The Roles of Tumor Induced Factor (TIF) in Stromal-Tumor Interactions

Lines of evidence indicate that stromal cell is one of the determinants in tumor formation by providing a favorable microenvironment for the growth of cancer cells. Cancer associated fibroblast (CAF) is a special form of stromal cells which are shown to be derived from bone marrow. Upon reaching the tumor, the bone marrow-derived mesenchymal stem cells differentiate into CAF, which secrets various growth factors and cytokines to promote cancer growth. Furthermore, genetic study shows that CAF displays p53 mutations and other genetic changes.;Tumor induced factor (TIF) is a CXC chemokine that is originally identified from a xenograft tumor. Sequence analysis suggests TIF is a family member of the Gro CXC chemokines, and exerts its cellular function via activating CXCR2 receptors. In order to investigate the functional roles of TIF, a stable cell line over-expressing TIF in hamster CHO-K1 was established.;To explore the cancer-stromal interactions in xenograft, mouse embryonic fibroblast (MEF) were used as a study model for CAF. MEF was sub-cultured by a conventional protocol that was used for developing the NIH3T3 cells. Based on the growth patterns and expressions of cell markers, growth of MEF can be divided into three stages: the early stage, the senescent stage and the immortalized stage. Our results suggested that MEF might mirror the various developmental stages of CAF.;To examine the contributions of MEF in tumorigenesis, CHO-K1 cells and MEF were co-injected into nude mice. Intriguingly, MEF that in senescent and immortalized stages, rather than in early stage, promoted tumor formation. A possibility arose that the contribution of senescent and immortalized MEF in promoted tumorigenesis may due to CD133 and CXCL1, as the expression of CD133 and CXCL1 in senescent and immortalized MEF were higher than that of MEF in early stage. Moreover, as MEF could gradually develop into a fibroblast promoted tumor formation, MEF could be used as a crucial model to illustrate the origination and development of CAF.;Surprisingly, in nude mice co-injected with immortalized MEF with TIF-overexpressing D12 cells, suppression instead of promotion of tumor growth was found. In order to explore the underlined mechanism of tumor suppression, recombinant TIF protein was purified based on a bacterial expression system. Using purified TIF protein to treat CHO-K1 cells and MEF, it was found that low concentration of TIF promoted Erk phosphorylation but high concentration of TIF suppressed it, which might resulted from desensitization of CXCR2 receptors. Reduction of Erk phosphorylation resulted in decreased proliferation in CHO-K1 cells and alleviated expression of CD133 in MEF, which could be the mechanisms for TIF-induced tumor suppression in nude mice.;Taken together, a CAF model was established to examine the function of TIF in tumor-fibroblast interactions. Mechanistic studies indicated that TIF-induced tumor suppression in nude mice was mediated via desensitization of CXCR2 receptors by high concentration of TIF in the tumor microenvironment.