||Associate Professor, Department of Nutrition Sciences
Associate Scientist, Nutrition Obesity Research Center
University of Alabama at Birmingham
Department of Nutrition Science
1675 University Blvd
311 Webb Nutrition Sciences Building
Birmingham, AL 35294-3360
Phone: (205) 975-6624
Fax: (205) 996-5775
Regulation of calcitriol on TPA-induced tumorigenesis and the involvment of osteopontin in tumorigenesis
Current Research Focus
Most initiated cells that contain genetic alterations by carcinogens or spontaneously can remain dormant and will not develop into tumors or progress to malignancies unless further internal or external factors, called tumor promoters, persistently stimulate these cells. Therefore, benign tumor and later malignancies commonly develop much later in life. The mechanisms of tumor promotion are not well understood. Our long-term goal is to understand how matrix proteins stimulated by tumor promoters might participate in the process of tumorigenesis. Specifically, we are studying the secreted, adhesive protein, osteopontin (OPN). OPN expression is elevated in several types of human cancers and premalignant tumors. Evidences from in vitro experiments suggest the role of OPN as a mediator of tumor promotion. Thus, we hypothesize that induced OPN acts as an autocrine or paracrine stimulus for tumor promotion of initiated cells. To test this hypothesis, we are using both in vitro and in vivo tumor promotion models.
In vitro studies from our laboratory and others using JB6 cells
To study the role of osteopontin (OPN) in tumor promotion in vitro, we are using the preneoplastic mouse promotable JB6 cell model established by Dr. Nancy Colburn in NCI. These cells do not form colonies in soft agar (an assay, which highly correlates with tumorigenesis in vivo), nor do they form tumors in nude mice. However, upon treatment with the skin tumor promoter, 12-0-tetradecanolyphorbol-13-acetate (TPA), JB6 cells are irreversibly transformed as indicated by the formation of colonies on soft agar. Concomitantly, TPA dramatically stimulates the synthesis and secretion of the phosphorylated glycoprotein, OPN (Smith and Dendhardt, J Cell Biochem, 1987, Craig et al., J Biol Chem, 1989; Chang and Prince, Cancer Res, 1991, 1993). Furthermore, TPA-induced OPN expression is mediated through protein kinase C and mitogen-activated protein kinase (Chang et al., Intl J Biochem & Cell Biol, 2002). We have previously shown that treatment of these cells with TPA, greatly enhanced the affinity or avidity of the cells to OPN mediated most likely through the integrin receptor, ?v?5 (Chang and Chambers, J. of Cell Biochem, 2000). To determine whether the induction of OPN is required for the transformation process of JB6 cell, we tested whether the addition of OPN (to mimic overexpression) stimulates cell transformation or the suppression of TPA-induced OPN expression by antisense OPN will suppress colony formation on soft agar and consequently, will the addition of OPN rescue this inhibition. Our findings showed that the addition of OPN dose-dependently stimulated colony formation and that suppression of TPA-induced OPN expression by antisense OPN inhibited colony formation, which was partially rescued by the addition of OPN. Thus, we have shown that the induction of OPN is required for TPA-induced transformation of JB6 cells (Chang et al., Carcinogenesis, 2003).
To examine the role of OPN in tumor promotion in vivo studies are in progress using the classical tumor promotion model, the two-stage mouse skin carcinogenesis system, on OPN null mice and inducible OPN transgenic mice. Previous studies on wild-type mice have shown that OPN mRNA is expressed by 13 h in the epidermis and also in papillomas and squamous cell carcinomas (Craig et al., Intl J Cancer, 1990; Hashimoto et al., Mol Carcinogen, 1990). We will examine the expression of OPN more closely at the early tumor promotion period and correlate that with early skin responsive parameters to TPA treatment and compare our findings to those of OPN null mice. We hope that our studies will provide more in sight on the role of the inducible form of OPN in tumor promotion. Interestingly, recently generated homozygous transgenic OPN mice showed high mortality rate resulting from spontaneous development of malignant tumors in various organs.
Expression of OPN in human skin
In addition to these studies, our laboratory is also exploring the expression of OPN in normal human epidermis, premalignant skin (actinic keratosis), squamous cell carcinomas (SSC) and basal cell carcinomas (BCC). Immunohistochemistry studies showed that OPN is expressed in the granular layer of normal skin, in actinic keratoses, SSC and only in more differentiated forms of BCC (Chang et al., Abstract presented at AACR, 2002). Further studies are in progress to verify our findings.
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