Improving cell therapy—experiments using transplanted telomerase-immortalized cells in immunodeficient mice

Abstract

Cell therapy is the use of stem cells and other types of cells in various therapies for age-related diseases. Two issues that must be addressed before cell therapy could be used routinely in medicine are improved efficacy of the transplanted cells and demonstrated long-term safety. Desirable genetic modifications that could be made to cells to be used for cell therapy include immortalization with hTERT (human telomerase reverse transcriptase). We have used a model for cell therapy in which transplantation of adrenocortical cells restores glucocorticoid and mineralocorticoid hormone levels in adrenalectomized immunodeficient mice. In this model, clones of cells that had been immortalized with hTERT were shown to be able to replace the function of the animals'adrenal glands by forming vascularized tissue structures when cells were transplanted beneath the capsule of the kidney. hTERT-modified cells showed no tendency for neoplastic changes. Moreover, a series of experiments showed that hTERT does not cooperate with known oncoproteins in tumorigenesis either in adrenocortical cells or in human fibroblasts. Nevertheless, hTERT was required for tumorigenesis when cells were implanted subcutaneously rather than in the subrenal capsule space. Changes in gene expression make hTERT-modified cells more robust. Understanding these changes is important so as to be able to separately control immortalization and other desirable properties of cells that could be used in cell therapy. Alternatively, desirable properties of transplants might be provided by cotransplanted mesenchymal cells: mesenchymal cell-assisted cell therapy. For both hTERT modification and mesenchymal cell-assisted cell therapy, genomics approaches will be needed to define what genetic modifications are desirable and safe in cells used in cell therapy.

Stem and progenitor cell-mediated tumor selective gene therapy

“…With respect to the first mechanism, several genes have been evaluated for their potential utility in immortalizing stem cells for use as delivery vehicles: the proto-oncogene v-myc, 3,70,72,[89][90][91] the catalytic component of the telomerase complex (human telomerase reverse transcriptase (hTERT)), human papillomavirus type 16 E6/E7 genes, Bmi-1 [92][93][94][95][96][97][98][99] and an N-terminal fragment of SV40 large T-antigen. 2,100 Characteristics of the resultant cell lines, each of which had extended life spans in culture, have been reviewed.…”

Estrogen deficiency leads to telomerase inhibition, telomere shortening and reduced cell proliferation in the adrenal gland of mice

“…In our recent studies in the ovarian tissue, we have found that estrogen regulates telomerase activity in the granulosa cells of ovarian follicles (manuscript submitted). Given that transplantation of adrenocortical cells expressing telomerase activity restores the adrenal hormone levels in adrenalectomized mice [36], it is likely that the adrenal gland preserves a population of telomerase-positive cells to retain cell proliferation capacity with differentiation for adequate function of the adrenal gland in mice. Consistent with the hypothesis that estrogen regulates telomerase activity and thereby tissue genesis and repair in a tissue-specific manner, elimination of estrogen causes a significant decrease of telomerase activity in association with the loss of tissue weight of the adrenal gland.…”

Cell vehicle targeting strategies

“…[119][120][121][122] Similarly, ectopic expression of the catalytic domain of human telomerase reverse transcriptase has been evaluated as a strategy to limit proliferative senescence in primary cell populations. 123,124 Despite their potential for enhancing therapeutic cell survival and proliferation, many of these gene products are also associated with malignant transformation. Therefore, nonconstitutive or pharmacologically regulated expression strategies may be required to ensure safety.…”