Dingzhi Wang

Activation of nuclear hormone receptor peroxisome proliferator–activated receptor-δ accelerates intestinal adenoma growth

We treated Apcmin mice, which are predisposed to intestinal polyposis, with a selective synthetic agonist of peroxisome proliferator–activated receptor-δ (PPAR-δ). Exposure of Apcmin mice to the PPAR-δ ligand GW501516 resulted in a significant increase in the number and size of intestinal polyps. The most prominent effect was on polyp size; mice treated with the PPAR-δ activator had a fivefold increase in the number of polyps larger than 2 mm. Our results implicate PPAR-δ in the regulation of intestinal adenoma growth.

The tumorigenic and angiogenic effects of MGSA/GRO proteins in melanoma.

Continuous expression of the MGSA/GROα, β, or γ chemokine bestows tumor‐forming capacity to the immortalized murine melanocyte cell line, melan‐a. The mechanism for this transformation is unclear, although both autocrine and paracrine processes are possible because melan‐a cells as well as endothelial cells express a low level of the receptor for this ligand. To further define the role of MGSA/GRO proteins in melanocyte transformation, two types of experiments were designed to neutralize the biological effects of MGSA/GRO in the transfected melan‐a clones: (1) the effect of neutralizing antiserum to MGSA/GRO proteins on melan‐a tumor growth was assessed; (2) the tumor‐forming capacity of melan‐a clones expressing ELR motif‐mutated forms of MGSA/GRO with compromised receptor affinity was compared to the tumor‐forming capacity of clones expressing wild‐type MGSA/GRO. These experiments revealed that SCID mice inoculated with MGSA/GROα‐ or γ‐expressing melan‐a cells and subsequently treated with antiserum to the respective chemokine exhibited decreased tumor growth. This reduction in tumor growth was accompanied by declining angiogenic activity in MGSA/GROγ‐expressing tumors. Moreover, athymic nude mice injected with melan‐a cells expressing ELR‐mutant forms of MGSA/GROα exhibited markedly impaired tumor‐forming capacity compared with those mice injected with melan‐a clones expressing wild‐type MGSA/GRO. These data suggest that continuous expression of MGSA/GRO proteins may facilitate tumor growth by stimulating the growth of microvessels into the tumor (paracrine) and by affecting melanocyte growth (autocrine). J. Leukoc. Biol. 67: 53–62; 2000.

Role of Clathrin-mediated Endocytosis in CXCR2 Sequestration, Resensitization, and Signal Transduction

CXCR2 is a seven-transmembrane receptor that transduces intracellular signals in response to the chemokines interleukin-8, melanoma growth-stimulatory activity/growth-regulatory protein, and other ELR motif-containing CXC chemokines by coupling to heterotrimeric GTP-binding proteins. In this study, we explored the mechanism responsible for ligand-induced CXCR2 endocytosis. Here, we demonstrate that dynamin, a component of clathrin-mediated endocytosis, is essential for CXCR2 endocytosis and resensitization. In HEK293 cells, dynamin I K44A, a dominant-negative mutant of dynamin that inhibits the clathrin-mediated endocytosis, blocks the ligand-stimulated CXCR2 sequestration. Furthermore, co-expression of dynamin I K44A significantly delays dephosphorylation of CXCR2 after ligand stimulation, suggesting that clathrin-mediated endocytosis plays an important role in receptor dephosphorylation and resensitization. In addition, ligand-mediated receptor down-regulation is attenuated when receptor internalization is inhibited by dynamin I K44A. Interestingly, inhibition of receptor endocytosis by dynamin I K44A does not affect the CXCR2-mediated stimulation of mitogen-activated protein kinase. Most significantly, our data indicate that the ligand-stimulated receptor endocytosis is required for CXCR2-mediated chemotaxis in HEK293 cells. Taken together, our findings suggest that clathrin-mediated CXCR2 internalization is crucial for receptor endocytosis, resensitization, and chemotaxis.

The role of chemokines in intestinal inflammation and cancer.

Chronic inflammation is a risk factor for several gastrointestinal malignancies, including colorectal cancer. Recent epidemiological studies and clinical trials demonstrate that long-term use of non-steroidal anti-inflammatory drugs (NSAIDs) markedly reduced the relative risk of colorectal cancer. Chronic inflammation associated with development of cancer is partly driven by the chemokine system. Chemokines are chemoattractant cytokines that recruit leukocytes from the circulatory system to local inflammatory sites. In this review, we highlight recent breakthroughs in our understanding of the role of chemokines in inflammatory bowel disease and colorectal cancer from animal models and human studies. These findings provide a rationale for the development of new anti-inflammatory therapeutic approaches for prevention and/or treatment of inflammatory bowel disease and colorectal cancer.

Peroxisome Proliferator-Activated Receptor β/δ Expression and Activation in Lung Cancer

Peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) is a ligand-binding inducible transcriptional factor linked to carcinogenesis. Important functions of PPARbeta/delta were demonstrated in series of human epithelial cancers; however, its role in lung cancer remains controversial. We investigated the differential expression level and localization of PPARbeta/delta in tumors and adjacent normal lung tissue, and the effect of PPARbeta/delta activation on lung cancer cell proliferation and apoptosis. PPARbeta/delta was expressed in all studied human non-small cell lung cancers, and strong PPARbeta/delta immunoreactivity was observed in epithelial cells of more than 75% of studied lung tumors. PPARbeta/delta expression was consistently limited to the cancer cells in tumor tissue, while in adjacent normal lung tissue it was limited predominantly to the mononuclear cells. We found that ligand-binding activation of PPARbeta/delta stimulates cell proliferation (an effect that was blocked by a dominant-negative construct of PPARbeta/delta), stimulates anchorage-independent cell growth, and inhibits apoptosis in lung cancer cell lines. Importantly, the activation of PPARbeta/delta induces Akt phosphorylation correlated with up-regulation of PDK1, down-regulation of PTEN, and increased expression of Bcl-xL and COX-2. These findings indicate that PPARbeta/delta exerts proliferative and anti-apoptotic effects via PI3K/Akt1 and COX-2 pathways. In conclusion, PPARbeta/delta is strongly expressed in the majority of lung cancers, and its activation induces proliferative and survival response in non-small cell lung cancer.

HMG-I(Y) Phosphorylation Status as a Nuclear Target Regulated through Insulin Receptor Substrate-1 and the I4R Motif of the Interleukin-4 Receptor

Interleukin (IL)-4 is a cytokine that regulates both the growth and differentiation of hematopoietic cells. Its ligand binding specificity and important signal transduction mechanisms are conferred by the IL-4 receptor α chain (IL-4Rα). The I4R is a tyrosine-containing motif within IL-4Rα that is critical for proliferative responses to IL-4. Although the I4R also contributes to gene regulation, nuclear targets directly regulated by this motif have not been described. It is shown here that the tyrosine at position 497 in the I4R is critical for regulation of the phosphorylation status of a set of nuclear proteins that includes HMG-I(Y), small non-histone chromosomal proteins involved in the control of gene expression in hematopoietic cell lines. Moreover, IL-4 is unable to induce HMG-I(Y) phosphorylation in insulin receptor substrate-1-deficient cells, and the inhibitor wortmannin completely blocks IL-4 regulation of HMG-I(Y) phosphorylation status but not activation of an IL-4 Stat protein. Taken together, these data indicate that HMG-I(Y) is a nuclear target whose phosphorylation status is regulated through the I4R motif via insulin receptor substrate proteins, independent of activation of the Stat pathway.