Induction of squamous differentiation of normal human bronchial epithelial cells by small amounts of serum

Abstract. Recently we reported a low calcium (110 μM) serum-free medium (LHC-1) for clonal growth of normal human bronchial epithelial (NHBE) cells. NHBE cells within colonies are small (mean surface area = 1,250 μ²) rarely migratory, have few tonofilaments, and multiply with an average population doubling time of 28 h. We have also noted that adding small amounts of blood-derived serum to LHC-1 medium (as little as 2%) significantly decreased the clonal growth rate. We have now found that the growth inhibiting effect of serum is due to the induction of squamous (terminal) differentiation. Serum quickly increases the size of the cells (mean surface area = 4,900 μ²). In addition, the cells acquire numerous desmosomal junctions and an extensive network of keratin bundles. In contrast, human lung carcinoma cells multiply rapidly at clonal density in LHC-1 medium containing as much as 8% serum.
Although high concentrations of calcium ions in the medium are known to induce squamous differentiation of epidermal keratinocytes in the absence of serum, high levels of Ca²⁺ (up to 1,000 μM) increased the number of desmosomal junctions, but did not significantly affect the clonal growth rate or size of the NHBE cells. However, high concentrations of calcium (above 450 μM) were found to potentiate serum differentiation-inducing activity. On the other hand, cholera toxin (10 ng/ml) inhibited the differentiation-inducing acitivy of serum. These results show that squamous differentiation of NHBE cells can be induced by serum and the potency of these serum factors can be modulated. In addition, the data show that lung carcinoma cells differ from their normal counterparts by not undergoing differentiation in the presence of serum.