Cyclic Nucleotides and Growth Regulation in BHK Cells

Abstract

Abstract

The role of cyclic nucleotides in the hormonal regulation of proliferation was evaluated in BHK cells. Sparse quiescent cultures in low (0.5%) serum could be stimulated to enter DNA synthesis by addition of physiological concentrations of insulin or epidermal growth factor (EGF). The activity of the two hormones together was greater than either alone, but was less than additive. Their effects were maintained for at least 3 days and led to as high as a sixfold increase in cell number. Dexamethasone was inhibitory under most conditions, but stimulated in the presence of high levels of insulin. NSILA-S (nonsuppress-able insulin-like activity) also stimulated DNA synthesis, whereas PGF_2α and several other hormones were ineffective. Serum, EGF, insulin, and NSILA-S all decreased cAMP levels. However, PGF_2α, which was not mitogenic, also decreased cAMP levels. Also, the combination of EGF and insulin lowered cAMP levels to the same extent as 10% serum but was much less effective in stimulating growth. Cyclic GMP levels were exceedingly low in BHK cells (3.5 fmole/mg protein). In exponentially dividing cultures, insulin and EGF had no effect on cGMP levels. Transformed BHK cells had sixfold-elevated cGMP levels, whereas cAMP levels were unchanged. Both cyclic nucleotides increased in confluent cultures. These studies suggest that a fall in cAMP accompanies growth activation but is not sufficient to induce DNA synthesis. In addition, they provide further support against a mitogenic role for cGMP.