Effects of dibutyryl cyclic adenosine 3':5'-monophosphate on the growth of cultured human small-cell lung carcinoma and the specific cellular activity of L-dopa decarboxylase.

High activity of L-dopa decarboxylase separates small (oat)-cell from non-small-cell lung cancer in cell culture. The present study investigates relationships between the specific cellular activity of this enzyme and: (a) cell growth kinetics of an established line (O-H-1) of human small cell lung carcinoma, and (b) responses of these cells to treatment with cyclic adenosine 3':5'-monophosphate and sodium butyrate. The O-H-1 cells, as for most other established small-cell lines, grow as suspended cell aggregates. During growth, the specific cellular activity of L-dopa decarboxylase parallels levels for [3H]thymidine labeling index and the ratio of cells in G2-M to those in G1-G0 phases of the cell cycle. Each of these parameters is 2- to 3-fold higher during exponential versus stationary growth. Continuous treatment with dibutyryl cyclic adenosine 3':5'-monophosphate (dcAMP; 0.1 or 1 mM) and 1 mM theophylline produces simultaneous cessation of cell growth and an increase in cellular L-dopa decarboxylase activity. During this period, analyses of DNA histograms reveal an increase in the number of cells in the G2-M phase; the rate of increase in the ratio of G2-M to G1-Go cells paralleled the rate of increase in specific activity of the enzyme. The effects of the dcAMP were promptly reversible; release of the apparent G2-M block preceded regrowth of the cells and was accompanied by a return of L-dopa decarboxylase activity to base-line levels. The changes in enzyme activity were specific for cyclic adenosine 3':5'-monophosphate; another cyclic adenosine 3':5'-monophosphate analogue, 8-bromo adenosine cyclic 3':5'-monophosphate yielded similar increases in L-dopa decarboxylase to those seen with dcAMP, while 0.01 to 1 mM butyrate alone produced the inhibition of cell growth but no changes in specific activity of L-dopa decarboxylase or percentage of cells in the different phases of the cell cycle. We conclude that the specific activity of L-dopa decarboxylase, a key neuroendocrine marker for cultured small-cell lung carcinoma, is highest during proliferative growth and/or when these cells are in the G2M phase of the cell cycle. The differential effects of dcAMP and sodium butyrate offer potential for exploring neuroendocrine differentiation in this important lung cancer and related endocrine neoplasms.