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The mechanisms of action of extracellular mitogens for normal human bronchial epithelial cells (NHBE) were investigated by observing their effects on selected biochemical pathways when the cells were incubated in serum-free media. We find that (a) epidermal growth factor (EGF) stimulates ornithine decarboxylase (ODC) activity and the rate of cell division without stimulating cAMP; (b) alone, pituitary extract (PEX) does not stimulate ODC activity, cAMP levels, or cell division; (c) when PEX is added to medium containing EGF there is a further increase in both ODC activity and the rate of cell division, again with no increase in cAMP levels; (d) in contrast, alone, L-epinephrine (EPI) stimulates an increase in both ODC and cAMP but does not stimulate cell division; (e) when EPI is added to medium containing both EGF and PEX a further increase in the rate of cell division is noted; (f) the specific inhibitor of ODC, alpha-(difluoromethyl)-ornithine (DMFO), also inhibits NHBE cell proliferation; and (g) the beta-adrenergic receptor antagonist propranolol inhibits the mitogenic action and ODC induction by EPI observed under condition e. We conclude that an increase in ODC activity is necessary but not sufficient for an increase in proliferation of NHBE cells. In contrast, cAMP stimulation is not necessary for an increase in NHBE cell division. However, in the presence of undefined factors in PEX, increases in cAMP levels result in a synergistic increase in the rate of EGF-stimulated clonal growth. By correlating the biochemical pathways invoked by EGF, PEX, EPI, and combinations thereof with their mitogenic actions, we have better defined the role each of these different mitogens plays in stimulating epithelial cell division.

The interaction of mitogenic factors on a single cell type and the comparative activity of a given factor in diverse cell types have been studied by applying the principles of Michaelis-Menten kinetics to clonal growth data. Such comparisons are facilitated by derivation of two parameters; Km mitogen, the mitogen concentration that gives half-maximal clonal growth and a theoretical maximal growth rate, RMAX T. Both parameters are analogous to the Km and VMAX as applied to enzymatic reactions. Use of these parameters permits meaningful comparisons between cells with different growth rates. Using kinetic analysis of dose-response data, we found that normal human epithelial cells require 200 times more fetal bovine serum protein (FBSP) than a malignant line to multiply at their respective half-maximal rates. Further, the Km FBSP of normal cells was reduced to that of the malignant line by the inclusion of growth factors (EGF or FGF, and hydrocortisone) in the medium. On the other hand, even though greater levels of serum were required when growth factors and hydrocortisone were not present, their inclusion did not alter RMAX T. Interactions between mitogenic factors were shown to be unidirectional. Although EGF reduced the Km FBSP, FBSP did not change the Km EGF. The same type of analysis revealed that hydrocortisone, which potentiated the mitogenic activity of EGF did not change the Km EGF. Kinetic analysis of cell growth should prove useful in studies on the relation between growth and tumor promotion as well as in the evaluation of growth-inhibiting chemotherapeutic agents.

A significant interindividual variation in the growth rates is found in normal cultured human mesothelial (NHM) cells derived from different donors. This variation is observed when the mesothelial cells are incubated in medium containing serum and when the potencies of several separate growth factors are measured by using defined media. Depending on the donor, gamma-interferon and interleukin-2 can be toxic, have no effect, or stimulate the growth rate of NHM cells. Cultured NHM cells can be induced to multiply by growth factors that are released by activated macrophages. Thus, interindividual variation in NHM cell growth control could play a role in the pathogenesis of mesothelioma for a person exposed to asbestos.