Effects of serum concentration on cellular responses of Chinese Hamster Ovary (CHO) cells were investigated to reduce serum concentration and produce EPO from lower serum media without a decrease of cell specific productivity. The cell specific productivity ($q_p$) and specific growth rate (μ) in 1.0% FBS were 4 and 5 fold lower than those in 10% FBS, respectively. Northern hybridization and MTT assay showed that transcription activity and cellular metabolic activity was proportional to serum concentration. The differences between 1.0% and 10% FBS culture in $\mu$ and $q_p$ were mainly due to the reduced cellular metabolism and mRNA synthesis activity. The relationship between cell cycle and specific productivity was unlike hybridoma which had the highest productivity in $G_0/G_1$ phase. On the contrary, the $G_0/G_1$ phase in CHO cell was the least productive phase. When the cells adapted in 10\% FBS were transferred to 1% FBS and cultured for 15 days by daily medium exchange, the specific productivity was constantly maintained over 15 days, regardless of a decrease of cellular metabolic activity.
For the long term production of EPO with lower serum media, CHO cells were cultured in spinner flasks containing each of two types of microcarrier, Cytodex (3g/L) and CutispherG (1g/L). Media were replaced by 0.7 v/v/d, reducing stepwise serum concentration from 10% to 0.1% FBS over 50 days. Viable cell density did not decrease regardless of low serum concentration. EPO production was dependent on serum concentration. EPO concentration in 0.1% FBS was 4 fold lower than that in 10% FBS. Cells on Cytodex3 detached and formed cell aggregate according to culture time. However, entrapped in CultisoerG did not form cell aggregates
Effect of sodium butyrate on EPO production was studied. Sodium butyrate, at millimolar concentration, when added to the culture medium in the inoculation period, arrested cell growth, but enhanced the specific productivity of EPO up to 12-fold, compared to that of culture without sodium butyrate. However, when sodium butyrate was added during the rapid proliferation of cells, sudden death of the cells was observed. The death of the cell was due to the induction of apoptosis, according to the assesment by intranucleosomal fragmentation assay and progression of apoptosis was observed by tracing the quantity of fragmented DNA. In order to evaluate the possibility of negative effect on glycosylation of EPO, the glycosylated pattern was analyzed by using 'Lectin-blotting' method. As a result, no specific negative effect could be observed on EPO expressed in culture containing sodium butyrate.
