Effects of phorbol ester tumor promoters and nerve growth factor on neurite outgrowth in cultured human neuroblastoma cells.

The effect of phorbol ester tumor promoters on neurite outgrowth was studied in cultured human neuroblastoma cell lines and in cultured embryonic chick and neonatal rat sympathetic ganglia. Promoters inhibited nerve growth factor (NGF)-stimulated neurite outgrowth in sympathetic ganglia while nonpromoting structural congeners did not, in keeping with previous results in embryonic sensory ganglia. In contradistinction, promoters reversibly enhanced neurite outgrowth in malignant SH-SY5Y neuroblastoma cells, whereas nonpromoting congeners were inactive. The potent promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) reversibly increased the proportion of SH-SY5Y cells with neurites and the average length of neurites; the effects of the combination of TPA and NGF were greater than that of either compound alone. The half-maximum response to TPA was at about 2 ng/ml (3 nM). The neurites were thinner, straighter, and less branched and showed more swellings resembling beads or varicosities in comparison with NGF-induced neurites. TPA transiently inhibited the cellular growth rate between Days 2 and 4. Thereafter, the rate was the same as in untreated cultures. The transient inhibition was due to causes other than degradation of TPA since fresh TPA was added. The effect of TPA on neurite outgrowth was not transient and was independent of effects on growth. TPA also enhanced neurite outgrowth in another NGF responsive line, LA-N-5, but not in two unresponsive lines, CHP-100 and CHP-134.