Tumorigenicity, metastasis and suppression of MHC class-I expression in murine fibroblasts transformed by mutant v-ras deficient in GTP binding

Tumors arise from the excessive repair of damaged stem cells

Although many hypotheses for tumorigenesis have been proposed, none can explain the occurrence and development of tumors comprehensively until now. We put forward a new hypothesis: tumors arise from the excessive repair of damaged stem cells. There are stem cells in all tissues and organs, and the stem cells have perfect damage repair mechanisms, including damage repair systems and repair-inhibiting systems. Tumors arise from the excessive repair of damaged stem cells, i.e., carcinogens induce stem cell damage, leading to overexpression of damage repair systems, and simultaneous inactivation of repair-inhibiting systems through genetic or non-genetic mechanisms, finally forming tumors. The outcome (forming clinically significant tumors or death) and development (tumor recurrence, metastasis or spontaneous healing) of the tumor cells depends on whether the injury and the excessive repair persists, whether immune surveillance function is normal and the tumor microenvironment is appropriate. This hypothesis not only addresses the issues of where tumor cells arise from, how tumors form and where they go, but also provides a reasonable explanation for many unresolved issues in tumor occurrence, development, metastasis or healing. In addition, this hypothesis could guide the early diagnosis, reasonable treatment and effective prevention of tumors.

Down-regulation of the MHC class I antigen-processing machinery after oncogenic transformation of murine fibroblasts

Malignant transformation is often associated with genetic alterations providing tumor cells with mechanisms for escape from immune surveillance. Human and murine tumors of various origin as well as in vitro models of viral and oncogenic transformation express reduced levels of major histocompatibility complex (MHC) class I antigens resulting in decreased sensitivity to MHC class I-restricted cytotoxic T lymphocyte (CTL)-mediated lysis. We here investigate whether the suppressed MHC class I surface expression of ras-transformed fibroblasts is due to dysregulation of the genes of the antigen-processing machinery, the peptide transporters TAP-1 and TAP-2 and the proteasome subunits LMP-2 and LMP-7, and whether it can be restored by gene transfer. In comparison to parental NIH3T3 cells, the ras oncogenic transformants revealed reduced TAP and LMP mRNA expression and impaired function of these genes, leading to deficient peptide transport and peptide loading of MHC class I molecules resulting in instable expression of the MHC class I complex on the cell surface. Enhanced H-2 surface expression due to stabilization of the MHC class I complex could be achieved by culturing ras transformants at low, unphysiological temperature (26 degrees C) or by loading these cells with either exogenous human beta2-microglobulin or MHC class I-binding peptide alone or in combination. Furthermore, interferon-gamma treatment was capable to enhance the expression of TAP, LMP and MHC class I molecules in both parental as well as ras-transformed fibroblasts. Stable transfection of the human TAP-1 cDNA into ras transformants caused a partial reconstitution of the peptide transport and an enhancement of the MHC class I surface expression, whereas the level of MHC class I biosynthesis was not affected by TAP-1 overexpression in parental cells. Together these results point to the existence of an association between oncogenic transformation and deficiencies in the MHC class I antigen-restricted immunosurveillance, suggesting intervention strategies involving specific MHC class I-binding peptides or transfection of the LMP and/or TAP genes to overcome the expression of the immune escape phenotype.

The effect of H-ras expression on tumorigenicity and immunogenicity of Balb/c 3T3 fibroblasts

In an attempt to define immunological parameters affected by the H-ras oncogene, we have used Balb/c 3T3 cells transfected with either H-ras (98/6), H-ras+v-myc (98/4v) or plasmid only (98/1). We found that while control and oncogene-transfected Balb/c 3T3 cells exhibit similar low sensitivity to lysis by natural killer (NK) cells, H-ras+v-myc-transfected cells could immunize syngeneic Balb/c mice and induce cytotoxic T cells (CTL) with broad specificity, that lysed all types of Balb/c 3T3 cells tested. Immunization of Balb/c mice with 98/4v cells prevented homologous tumor formation and partially inhibited the formation of tumors derived from H-ras-transfected cells. 98/6 cells were not immunogenic in vivo and did not protect the animals from a challenge of 98/6 cells. The results suggested that CTLs but not NK effector cells were important for eliciting in vivo tumor rejection of H-ras+v-myc-transfected cells. In contrast, antigens eliciting the cytotoxic T-cell response, and possibly also the in vivo tumor cell rejection response, were expressed on all cell types tested but were immunogenic only on the surface of 98/4v cells. We further determined major histocompatibility complex (MHC) class-I molecule expression on the outer cell surface and found that H-2K was down-regulated in H-ras-transfected cells. The results support the observation that oncogenes can down-regulate specific MHC antigens, thereby preventing presentation of tumor antigens and allowing tumor escape from immune recognition.

Sequential changes in MHC antigen expression induced by the v-Ki-ras oncogene

A series of early-passage cell lines were transformed with the v-Ki-ras oncogene with the aim of examining the effect of an activated ras gene on the ability of these cells to express major histocompatibility complex (MHC) antigens. These cell lines were found to undergo multiple phenotypic changes upon transformation and subsequent proliferation. At early passage, the predominant effect of ras was an increased ability to express class II antigens when induced with interferon gamma (IFN gamma). For class I antigens, maximum levels of expression induced with IFN gamma were largely unaffected, however, decreased sensitivity to induction with this lymphokine was noted. With subsequent in vitro or in vivo passage, both class I and class II antigen inducibility was attenuated. The latter phenotypic change was found to be transferable by coculture, implicating a soluble IFN gamma antagonist. Conditioned media from ras-transformed cells treated to activate their latent transforming growth factor beta (TGF beta) content mediated similar changes in MHC antigen inducibility, suggesting that TGF beta may be involved in modulating MHC antigen expression in ras-transformed cells.

Natural history of HLA expression during tumour development

HLA expression is frequently altered in tumours compared to the tissue from which they originate. Given the central role of MHC products in the restriction of T-cell recognition, regulation of tumour HLA expression might be a strategy for the evasion of immune surveillance by the malignant cells. Federico Garrido, Peter Stern and colleagues present data from a variety of tumour types, suggesting that HLA class I alterations may occur at a particular step between the development of an in situ lesion and an invasive carcinoma.

The H-ras oncogene regulates expression of 70- and 45-kDa cell-surface molecules whose expression correlates with tumor-cell immunogenicity

The effects of the H-ras oncogene on fibroblast cell tumorigenicity and immunogenicity was studied in transfectants of the BALB/c 3T3 clone A31 fibroblastoid cell-line. Cells that were transfected with MC29-LTR-H-ras (98/6) or MC29-LTR-v-myc + H-ras (98/4v) and were inoculated into syngeneic BALB/c mice were tumorigenic in 100% and 60% of animals respectively. By contrast, transfectants containing the pSV2neo plasmid alone (98/1) displayed normal characteristics both in vitro and in vivo. Inoculation of mice with mitomycin-C-treated 98/1 or 98/4v cells induced an effective protective immunity to a challenge of live 98/4v cells, and a partial immunity against 98/6 cells. Mitomycin-C-treated 98/6 cells failed to render immunity against a challenge of either 98/6 or 98/4v cells. To correlate immunogenicity and tumorigenicity of the different cell types with cell-surface-antigen expression, we prepared MAbs against 98/4v cells in syngeneic mice. Immunohistochemical and immunoblot analysis revealed that MAbs 102 and 104 recognized 2 protein band of 70 and 45 kDa respectively, which were expressed predominantly in 98/1 and 98/4v cells. A third immunoreactive protein band of 44 kDa that reacted with MAb 6 was expressed at a similar cell-surface density on all cell types. Cell-differentiation-inducing agents, such as DMSO, retinoic acid or sodium butyrate, were all found to induce 98/6 cell flattening and morphological changes toward a normal phenotype that were followed by up-regulation of the 70- and 45-kDa antigens. The results suggest that regulation of expression of the 70- and 45-kDa molecules is affected by H-ras, and that expression of these cell-surface molecules may be relevant to tumor cell immunogenicity.