Colony morphology and heritability of anchorage-independent growth among spontaneously transformed Balb/3T3 cells.

A clone of spontaneously transformed Balb/3T3 cells produced four different types of colonies in agar. Flat colonies arose at the interface between bottom and top layers of agar, two types of somewhat flattened compact colonies arose within the top layer, and mixed colonies developed when the compact colonies contacted the interface. The proportions of different colony types could be altered by varying the volume, concentration, and gelling rate of the agar. Spherical colonies only appeared in ungelled medium. None of the colony types bred true, showing that their morphologies depended on local conditions. When cells from a subpopulation with low colony-forming efficiency in agar (CFE) were cloned on plastic, all clones formed colonies in agar and did so with about the same CFE as the parental population. Colonies isolated from agar had no higher CFE than the parental population. The results indicate a great phenotypic heterogeneity in anchorage-independent growth, even in cloned populations of transformed cells.

Tumor progression in nude mice and its representation in cell culture.

Varying dilutions containing from 10(6) to 10(3) spontaneously transformed Balb/3T3 cells were inoculated into nude mice [N:NIH(S)II]. Less than half the mice inoculated with 10(3) cells developed tumors. The higher concentrations of cells produced visible tumors in all mice within 2-3 weeks, and these tumors grew rapidly to large sizes. Some tumors initiated by the lower concentrations of cells arose quickly, but others were greatly delayed in onset, then grew slowly, if at all, for several weeks before a rapid acceleration. The delayed acceleration can be considered a form of tumor progression. When first explanted into culture, cells from the early tumors multiplied somewhat more slowly than the parental cells that initiated the tumors, but narrowed the gap in a few weekly passages. By contrast, only a small fraction (less than or equal to 0.001) of cells from the longest delayed tumors could sustain multiplication in culture, although flow cytometry revealed them to have been a rapidly multiplying population when explanted. A relatively large fraction of these explanted tumor cells incorporated a 1-hour pulse of [3H]thymidine into DNA, although at a low rate. The shift to culture apparently slowed progress through the S-period of the cell cycle. The multiplication rate of cell populations from the delayed tumors increased in successive passages in culture. There was great heterogeneity in growth capacity among clones of the tumor cells. The growth rates of some clones declined to the point of extinction, those of others remained constant for several weeks, while those of still others steadily increased in growth rate. The low initial cloning efficiency of cells from the delayed tumors and the heterogeneity of growth rates among the clonable cells indicate that selection plays a major role in the increase of the growth capacity of the cell population. The steady increase in growth rates within clones suggests that physiological adaptation also contributes to the progressive growth of the tumor populations in culture. The results constitute a rationale for using the progressive growth of cells in culture as a model system for discriminating the types of cellular changes that underlie tumor progression.

Heritable variations in growth potential and morphology within a clone of Balb/3T3 cells and their relation to tumor formation.

A nontransformed clone and a spontaneously transformed clone were isolated from a twice-recloned line of Balb/3T3 cells. At different times two sublines were initiated from the nontransformed clone, and three were initiated from the transformed clone. The sublines were maintained in parallel passages under the same conditions. Each subline was distinctive in appearance and fell into the same rank order in a variety of growth parameters in vitro. Colony formation in agar and tumor formation in mice occurred only in the morphologically transformed sublines, but there was no quantitative correlation between the two properties or with the rate of glucose utilization. Two of the cell populations derived from noninbred NIH nude mouse tumors of the 3 transformed sublines differed in agar colony formation from the parental sublines. The results indicate that there is an immense capacity for variation in cultured animal cells involving many unrelated characteristics expressed in a way that is difficult, if not impossible, to explain by conventional genetic models.

Dynamics of tumor growth and cellular adaptation after inoculation into nude mice of varying numbers of transformed 3T3 cells and of readaptation to culture of the tumor cells.

Decimal dilutions containing 5 X 10(5) to 5 X 10(1) cells were inoculated s.c. into nude mice and the course of tumor development was recorded. The highest concentration of cells produced rapidly growing poorly differentiated sarcomas within 2-3 weeks of their inoculation. Upon explantation the resultant tumors yielded cells which multiplied on plastic almost as rapidly as did their progenitors used to initiate the tumors, and had as high a colony forming efficiency in agar as long as tryptose phosphate broth was omitted from the agar medium. Tumors initiated by the lower concentrations of cells were disproportionately delayed in their appearance and tended to increase in size at a low rate. At least one tumor regressed and one which apparently regressed appeared again at a later time. These changes are characteristically described under the rubric of tumor regression. Host reactive cells such as neutrophils, eosinophils, macrophages, and fibroblasts were observed in some tumors. One of the tumors was a low grade hemangiosarcoma, another a well-differentiated fibrosarcoma, and the rest poorly differentiated sarcomas. Cells from two tumors initiated by 500 and 5000 cells multiplied slowly in early passages in culture, particularly when seeded at low densities at which they appeared to sustain cumulative damage even when multiplying. In later passages, the "low dose" tumor cells gained the capacity to multiply in culture after seeding at low densities, but it took up to 50 cell generations to reach this capacity. The loss of growth capacity on plastic of cells from the low dose tumors and its subsequent restoration by passaging in culture may provide a quantitative method for analyzing the type of cellular change which underlies tumor progression.

Selection and adaptation for rapid growth in culture of cells from delayed sarcomas in nude mice.

More than 1000 cells of a spontaneously transformed line of BALB/3T3 cells are required to initiate tumors in half the nude mice inoculated s.c., although the cells clone with an efficiency approaching 100% in culture. The cells of two tumors with prolonged latent periods, initiated by 2 X 10(4) and 5 X 10(3) cells, were chosen for detailed clonal analysis in culture. The cells from the tumors grew very poorly in culture in the first passages, but with increasing speed and efficiency in later passages. Cells derived directly from the two tumors cloned in agar with an efficiency of 0.01 and 0.002%. The growth rates on plastic of the rare successful clones derived from agar were generally low but extremely varied. Some clones lost the capacity for multiplication in a few passages, while others persisted but fluctuated unpredictably in growth rate in the early passages. The graded increase in growth rate of the uncloned tumor cell populations was probably the result of selection of the more rapidly growing clones. One of the slower-growing clones was subcloned. About half of the subclones grew at a slower rate than the parental clone. These, however, increased progressively in growth rate over six successive weekly passages, suggesting the occurrence of a gradual physiological adaptation. We conclude that selection of fast-growing clones contributes a major part of the gradually improving growth of tumor cell populations in culture, but that a physiological adaptation extending over many cell generations makes a significant contribution. The mechanism in either case is unknown, and indeed there may not be a unique mechanism in the scientifically rigorous sense.

High-frequency variation and population drift in a newly transformed clone of BALB/3T3 cells.

During repeated passage of BALB/3T3 cells and testing for anchorage-independent growth, a single transformed clone was isolated from agar, and five subclones were derived from it. These subclones differed from one another in morphology on a solid substratum, efficiency and size of colony formation in agar, and rate of tumor formation in nude mice. With weekly passage over a period of 6 months, the differences in morphology and growth in agar gradually decreased. The subclone which produced the fastest-growing tumors in nude mice after 4 weeks of culture produced the slowest-growing tumors after 18 weeks, and a change in the opposite direction was made by another subclone. There was no difference among the subclones in growth rate on plastic. The distribution of chromosome numbers was heterogeneous but overlapping in all the primary subclones at 16 and 24 weeks, with no statistically significant difference in the mean number of chromosomes per subclone. An extremely high degree of variation must have occurred to produce the multiple differences between the subclones, and the same type of variation could have been responsible for the subsequent changes with repeated passage. The high frequency and graded nature of the changes and the concurrent involvement of several traits suggest an epigenetic basis for the variation.

Inheritance of acquired changes in growth capacity of spontaneously transformed BALB/3T3 cells propagated in mice and in culture.

Five subclones were derived from a spontaneously transformed BALB/3T3 clone soon after its isolation. Despite their common clonal origin, the subclones were different from each other in appearance, colony-forming efficiency in agar (CFEag), and rate of tumor formation in mice. A comparison of growth properties during repeated passages in culture was made between the cells derived from the tumors and the cells used to initiate the tumors. In most cases, the tumor-derived cells had a much lower CFEag than did their parental in vitro-propagated cells, and the CFEag was restored slowly to the original level or remained at a reduced level during the period of study. In a few cases, the tumor-derived cells had almost as high a CFEag as their parental cells or were quickly restored to this level during cultivation. It was shown by karyotypic and clonal analysis that the reduced CFEag of the tumor-derived cells arose from a change in the transformed cells; i.e., it was not due to the presence of normal host cells in the explanted tumor. Clones of the tumor-derived cells tended to show the same patterns of change in CFEag as the uncloned tumor cell populations, but there were cases of individual variation in pattern among the tumor cell clones. Tumor cells with greatly reduced CFEag also grew a little more slowly on plastic than did the parental nontumor cells, and their growth rate tended to increase along with CFEag in long-term culture. Clonal analysis of one of the five original subclones failed to reveal cells which had the CFEag properties of its progeny tumor cells. This suggests that the reduction of CFEag during tumor formation arose by adaptation rather than selection of preexisting variants. A similar conclusion was drawn about the restoration of CFEag during cultivation of the tumor-derived cells. Although the decrease in CFEag which accompanied tumor formation varied in magnitude and stability, some tumor cell populations retained their reduced capacity through months of weekly passaging in culture, involving up to 100 cell divisions. The results are therefore consistent with the heretical notion of inheritance of acquired characteristics. In addition, the wide variation of in vitro growth capacity among tumors initiated by different subclones, and even among tumors initiated by the same subclone, raises the possibility that the complete chain of causality underlying the variation is intrinsically indeterminate.

Nondestructive measurements of the properties of healing burn scars.

A testing protocol and the requisite instrumentation have been developed to nondestructively monitor the temporal and mechanical properties of maturing scar. The maturing scar can become progressively and unpredictably adherent or contractured, producing varying degrees of functional impairment. By plotting these mechanical changes as a temporal function of limb motion history, more accurate prediction and control of the ultimate scarring may result. These same techniques could also be used to study normal skin aging. Extrapolation could be made to connective tissue scars in tendons, ligaments, and other structural elements. Scar contractures may develop slowly along lines of tension or areas of maximum skin defect in large maturing scars once the patient has recovered sufficiently to exercise the underlying joints. Since the present endeavor to monitor potential contractures requires measurement of the "in-plane" stresses and strains, we have chosen to utilize an in situ strip biaxial test configuration.

Self-normalization of highly transformed 3T3 cells through maximized contact interaction.

Nontransformed and moderately and highly transformed BALB/c 3T3 cells maintained on small coverslips in a large volume of medium multiplied to 2, 3, and 4 times higher population density, respectively, than they did in conventional cultures. Deprivation of Mg2+ caused highly transformed cells on coverslips to assume the appearance of nontransformed cells, decrease their rate of multiplication, and stop further growth at a much lower saturation density than the same cells in physiological Mg2+. The latter cells reached a saturation density of 10(6)/cm2 and their rate of DNA synthesis decreased progressively with increased crowding. At saturation density, cells in physiological Mg2+ took on an appearance and arrangement similar to normal fibroblasts. They developed a high requirement for serum to initiate DNA synthesis. When transferred at low density, they flattened out on a plastic surface and maintained the appearance of nontransformed cells for approximately 1 day. Onset of DNA synthesis and multiplication in the transferred cells was delayed for periods characteristic of quiescent nontransformed cells stimulated by fresh medium or transfer. Cells from crowded coverslips were approximately 1/10th as efficient at colony formation when suspended in agar as cells from uncrowded coverslips. They also had a significantly lower Mg2+ content. The crowded cells returned to their transformed morphological and growth behavior 2 to 3 days after transfer at low density. We conclude that a very high degree of crowding causes highly transformed cells to revert to the phenotype of nontransformed cells. Other treatments such as deprivation of Mg2+ or inorganic orthophosphate can achieve similar results. It appears that a balanced reduction in rates of metabolism and multiplication can restore the normal phenotype to transformed cells, implying that they differ only quantitatively from nontransformed cells. The putative role of Mg2+ in the regulation of multiplication and in transformation of animal cells is discussed.

Solute concentration effects on the expression of cellular heterogeneity of anchorage-independent growth among spontaneously transformed BALB/c3T3 cells.

Clones were derived in culture from a tumor initiated by spontaneously transformed 3T3 cells and tested for their colony-forming efficiency in agar (CFEag). Incubation of petri dish cultures was done in subsaturation humidity to minimize mold contamination. There was great variation in CFEag between clones but also, under certain conditions, within clones. The most prominent condition that generated phenotypic diversity in CFEag was partial evaporation of the medium, which may occur during the protracted development of a mass population from a single cell. Evaporation was disproportionately great in 35-mm dishes and peripheral wells of multiwell plates. If the supraphysiological solute concentration resulting from evaporation was greater than 133% of normal, there was progressive suppression of cell growth in the succeeding transfer in agar or on plastic, even if isotonic medium was substituted 1 d before transfer. The effect of supraphysiological concentrations of all the solutes of the medium could be reproduced by simply increasing the NaCl concentration. Damaged cells were restored to their full growth potential after 3 d in isotonic medium. When nontransformed cells were chronically exposed to increased salt, irreversible increases in 2-deoxyglucose uptake were produced. With continued exposure of these cells to high salt, they became morphologically transformed, produced colonies in agar with high efficiency, and formed sarcomas when inoculated into nude mice.

Ionizing radiation accelerates aortic lesion formation in fat-fed mice via SOD-inhibitable processes.

Ionizing radiation promotes formation of reactive oxygen species, including the superoxide anion (O2-). To evaluate whether O2- or O2--mediated perturbations may contribute to the known atherogenic effects of radiation, we examined aortic lesion formation in irradiated C57BL/6 mice and evaluated the effects of CuZn-superoxide dismutase (CuZn-SOD) overexpression. Ten-week-old mice were exposed to a 2-, 4-, or 8-Gy dose of 250-keV x-rays to the upper thorax and then placed on a high-fat diet for 18 weeks. Based on quantitative lipid staining of serial sections of the proximal aorta, mean lesion area was increased with increasing radiation dose and was 3-fold greater in 8-Gy-irradiated than sham-irradiated mice (7800+/-2140 versus 2635+/-709 micrometer(2), P<0.05). These effects were absolutely dependent on a high-fat diet, which had to be introduced within 1 to 2 weeks of the radiation exposure, suggesting the early involvement of atherogenic lipoproteins that were elevated in response to the diet. The importance of radiation-induced oxidative stress was supported by the observation of a 2-fold lower mean lesion area in irradiated CuZn-SOD transgenic mice than in their irradiated, nontransgenic littermates (3026+/-1590 versus 6102+/-1834 micrometer(2), P<0.05). Lucigenin-enhanced chemiluminescence, used as an index of aortic O2- concentrations, was significantly elevated in the postradiation period, and this response was reduced in CuZn-SOD transgenics. On the basis of these results, we propose that radiation may be a useful tool for initiating oxidative or redox-regulated events that promote atherogenesis and for testing the antiatherogenic properties of antioxidants.

HDL antioxidant effects as assessed using a nonexchangeable probe to monitor particle-specific peroxidative stress in LDL-HDL mixtures.

High density lipoproteins (HDL) have been reported to inhibit oxidation of low density lipoproteins (LDL) based in part on observations that oxidative changes occur more slowly in LDL-HDL mixtures than in LDL alone. In the current studies, we developed an approach to discern particle-specific oxidation kinetics within mixed particle systems using the oxidation-labile fluorescent probe parinaric acid cholesteryl ester (PnCE) and applied this to the study of HDL inhibition effects. PnCE was introduced into acceptor lipoproteins by cholesteryl ester transfer protein (CETP)-mediated transfer from donor microemulsions. Incubation of PnCE-containing LDL and HDL with non-probe-containing HDL and LDL, respectively, followed by measurement of reisolated fractions, indicated that PnCE does not transfer appreciably between lipoprotein fractions. Oxidative loss of lipoprotein-associated PnCE occurred essentially in tandem with changes in conjugated dienes, suggesting that PnCE loss reflects the course of peroxidation of endogenous lipoprotein lipids. Using PnCE to separately monitor LDL- and HDL-specific oxidation within LDL-HDL mixtures, we obtained direct evidence that HDL inhibits both Cu(2+)- and Fe(3+)-induced peroxidation of LDL-associated lipids. Notably, in the presence of Cu2+, loss of HDL-associated PnCE fluorescence also was inhibited in LDL-HDL co-incubations, suggesting that LDL exert an antioxidant effect under these conditions as well. Thus, results obtained using this new methodology are consistent with previously reported antioxidant effects of HDL, but indicate that the behavior of individual lipoprotein particles may be more complicated than can be predicted from the collective behavior of the lipoprotein mixture.

Selective resistance of LDL core lipids to iron-mediated oxidation. Implications for the biological properties of iron-oxidized LDL.

Although the nature and consequences of oxidative changes in the chemical constituents of low density lipoproteins (LDLs) have been extensively examined, the physical dynamics of LDL oxidation and the influence of physical organization on the biological effects of oxidized LDLs have remained relatively unexplored. To address these issues, in the present studies we monitored surface- and core-specific peroxidative stress relative to temporal changes in conjugated dienes (CDs), particle charge (an index of oxidative protein modification), and LDL-macrophage interactions. Peroxidative stress in LDL surface and core compartments was evaluated with the site-specific, oxidation-labile fluorescent probes parinaric acid (PnA) and PnA cholesteryl ester (PnCE), respectively. When oxidation was initiated by Cu2+, oxidative loss of the core probe (PnCE) closely followed that of the surface probe (PnA), as indicated by the time to 50% probe depletion (t1/2; 15.5 +/- 7.8 and 30.4 +/- 12 minutes for PnA and PnCE, respectively). Both probes were more resistant in LDL exposed to Fe3+ (t1/2, 53.2 +/- 8.1 and 346.7 +/- 155.4 minutes), although core probe resistance was much greater with this oxidant (PnCE t1/2/PnA t1/2 5.8 vs 2.0 for Cu2+). Despite differences in the rate and extent of oxidative changes in Cu(2+)- versus Fe(3+)-exposed LDLs, PnCE loss occurred in close correspondence with CD formation and appeared to precede changes in particle charge under both conditions. Exposure of LDLs to hemin, a lipophilic Fe(3+)-containing porphyrin that becomes incorporated into the LDL particle, resulted in rapid loss of PnCE and simultaneous changes in particle, charge, even at concentrations that yielded increases in CDs and thiobarbituric acid-reactive substances similar to those obtained with free Fe3+. These results suggest that oxidation of the LDL hydrophobic core occurs in conjunction with accelerated formation of CDs and may be essential for LDL protein modification. In accordance with the known effects of oxidative protein modifications on LDL receptor recognition, exposure of LDLs to Cu2+ and hemin but not Fe3+ produced particles that were readily processed by macrophages. Thus, the physical site of oxidative injury appears to be a critical determinant of the chemical and biological properties of LDLs, particularly when oxidized by Fe3+.

Increased low density lipoprotein degradation in aorta of irradiated mice is inhibited by preenrichment of low density lipoprotein with alpha-tocopherol.

We previously reported that upper thoracic exposure to ionizing radiation (IR) accelerates fatty streak formation in C57BL/6 mice and that such effects are inhibited by overexpression of the antioxidant enzyme CuZn-superoxide dismutase (SOD). Notably, IR-accelerated lesion formation is strictly dependent on a high fat diet (i.e., atherogenic lipoproteins) but does not involve alterations in circulating lipid or lipoprotein levels. We thus proposed that IR promotes changes in the artery wall that enhance the deposition of lipoprotein lipids. To address this hypothesis, we examined the effects of IR on aortic accumulation and degradation of low density lipoproteins (LDL). Ten-week-old C57BL/6 mice were exposed to a single (8-Gy) dose of (60)Co radiation to the upper thoracic area or were sham irradiated (controls) and were then placed on the high fat diet. Five days postexposure, the mice received either (125)I-labeled LDL ((125)I-LDL) (which was used to measure intact LDL) or (125)I-labeled tyramine cellobiose ((125)I-TC)-LDL (which was used to measure both intact and cell-degraded LDL) via tail vein injection. On the basis of trichloroacetic acid (TCA)-precipitable counts in retroorbital blood samples, > or =95% of donor LDL was cleared within 24 h and there were no differences in time-averaged plasma concentrations of the two forms of LDL among irradiated and control mice. Aortic values increased markedly within the first hour and thereafter exhibited a slow increase up to 24 h. There were no differences between irradiated and control mice at 1 h, when values primarily reflected LDL entry, but a divergence was observed thereafter. At 24 h, (125)I-TC-associated counts were 1.8-fold higher in irradiated mice (P = 0.10). In contrast, (125)I-LDL-associated counts were 30% lower in irradiated mice (P< 0.05), suggesting that most of the retained (125)I-TC was associated with LDL degradation products. Consistent with the proposed involvement of oxidative or redox-regulated events, IR-induced LDL degradation was lower in SOD-transgenic than wild-type mice (P<0.05). The importance of LDL oxidation was suggested by observations that IR-induced LDL degradation was significantly reduced by preenriching LDL with alpha-tocopherol. On the basis of these results, we propose that IR elicits SOD-inhibitable changes in the artery wall that enhance LDL oxidation and degradation leading to the deposition of LDL-borne lipids. These studies provide additional support for the role of oxidation in lipoprotein lipid deposition and atherogenesis and suggest that IR promotes an arterial environment that stimulates this process in vivo.