Tumor promoters retard the loss of a transient subpopulation of cells in low passage Syrian hamster cell cultures

Enhancement of the morphological transformation of Syrian hamster embryo (SHE) cells by reducing incubation time of the target cells

Syrian hamster embryo (SHE) cell transformation has been used for many years to study chemical carcinogenesis in vitro. It has been shown that this assay is probably the most predictive short-term test system for identifying rodent carcinogens. Although most of the operational difficulties encountered in the early stage of application of this assay have been overcome by culturing the SHE cells under slightly acidic conditions (pH 6.7), a relatively low level of induction of morphological transformation (MT) by known carcinogens still occurs for many cell isolates. In order to improve the response of this assay system to known carcinogens, the effect of incubation time of target SHE cells on the frequency of morphological transformation induced by benzo(a)pyrene (BaP) was investigated. It was shown that the morphological transformation frequency induced by BaP increased significantly (1.4-2.5-fold) when the incubation time of target cells was reduced from the usual 24h to less than 6h prior to seeding onto feeder layers. This improvement in sensitivity was consistent for different cell isolates. In addition, the enhanced response appeared to be a property of carcinogens because treatment with two non-carcinogens, l-ascorbic acid and 4-nitro-o-phenylenediamine, did not induce significant increases in the transformation frequency under the shortened incubation period for target cells. These results suggest that the response of the SHE cell transformation assay may be improved by optimizing the incubation time of the target SHE cells. In addition, the results of the present study provide further evidence to support the idea that morphological transformation of SHE cells results from a block of cellular differentiation of stem or stem-like cells.

The pH 6.7 Syrian hamster embryo cell transformation assay for assessing the carcinogenic potential of chemicals

Cell transformation models have been established for studying the cellular and molecular basis of the neoplastic process. Transformation models have also been utilized extensively for studying mechanisms of chemical carcinogenesis and, to a lesser degree, screening chemicals for their carcinogenic potential. Complexities associated with the conduct of cell transformation assays have been a significant factor in discouraging broad use of this approach despite their reported good predictivity for carcinogenicity. We previously reported that many of the experimental difficulties with the Syrian hamster embryo (SHE) cell transformation assay could be reduced or eliminated by culturing these cells at pH 6.7 culture conditions compared to the historically used pH 7.1-7.3. We and others have shown that morphological transformation (MT), the earliest recognizable phenotype in the multi-step transformation process and the endpoint used in the standard assay to indicate a chemical's transforming activity, represents a pre-neoplastic stage in this model system. In the collaborative study reported here, in which approx. 50% of the chemicals were tested under code in one laboratory (Hazelton) and the other 50% evaluated by several investigators in the second laboratory (P & G), we have evaluated 56 chemicals (30 carcinogens, 18 non-carcinogens, 8 of inconclusive carcinogenic activity) in the SHE cell transformation assay conducted at pH 6.7 culture conditions with a standardized, Good Laboratory Practices-quality protocol. An overall concordance of 85% (41/48) between SHE cell transformation and rodent bioassay results was observed with assay sensitivity of 87% (26/30) and specificity of 83% (15/18), respectively. The assay exhibited a sensitivity of 78% (14/18) for Salmonella assay negative carcinogens, supporting its value for detecting non-mutagenic carcinogens. For maximum assay sensitivity, two exposure durations were required, namely a 24-h exposure and a 7-day exposure assay. Depending on the duration of chemical treatment required to induce transformation, insight into the mechanism of transformation induction may also be gained. Based on the data reported here, as well as the larger historical dataset reviewed by Isfort et al. (1996), we conclude that the SHE cell transformation assay provides an improved method for screening chemicals for carcinogenicity relative to current standard genotoxicity assays.

Gestation stage-specific frequency of adipogenic cells in Syrian hamster cell cultures

High frequencies (up to 50%) of spontaneous adipocyte differentiation are observed in cultures of 9 day gestation Syrian hamster embryos (E9 cells) within six to eight population doublings after primary culture. This is in contrast to the absence of adipogenic cells in primary cultures derived from later gestation age Syrian hamster tissue. In addition, E9 primary cultures contain a transient subpopulation of presumptive mesenchymal stem or progenitor cells that lack density dependent inhibition of growth [contact-insensitive (CS-) cells]. Analysis of the temporal pattern of expression of the CS- and adipocyte phenotypes during the proliferative life span of E9 cells demonstrates that maximal expression of the CS- phenotype precedes maximal expression of adipocyte differentiation. In addition, lipid accumulation appears to occur primarily, if not exclusively, in the contact-sensitive (CS+) cells that are derived from CS- cells. These observations suggest that primary E9 cultures contain either adipoblasts or primordial mesenchymal cells that become determined to the adipocyte lineage early during the in vitro life span of the cultures, and that the CS- phenotype may be a marker for these earlier developmental cell stages.

Phorbol ester-induced reorganization of the cytoskeleton in human fibroblasts during ageing in vitro

Phorbol esters induce drastic morphological alterations in cells of different origin by altering the conformation and the interrelationship of the elements of the cytoskeletal system. Treatment of early passage (young) and late passage (senescent) human fibroblasts MRC-5 with phorbol-12-myristate-13-acetate (PMA) results in the rearrangement of actin and tubulin filaments. PMA brings about the disorientation and diffusion of the heavily criss-crossed network of actin and microtubulin fibres characteristic of senescent cells suggesting thereby an increased sensitivity of senescent cells to phorbol esters. Since phorbol esters are known to be specific activators of protein kinase C (PKC), the PMA-induced modulation of the cytoskeleton patterns in ageing fibroblasts provides further support for the view that the effectiveness of the signalling mechanisms is retained during cellular ageing.

Age-related differences in promoter-induced extension of in vitro proliferative life span of Syrian hamster fibroblasts

The proliferative capacity of Syrian hamster dermal fibroblasts has been previously shown to be inversely related to the age of the donor (Mech. Ageing and Dev., 34 (1986) 151). The present study demonstrates an inverse correlation between in vivo age and the in vitro morphological and proliferative response of Syrian hamster dermal fibroblasts to the tumor promoter phorbol-12,13-didecanoate. Treatment of fetal fibroblasts with promoter increased the proliferative life span of the cultures by approximately 2-fold, but did not increase the frequency of conversion to established cell lines. Neonatal and young adult fibroblasts exhibited intermediate responses to promoter treatment, showing 54.9% and 33.1% extension, respectively. In contrast, promoter treatment had no significant effect on aged adult fibroblasts. Maximal extension required continual treatment beginning in primary culture or at passage 1. Promoter-induced extension of proliferative life span appears to be mediated through the prolonged maintenance of small, highly proliferative cells that are present in primary cultures of these cells.

Ultrastructure of dermal fibroblasts during development and aging: relationship to in vitro senescence of dermal fibroblasts

One approach to understanding the relationship between in vitro cellular senescence and in vivo aging is to define the development and aging of cells in vivo and then to compare these in vivo properties with the in vitro behavior of the same cells. The Syrian hamster is being used as an experimental aging model to investigate the intrinsic developmental program of dermal fibroblasts in vivo (proliferation, extracellular matrix (ECM) production, quiescence, and reactivation during wound repair) in order to determine whether the in vivo differentiation program and mature function of these cells is related to their in vitro proliferation and senescence pattern. The ultrastructure of dermal fibroblasts from midfetal development through old age is described, and a working hypothesis of the development and aging of dermal fibroblasts is proposed as a framework for further evaluation of the relationship between in vitro proliferative capacity of dermal fibroblasts and in vivo developmental and age-related changes in the dermis.

Two classes of continuous cell lines established from Syrian hamster 9 day gestation embryos: preneoplastic cells and progenitor cells

Primary cultures of 9-d-gestation Syrian hamster embryo (E9) cells are distinct from primary cultures of later gestational age in terms of their growth and differentiation. First, primary E9 cell cultures express multiple mesenchymal differentiation lineages (e.g., adipocyte, myoblast) only rarely seen in cultures of 13-d-gestation fetal (F13) cells. Second, although most primary E9 cultures have a limited in vitro proliferative life span and exhibit cellular senescence similar to primary cultures of F13 cells, E9 cultures seem to have higher frequency of escape from senescence and conversion to continuous cell lines compared to F13 cells. Moreover, this frequency can be further increased 4- to 5-fold by continuous exposure of the E9 cells to tumor promoters or epidermal growth factor. Eleven continuous cell lines have been isolated from untreated, promoter-treated, or epidermal growth factor-treated primary E9 cultures. Seven of these are neoplastic or preneoplastic. However, the remaining four do not show any evidence of being in neoplastic progression and three of these continue to express the same differentiated phenotype observed in ther parental primary cell cultures.