Mechanisms of H1o accumulation in mouse neuroblastoma cells differ with different treatments.

The accumulation of histone H1o in mouse NIE-115 neuroblastoma cells was measured during the course of three treatments that block cell division. Over the course of 12 days, these treatments, 5 mM butyrate, 2% dimethyl sulfoxide, and serum withdrawal, all resulted in decreased levels of DNA synthesis and increased levels of H1o (in absolute terms and relative to the other H1 histones, H1abc). However, the increase in H1o differed comparing butyrate treatment, where there was a 6-fold increase in the H1o/H1abc ratio, with the other two treatments which both had only 3-fold increases in the H1o/H1abc ratio. The mechanism for increasing H1o differed for each of the three treatments and involved differential changes in both synthesis and degradation of the H1 subfractions to favor H1o accumulation on the chromatin. Despite the obvious correlation of the increase in H1o levels with the inhibition of DNA replication, we also showed that increases in H1o can occur without any change in DNA synthesis when cells are switched from media containing dimethyl sulfoxide to media with butyrate as the blocking agent. Finally, there was no correlation between the production of neurites in this cell line and H1o accumulation, arguing against simple, direct involvement of H1o in differentiation.