induced Cell Death by Targeting the Molecular Chaperone Glucose-Regulated Protein 78

Many beneficial properties have been attributed to (-)-epigallocatechin gallate (EGCG), including chemopreventive, anticarcinogenic, and antioxidant actions. In this study, we investigated the effects of EGCG on the function of glucose-regulated protein 78 (GRP78), which is associated with the multidrug resistance phenotype of many types of cancer cells. Our investigation was directed at elucidating the mechanism of the EGCG and GRP78 interaction and providing evidence about whether EGCG modulates the activity of anticancer drugs through the inhibition of GRP78 function. We found that EGCG directly interacted with GRP78 at the ATP-binding site of protein and regulated its function by competing with ATP binding, resulting in the inhibition of ATPase activity. EGCG binding caused the conversion of GRP78 from its active monomer to the inactive dimer and oligomer forms. Further, we showed that EGCG interfered with the formation of the antiapoptotic GRP78-caspase-7 complex, which resulted in an increased etoposide-induced apoptosis in cancer cells. We also showed that EGCG significantly suppressed the transformed phenotype of breast cancer cells treated with etoposide. Overall, these results strongly suggested that EGCG could prevent the antiapoptotic effect of GRP78, which usually suppresses the caspase-mediated cell death pathways in drug-treated cancer cells, contributing to the development of drug resistance.

The functional contrariety of JNK.

The JNK proteins are activated by multiple and diverse stimuli, leading to varied and seemingly contradictory cellular responses. In particular, JNKs have been reported to have a role in the induction of apoptosis, but have also been implicated in enhancing cell survival and proliferation. Thus the JNK proteins seem to represent an archetype of contrariety of intracellular signaling. The opposing roles of JNKs have been attributed to the observation that JNKs activate different substrates based on specific stimulus, cell type or temporal aspects. Because of their analogous expression in apparently almost every tissue, JNK1 and JNK2 have most often been considered to have overlapping or redundant functions. In spite of this assessment, research evidence suggests that the functions of JNKs should be addressed in a manner that differentiates between their precise contributions. Specifically in this review, we examine evidence regarding whether the JNKs proteins might play distinctive roles in cellular processes associated with carcinogenesis.

Requirement of Erk, but Not JNK, for Arsenite-induced Cell Transformation

Trivalent arsenic (arsenite, As3+) is a human carcinogen, which is associated with cancers of skin, lung, liver, and bladder. However, the mechanism by which arsenite causes cancer is not well understood. In this study, we found that exposure of Cl 41 cells, a well characterized mouse epidermal cell model for tumor promotion, to a low concentration of arsenite (<25 μm) induces cell transformation. Interestingly, arsenite induces Erk phosphorylation and increased Erk activity at doses ranging from 0.8 to 200 μm, while higher doses (more than 50 μm) are required for activation of JNK. Arsenite-induced Erk activation was markedly inhibited by introduction of dominant negative Erk2 into cells, while expression of dominant negative Erk2 did not show inhibition of JNK and MEK1/2. Furthermore, arsenite-induced cell transformation was blocked in cells expressing the dominant negative Erk2. In contrast, overexpression of dominant negative JNK1 was shown to increase cell transformation even though it inhibits arsenite-induced JNK activation. Our results not only show that arsenite induces Erk activation, but also for the first time demonstrates that activation of Erk, but not JNK, by arsenite is required for its effects on cell transformation.