PubMed (a service of the National Library of Medicine) provides a comprehensive list of Dr. Dong's recent publications.

Cancer is one of the leading causes of death in today’s world. In order to facilitate the development of chemopreventive and chemotherapeutic agents that specifically target molecules important in cancer development, we must know the enemy – we must understand carcinogenesis. The prevailing thought today is that cancer may be prevented or treated by targeting specific cancer genes, signaling proteins and transcription factors.

Cancer is a multistage process, consisting of initiation, promotion and progression stages. Although each stage 6 may be a possible target for chemopreventive agents, because of its extensive length, the promotion stage has the most potential to be reversed. By focusing on the molecular mechanisms explaining how normal cells can undergo neoplastic transformation induced by tumor promoters, we have discovered that several specific transcription factors and protein kinases are critical factors in cancer development and significant targets for cancer prevention and treatment.

Another major goal is to identify anticancer agents that have low toxicity with fewer adverse side effects, which may be used alone or in combination with traditional chemotherapeutic agents to prevent or treat cancer. Many dietary factors have potent anticancer activities that work through, as yet, unknown mechanisms. Over the years, we have been working to identify those mechanisms through our work with signal transduction pathways. Signal transduction is the process by which information from a stimulus outside the cell is transmitted from the cell membrane (e.g., through its receptor) into the cell and along an intracellular chain of signaling molecules to stimulate a response. Various dietary factors, including many isolated from green and black tea, potatoes, broccoli, peanuts, ginger root and rice, can have effects on key signaling molecules crucial in cancer development and prevention.

The expression of genes transcriptionally induced by tumor promoters, such as arsenic, EGF, TPA, or UV, is required to implement the process of tumor promotion. Many transcription factors involved in tumorigenesis are tightly regulated by the mitogen-activated protein (MAP) kinase signaling cascades. The MAP kinase signaling pathways are critical for activator protein-1 (AP-1) activation. AP-1 is known to be involved in cancer development and suppressing its activation can inhibit tumorigenesis. An important group of AP-1 activators are the c-Jun N-terminal kinases 1, 2, and 3.

UV irradiation is categorized by wavelength as UVA I (340-400 nm), UVAII (320-340 nm), UVB (280-320) nm), and UVC (180-280 nm). In mouse skin, UV light acts as both an initiator, presumably by causing DNA damage leading to gene mutations, and as a tumor promoter. The mechanisms behind the tumor promoting ability of UV are areas of intense study in our laboratory. Numerous oncogenic and/or protective signaling pathways are activated in UV-induced carcinogenesis. Very little is known about UV-induced phosphorylation of histones, proteins that are very important in the packaging of DNA. However, recent data suggests that MAP kinases have a distinct and key role in mediating that process. Histone H3 is a basic component of the transcriptional machinery and a structural protein of the nucleosome. Phosphorylation of histone H3 probably has an important role in immediate early gene expression, chromatin remodeling, and chromosome condensation during mitosis. UVB irradiation was shown to markedly induce phosphorylation of histone H3 at serine 28 in JB6 Cl41 cells and the phosphorylation was mediated in varying degrees by ERKs, p38 kinase and JNKs.

We found that the UV-induced signal transduction pathways are mediated primarily through signaling cascades involving the MAP kinases, resulting in the modification of transcription factors, including AP-1, nuclear factor kappaB (NF-kappaB), signal transducer and activator of transcription (STATs), p53 and nuclear factor of activated T cells (NFAT). We also found that histone phos-phorylation is critical to mediate UV or other tumor promotion induced apoptosis and cancer formation.

In addition to the study of UV as a carcinogen and tumor promoter, this laboratory has focused on the elucidation of mechanisms to explain the paradox of arsenic. Arsenic’s enigmatic effects make the elucidation of the exact mechanisms of its cancer-causing effects difficult to discern. Arsenic cannot be precisely classified as a carcinogen because it has not been shown conclusively to be an initiating or a promoting agent of carcinogenesis in animals. Furthermore, in contrast to classic tumor promoting agents, its effects are not reversible. We showed that exposure of JB6 Cl41 cells to arsenite induced cell transformation and phosphorylation and activation of ERKS and JNKs. Of particular interest was the finding that arsenite-induced ERKs activation and cell transformation were blocked in cells expressing the dominant negative ERK2. In contrast, overexpression of a dominant negative JNK1 inhibited arsenite-induced JNK activation but increased arsenite-induced cell transformation. These results demonstrate that activation of ERKs, but not JNKs, by arsenite is required for arsenite’s effects on cell transformation. The results of these two studies support the hypothesis that the induction of ERKs by arsenic may promote arsenic’s carcinogenic effects, whereas induction of JNKs by arsenic may enhance its apoptotic activity and therefore its anticarcinogenic effects.

Further investigations indicated that MSK1, a downstream kinase of p38 and ERK MAP kinases directly monitors UVB-induced phosphorylation of histone H3 at serine 28. We recently found that Fyn, a member of the Src kinase family, is also involved in the UVB-induced phosphorylation of histone H3 at serine 10. Very little is known about the role of histone H3 phosphorylation in malignant transformation and cancer development. We examined the function of H3 phosphorylation in cell transformation in vivo. Introduction of small interfering (si) RNA-H3 into JB6 cells resulted in decreased epidermal growth factor (EGF)-induced cell transformation. In contrast, wild-type histone H3 (H3 WT)-overexpressing cells markedly stimulated EGF-induced cell transformation, whereas the H3 mutant S10A cells suppressed transformation. When H3 WT was overexpressed, EGF induction of c-fos and c-jun promoter activity was significantly increased compared with control cells but not in the H3 mutant S10Aor S28Acells. In addition, AP-1 activity in H3 WT-overexpressing cells was markedly up-regulated by EGF in contrast to the H3 mutant S10Aor S28A cells. These results indicated that the phosphorylation of histone H3 at Ser(10) is an essential regulatory mechanism for EGF-induced neoplastic cell transformation.

We have also focused on the effects of tea in inhibiting carcinogenesis. We have reported that (-)-epigallocatechin- 3-gallate (EGCG) from green tea or theaflavins (TFs ) from black tea inhibit tumor promoter induced AP-1, NF-kappaB activation, MAP kinase activation and cell transformation. We also showed an inhibitory effect of TFs and EGCG on UVB-induced STAT1 (Ser727), ERKs, JNKs, PDK1 and p90RSK2 phosphorylation. We have shown that EGCG or theaflavins block arsenite induced apoptosis of JB6 cells. Searching for the EGCG “receptor” or high affinity proteins that bind to EGCG is the first step in understanding the molecular and biochemical mechanism of the anticancer effects of tea polyphenols. Recently, we identified the intermediate filament protein, vimentin and insulin-like growth factor receptor 1 (IGF-1R), as novel EGCG-binding proteins. Intermediate filament (IF) proteins, such as vimentin, have an important functional involvement in cell division and proliferation. EGCG has been reported to inhibit cell proliferation of a variety of cell lines and in our work, when vimentin expression was suppressed, cell growth was inhibited.

We continue to enjoy our productive collaborations with Drs. C.S. Yang and Allan Conney at Rutgers University, and Drs. Tim Bowden and David Alberts at the University of Arizona on tea research and skin cancer prevention, respectively. More recently, we have worked with Dr. Yuan-Ping Pang at Mayo Clinic to use high-performance computers, modern chemical synthesis and cancer biology to block JNK and develop anti-cancer drugs. Further, we have teamed with IBM and its Blue Gene group under the leadership of Mike Good and the University of Minnesota Super Computing Institute to use the world’s fastest computer to understand complex diseases like cancer; and to screen anti-cancer drugs for cancer prevention and treatment. In collaboration with Dr. Paul Limburg (Mayo Clinic Rochester), we will conduct clinical trials to use cancer preventive agents developed in our institute.

In summary, we address fundamental questions concerning the response of animal and/or human cells to carcinogens and tumor promoters such as UV light, arsenic, TPAand growth factors. We have established a series of necessary models or systems, such as the overagar assay for cell transformation, gene knockout mice, transcription factor/luciferase promoter stably transfected cells and transgenic mice, as well as gene knockdown (siRNA) or dominant negative mutant stably transfected cell lines. These models have been extensively utilized to examine the tumor promoter-induced signal transduction pathways and their role in cell neoplastic transformation. We have systematically studied the signal transduction networks induced by UVA, UVB and UVC. We have described the critical roles of MAP kinases at all three unique wavelengths of UV-induced signal transduction pathways. We have demonstrated the distinctive role of JNK1 and JNK2 in cancer development and have discovered several novel kinases for p53, histone H3, and histone H2AX. Such studies have provided the basis for the carcinogenic process caused by environmental carcinogens and molecular mechanisms for cancer prevention.

Further, we have identified key molecular targets for screening novel natural anticancer drugs with fewer side effects. Nutritional or dietary factors have attracted a great deal of interest because of their perceived ability to act as highly effective chemopreventive agents. They are perceived as being generally safe and may have efficacy as chemopreventive agents by preventing or reversing premalignant lesions and/or reducing second primary tumor incidence. Many of these compounds appear to act on multiple tumor promoter-stimulated cellular pathways. Some of the most interesting and well-documented are resveratrol and components of tea, EGCG, theaflavins and caffeine. Other potentially effective dietary compounds include inositol hexaphosphate, PEITC, ginger and CAPE. Acontinuing emphasis on obtaining rigorous research data and critical analysis of those data regarding these and other food factors is vital to determine the molecular basis and long-term effectiveness and safety of these compounds as chemopreventive agents. Large-scale animal and molecular biology studies are needed to address the bioavailability, toxicity, molecular target, signal transduction pathways, and side effects of dietary factors. Clinical trials based on clear mechanistic studies are also needed to assess the effectiveness of these dietary factors in the human population.

Other Professional Activities

Zigang Dong
Grant Reviewer, National Institutes of Health
Manuscript Review Editor, Molecular Carcinogenesis,
Carcinogenesis, Journal of Biochemistry and Molecular Biology