The mechanism of regulation of fibroblastic cell replication. II. Participation of the nucleoli

Inhibition of morphological transformation induced with N-methyl-N'-nitro-N-nitrosoguanidine in cultures of hamster embryo cells by 5'-bromo-2'-deoxyuridine-photolysis

The present study was performed in order to determine whether type III transformed foci induced by N-methyl-N'-nitro-N-nitrosoguanidine originate from the small subpopulation of cells stimulated by the carcinogen to enter DNA synthesis. During the last 30 min of variable treatment periods using different doses of N-methyl-N'-nitro-N-nitrosoguanidine, administered alone or in association with the thymidine analogue, 5'-bromo-2'-deoxyuridine (0.98 x 10(-5)M), the density-inhibited monolayers of hamster embryo cells were exposed to fluorescent light and then assayed for abnormal growth patterns by the focus formation method. Mock-irradiated cultures as well as monolayers whose medium lacked N-methyl-N'-nitro-N-nitro-soguanidine, 5'-bromo-2'-deoxyuridine, or both, served as controls. The cytotoxicity of 5'-bromo-2'- deoxyuridine + N-methyl-N'-nitro-N-nitrosoguanidine + photolysis (BMP) protocol on confluent as well as logarithmically growing hamster embryo cells was estimated in single-cell survival experiments. Plating efficiency determinations have demonstrated that, unlike their actively growing counterparts, confluent hamster embryo cell monolayers are extremely resistant to the cytotoxic effects of the BMP protocol. The quantitative transformation assays indicated that: (1) in non-illuminated cultures addition of 5'-bromo-2'-deoxyuridine to carcinogen-containing medium does affect transformation frequency of hamster embryo cells in the sense that the incidence of type III foci did not subside at later intervals during the post-carcinogen administration period as it did in the absence of the analogue; (2) irradiation of N-methyl-N'-nitro-N-nitrosoguanidine and halogenated pyrimidine analogue-treated cultures with fluorescent light practically suppressed transformation; (3) analogue-added and analogue-removed experiments pointed out that the event(s) on which 5'-bromo-2'-deoxyuridine fluorescent light sensitization of morphological transformation largely depends, takes place between 5 and 15 h after N-methyl-N'-nitro-N-nitrosoguanidine administration, i.e., during the period of maximal carcinogen-stimulated DNA synthesis; and (4) neither fluorescent light nor 5'-bromo-2'-deoxyuridine, singly or in combination, were able to transform cultures of hamster embryo cells. These findings are strong indirect arguments for the concept that carcinogen-induced DNA synthesis and the initiation of transformed clones are causally related.

Secondary activities of diverse inhibitors potentiate the response of hamster embryo cultures to a mitotic stimulus

The reversal of the density dependent inhibition of replication (DDIR) of Syrian hamster embryo cultures by fresh medium containg 30-50% fetal bovine serum was poorly synchronized. There were two waves of DNA synthesis eight to nine hours apart, which by examination of autoradiograms of cultures pulse-labeled with 3H-thymidine, were found to involve 55% and 45% of the cell population. The second wave was found to be due to a subpopulation of epithelioid cells set in the predominantly fibroblastic cell cultures. Pretreatment of the DDIR cultures with hydroxyurea (HU) or arabinosyl cytosine (ara-C), followed by serum stimulation in the absence of the drugs, led to an enhancement of the synchrony. The effect increased with lengthening of the contact with antimetabolites, to a maximum after 20 hours' pre-exposure, and was in part due to the shortening of the GI phase of the epithelioid elements, and in part to increasing the synchrony of the fibroblastic cells. The resulting synchrony involved some 95% of the cells in simultaneous DNA synthesis after a median Gl period of 12 hours. The effect had no relationship to the role of HU and ara-C as specific inhibitors of DNA synthesis since the cultures were mitotically quiescent, and a similar enhanced response could be induced in DDIR cultures by prestimulation exposure lasting only two hours to cycloheximide (cyx), an inhibitor of protein synthesis. Pre-exposure of DDIR cultures to actinomycin D did not potentiate the cell response. A survey of the known secondary inhibitions caused by these three antimetabolites suggests that they all may cause deficiencies in the glycolipid or glycoprotein moieties of the cell surface. These observation provide a use ful, simple means of improving synchrony in these systems and may prove to be a useful probe for investigating the role of the cell surface in regulating cell replication.

Effects of withdrawal of a serum stimulus on the protein-synthesizing machinery of cultured fibroblasts

3T6 cells resting in medium containing 0.5% serum were stimulated to prepare for multiplication by the addition of medium containing 10% serum. After a number of hours, when the rate of preribosomal RNA synthesis, total RNA content (mainly ribosomal), and the cytoplasmic content of poly A (a measure of poly A(+) mRNA) were considerably elevated, the serum-rich medium was withdrawn, and the original medium replaced. The rate of preribosomal RNA synthesis began to drop within 30 minutes, but required a much longer time to fall to a new resting level. When the serum-rich medium was withdrawn after 12 hours of stimulation, the total RNA content required 12-18 hours to fall to the resting level, whereas cytoplasmic poly A content and the rate of protein synthesis declined more rapidly, reaching a new resting level within eight hours. During the 12 hours following withdrawal of the serum-rich medium an appreciable fraction of the cells initiated DNA synthesis. Presumably, the cellular preparations for DNA synthesis cannot be immediately reversed because of the inertial factors related to the protein-synthesizing machinery.