| Abstract |
Three major groups of genes may be used for cancer gene therapy: (i)
oncogenes and tumor suppressor genes; (ii) genes involved in the control
of tumor progression and metastasis; and (iii) genes encoding proteins
protecting the organism from tumor cells. Each group contains numerous
genes, and the discovery of new important genes is an exciting prospect in
cancer research. We are working on the search and characterization of the
genes over-or under-expressed in metastatic comparing to non-metastatic
tumors of the same origin. Two mouse systems are being used: (i) VMR-0
(non-metastatic mammary adenocarcinoma cells) -VMR-100-Liv and VMR-100-Ov
cells (metastatic preferentially to the liver or ovaries, respectively);
and (ii) CSML-0 -CSML-100 (mammary adenocarcinoma cells non-metastatic and
metastatic to the lungs, respectively). Several different genes were found
to be over-expressed in metastatic cells, but only few of them were shown
to be necessary and sufficient for maintaining the metastatic phenotype
using stably transfected cells and/or transgenic animals. Among them are
the mts1 and c-met genes. The mts1 gene, encoding a calcium-binding
protein of 101 amino acids of the S-100 family, was extensively
characterized. Its expression induced a number of changes in cell
functions connected with cytoskeleton features, attachment properties of
the cell, mesenchyme formation and possibly tumor vascularization. As a
multifunctional regulator, the mts1 gene is a promising target for gene
therapy of cancer. Other genes identified are over-expressed only in few
metastatic tumors and do not seem to be connected directly with the
acquisition of the metastatic phenotype. However, during the transfection
experiments some interesting features emerged for these genes, raising the
possibility of their exploitation in cancer gene therapy. The most
interesting is the tag7 gene encoding a new cytokine, 182 amino acids
long, with a far distant relation to cytokines of the TNF-Lymphotoxin
family. The tag7 gene is expressed in lymphoid cells, in a limited set of
other normal cells, and in few cancer cells including myelomas. The Tag7
protein is secreted to the culture medium and possesses a strong cytotoxic
activity inducing apoptosis. VMR-0 cells were stably transfected with a
construct containing the tag7 gene under control of the CMV promoter. The
original VMR-0 tumors killed mice in one month after subcutaneous
transplantation; animals displayed large necrotic foci at this stage.
However, the VMR-0/tag7 cells, synthesizing very low amounts of Tag7
protein, exhibited dramatically different growth properties: they grew
much slower; even after 4 months, no mice were killed by tumors arising
from the transplanted cells and no necrotic foci were formed. Histological
analysis of VMR-0/tag7 tumors showed a strong inhibition in mitotic rates
and an enhanced rate of apoptosis compared to VMR-0 tumors. The tumors
induced by transplantation of a mixture of VMR-0 and VMR-0/tag7 cells also
grew much slower than VMR-0 cells alone, suggesting an activation of the
immune system against tumor (tumor vaccination effect), which may be
mediated through induction of CTL cells. Experiments with nude mice gave
similar results. In fact at later stages of development in nude mice,
VMR-0/tag7 tumors were completely eradicated. It seems that the effect of
tag7 expression is complex and includes 381 Georgiev et al: Genes involved in
the control of tumor progression in gene therapy activation of an immune
response as well as a direct cytotoxicity. The higher tag7 expression in
culture cells is incompatible with cell survival. Experiments are in
progress for further elucidating the role of Tag7 and its exploitation for
the development of tumor vaccines.
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