Density-dependent regulation of growth in L cell suspension cultures. II. Synthesis of total protein and collagen in presence of rapidly declining oxygen tensions

Plateau-phase cultures: an experimental model for identifying drugs which are bioactivated within the microenvironment of solid tumours

A commonly used technique for evaluating potential bioreductive drugs is the determination of hypoxic cytotoxicity ratios in vitro. This experimental model, however, does not accurately mimic the tumour microenvironment, as other factors (such as reduced pH, poor nutrient status, low cell proliferation rates and high catabolite concentrations) are not incorporated into the design of the assay. Plateau-phase monolayer cultures possess many of these characteristics, and this study compared the response of plateau-phase and exponentially growing human colon carcinoma cells (DLD-1) with a series of standard and bioreductive compounds. All drugs tested were added directly to conditioned medium and three patterns of chemosensitivity were observed. In the case of doxorubicin, vinblastine and 5-fluorouracil, exponentially growing cells were significantly more responsive than plateau-phase cultures. ThioTEPA and MeDZQ (2,5-diaziridinyl-1, 4-benzoquinone) were equally cytotoxic to both populations of cells. Tirapazamine (SR4233), RSU 1069, mitomycin C and EO-9, however, were preferentially toxic towards plateau-phase compared with exponentially growing cells. While the exact mechanisms responsible for these observations in each case are not known, this study suggests that plateau-phase cultures may prove to be a useful experimental model in the evaluation of drugs designed to work preferentially within the tumour microenvironment.

Modulation of epithelial cell proliferation in culture by dissolved oxygen

Modulation of epithelial cell proliferation by the dissolved oxygen concentration (PO2) of the growth medium was assessed with primary human foreskin epithelium and a continuous monkey kidney epithelial cell line (LLC-MK2). Direct measurement of the growth medium PO2 provides the first quantitative evaluation of epithelial cell proliferation as a function of PO2. Sustained proliferation of LLC-MK2 cells occurs in serum-free medium equilibrated with a gas phase containing 18% or 30% O2 v/v. Mid-logarithmic phase cultures rapidly consume dissolved oxygen; this results in a 60-70 mm Hg decline in PO2, and leads to a stable growth medium PO2 between 70 and 100 mm Hg, well above anoxic values. In contrast, if culture medium is equilibrated with a gass phase containing 0% or 1% O2 v/v to yield a growth medium PO2 - approximately 20-40 mm Hg, proliferation of LLC-MK2 and primary foreskin epithelial cells is retarded, and LLC-MK2 cells use little dissolved oxygen. Gentle, continuous rocking to prevent diffusion gradient formation enhances proliferation slightly at the higher PO2, but neither periodic fluid renewals nor continued rocking stimulates cells retarded by a lowered oxygen concentration to resume proliferation. The data collectively demonstrate that epithelial cell proliferation requires a PO2 greater than 40 mm Hg, and threshold requirements are probably closer to 70 mm Hg. Glycolysis continues at a PO2 insufficient for proliferation, but more lactic acid accumulates in actively proliferating cultures than in cultures equilibrated with 0% oxygen. We conclude that epithelial cells in vitro both consume more oxygen and require a higher PO2 for continued proliferation, and that the oxygen requirement for epithelial cell proliferation exceeds that of a comparable population of fibroblasts for which low oxygen may enhance survival and proliferation.

The effect of hypoxia on collagen synthesis in cultured 3T6 fibroblasts and its relationship to the mode of action of ascorbate

When exposed to low oxygen tension, in the absence of added ascorbic acid 3T6 mouse fibroblast cultures in late log phase respond by increased lactate production and increased hydroxylation of proline in nascent collagen, which is paralleled by an increase in prolyl hydroxylase activity. After 6 h recovery from the anoxic stimulus, however, cultures still yield more prolyl hydroxylase than controls, but the effect on hydroxylation of nascent collagen has disappeared. These observations help to dissect the dual role of ascorbate which can stimulate hydroxylation both by increasing the amount of active enzyme and by a cofactor-like role; in addition, these observations may be relevant to wound healing.

Properties of growth-related acetylcholinesterase in a cell line of fibroblastic origin

We have previously reported the presence and regulation of an acetylcholine-hydrolyzing enzyme in high density suspension cultures of WRL-10A fibroblasts where its activity increases 100-fold when growth is arrested. Substrate specificity, substrate inhibition, and product identification studies indicate that this enzyme is acetylcholinesterase (AChE, EC 3.1.1.7). Treatment of whole cells with 5 mM diazotized sulfanilic acid revealed that most of the AChE is located on the external surface of the cell membrane. It was also found that the enzyme is released in the medium at a rate of 0.5 U/h/mg cell protein and that within a 24-h period the de novo synthesized and liberated AChE is equivalent to 90% of the activity associated with the cells. No similar synthesis of AChE was found in six order fibroblastic cell lines examined. These and related findings indicating that acetylcholine is also present in high density populations of WRL-10A cells suggest that this unique phenotype may be used profitably in exploring further the relationship between components of the cholinergic system and non-neuronal cell growth.