Nutrient uptake and control of animal cell proliferation.

The division of fibroblast-like cells in culture can be regulated by cell density, serum, and various growth factors. This system has been widely utilized as a model to study the regulation of cell proliferation. There are many physiological and metabolic changes that correlate with the proliferative state of the cell. These include changes in morphology, cyclic nucleotide levels, enzyme activities, and certain cell surface properties such as nutrient uptake and chemical composition of the plasma membrane. Of primary concern is determination of which changes might be critical links in the control of cell proliferation and which ones are simply correlated but not causally involved with cell growth. We have discussed evidence which has strongly suggested a fundamental role for uptake of certain nutrients in the regulation of cell growth. In addition, we have presented several methods allowing a critical analysis of a putative cause and effect relationship between nutrient uptake and growth control. One method involves a dose-response study of the effect of a mitogen on uptake and DNA synthesis, while a second method involves search for a particular mitogen that may, under the appropriate conditions, stimulate cell division without stimulating uptake. These two methods are limited, however, since they are not always applicable to any given nutrient or mitogen. A third method which is not limited in its applications involves varying the concentration of a particular nutrient in the medium to control its uptake. In the case of orthophosphate (Pi) or glucose, we have used this “nutrient concentration” method to demonstrate that under normal culture conditions, uptake of these nutrients is not a causal event in the regulation of cell division. We considered the possibility that intracellular nutrient availability might control cell growth, even under conditions where uptake did not. For Pi and glucose, we assumed intracellular pool size to be an accurate indicator of intracellular nutrient availability and measured these pools under a variety of proliferative conditions. These studies revealed, however, no correlation between pool size and proliferative state of the cells. This clearly demonstrates that for Pi and glucose, intracellular pool sizes are not causally involved in the control of growth. The possibility remains, however, that if these nutrients are compartmentalized within the cell, intracellular pool sizes may not be an accurate indicator of nutrient availability. For Pi and glucose there are many interesting questions that remain to be answered about the transport mechanisms for these nutrients. For some other nutrients, particularly K+ and amino acids, in addition to questions dealing with the nature of transport mechanisms, the question of uptake involvement in the control of proliferation remains entirely open. As with Pi and glucose, many observations strongly suggest a fundamental relationship between amino acid or K+ uptake and control of cell growth. We suggest that the “nutrient concentration” technique used in our studies to analyze Pi and glucose uptake is applicable to any nutrient and should, therefore, prove extremely useful for studying the involvement of any uptake change in the regulation of cell proliferation.