Gastrointestinal Carcinoma Antigen GA733: Target for Immunodestruction and Potential Modifier of Invasiveness and Chemoresponsiveness

Ligation of cell surface receptors for extracellular matrices and for other cells, including members of the integrin family of receptors, CD44, and the cadherins, generates downstream signals necessary for cellular differentiation, gene expression, the induction of ion fluxes, and the regulation of tumor progression, invasion, and metastasis (1–5). The role of altered expression of members of the integrin family and the E-cadherin family of receptors in tumor development and progression has been intensely studied during the last 10 years (6–8). The report by Basak et al. (9) appearing in this issue of the Journal focuses on the ability of a less well understood homophilic adhesion receptor, the human gastrointestinal carcinoma antigen GA733, to alter growth, invasion, and metastasis of cells of a colorectal carcinoma cell line. In earlier studies (10–15), GA733 was shown to be a potential target for passive and active immunotherapy in patients with advanced colorectal carcinoma. In a randomized phase II clinical trial (12), 189 patients with colorectal cancer (Dukes' stage C) were assigned to receive either postoperative treatment with monoclonal antibody (MAb) 17-1A, which is directed against the GA733 antigen, or no postoperative therapy. After a median follow-up of 5 years, the overall death rate was reduced by 30%, and the rate of recurrence was reduced by 27% in patients receiving the antibody. The beneficial effect was most pronounced for patients with distant metastases at first relapse. Basak et al. (9) now compare the effect of re-expressing either the human antigen GA733 or its murine homologue, murine epithelial glycoprotein (mEGP), in the mEGP-negative colorectal carcinoma cell line CT-26 with respect to tumor cell behaviors, such as adhesion, anchorage-dependent and anchorage- independent growth, invasiveness, and metastasis. Basak et al. demonstrate the function of mEGP as a homotypic adhesion receptor similar to that of its human homologue. They report that CT-26 cells expressing mEGP adhered to full-length mEGP and to a recombinant mEGP extracellular domain. In contrast, human GA733-expressing CT-26 cells adhered to full-length GA733 protein but failed to adhere to the recombinant GA733 extracellular domain. These findings suggest that differences in the structures of the mouse and the human proteins, resulting from amino acid differences or from the alterations in the posttranslational modification of the human protein expressed in murine cells, may produce altered adhesive function. Expression of mEGP in murine CT-26 cells resulted in increased anchoragedependent cell growth, decreased anchorage-independent cell growth in soft agar, decreased invasiveness in vitro, and decreased metastasis in syngeneic immunodeficient and immunocompetent mice. Somewhat surprisingly, Basak et al. found that expression of human GA733 in the same murine colorectal carcinoma cell line had no effect on either anchorage-dependent or anchorage-independent cell growth, but it did diminish invasiveness in vitro. Thus, Basak et al. (9) describe another cell-cell adhesion molecule that, like E-cadherin, decreases the metastatic potential of tumor cells in vivo. As previously shown for E-cadherin, increased cell-cell adhesion may diminish a cell's motile and invasive behavior. The data presented by Basak et al. raise a number of interesting thoughts, including the following: Although expression of either mEGP or human GA733 in CT-26 murine colorectal carcinoma cells diminished invasiveness in vitro, only expression of mEGP altered anchoragedependent and anchorage-independent growth of these cells. Since mEGP and GA733 are 82% identical at the amino acid sequence level (16), these findings suggest that minor differences in structure in either the extracellular or the intracellular domains drastically affect both adhesion and cell growth. Perhaps the cytoplasmic tails of the murine and the human proteins differ in their ability to interact with cytoskeletal scaffolds or signaling molecules, and, therefore, they affect adhesion and/or growth differently. The difference between the two species may provide clues to the regulation of growth inhibition. As Basak et al. pointed out, expression of GA733 in a GA733-negative human colorectal carcinoma cell line will begin to address this possibility. Although GA733 is expressed at low levels in normal human tissue and its expression is increased in colon cancer in vivo (10,17–20), expression of mEGP and GA733 decreased the invasiveness of CT-26 colorectal cancer cells in vitro, and expression of mEGP decreased metastasis in vivo. In this regard, it should be noted that molecular interactions that influence one aspect of tumorigenesis do not necessarily influence other parameters. The recently described (21–25) molecular changes associated with colon cancer progression provide an illustrative example. Alterations in the gene for adenomatous polyposis coli (APC) lead to increased cell proliferation and result in the development of adenomatous polyposis (24,25). Only in this background of mutant APC do alterations in the SMAD 4 (DPC4) gene lead to tumor progression and a malignant phenotype, i.e., invasive behavior (21). In addition, several studies (26–32) have demonstrated a role for the a6b4 integrin in increasing cellular invasiveness while at the same time resulting in growth arrest via activation of the cyclin-dependent kinase inhibitor p21 (WAF1/ CIP1). In a phase II trial using MAb 17-1A against GA733 (11), patients who were randomly assigned to receive postoperative therapy had significantly lower rates of death and tumor recurrence than the control patients. Results from a number of laboratories (10–15) have demonstrated an immunomodulatory role for MAb 17-1A in eliciting an antigen-specific response, the production of anti-idiotypic antibodies, and antibody-dependent cellular cytotoxicity. These studies suggest that the beneficial effect of MAb 17-1A is primarily due to the targeting of cancer cells for immunodestruction. The experiments reported by Basak et al. (9) raise at least one other possibility in targeting GA733 in vivo. Although GA733 is typically expressed at high levels in all stages of colon cancer, high-level expression of either this protein or its mouse homologue in CT-26 murine colorectal cancer cells suppresses invasion and/or metastasis in the murine environment. Litvinov et al. (33) have suggested that the GA733 molecule may become the predominant cell-cell adhesion receptor used by cancer cells as E-cadherin expression is diminished. Increased expression of GA733 relative to that of Ecadherin may result in the disorganization of normal cellular architecture and in the formation of multicellular spheroids. The formation of multicellular aggregates of tumor cells increases their resistance to chemotherapeutic agents, a phenomenon described 25 years ago as the “contact effect.” Kerbel et al. (34) recently demonstrated that the so-called “contact effect” was due to a reduced growth fraction of cells in tumor spheroids, resulting from the up-regulation (i.e., increased expression) of the cyclin-dependent kinase inhibitor p27kip1. Additional experiments (34,35) showed that disruption of tumor spheroids resulted in an increased growth fraction and increased chemosensitivity of the tumor cells both in vitro and in vivo. These studies suggest that the use of antibodies that disrupt cell-cell adhesion and prevent the formation of spheroids could render tumor cells more sensitive to chemotherapeutic agents. Additional clinical trials using inhibitory antibodies directed against GA733 in combination with chemotherapy may show a marked improvement in clinical response.