Analysis of cell-cycle-dependent production of tissue plasminogen activator analogue (pamiteplase) by CHO cells

The tissue plasminogen activator analogue (pamiteplase) gene was introduced into CHO cells, and several methotorexate (MTX) resistant clones were isolated. In a medium turnover culture of the recombinant CHO cells under varied serum concentrations, the specific growth rate of cells and the specific production rate of pamiteplase showed positive dependencies on serum concentrations; however, the relationship between them was not linear. To investigate the relationship between cell growth and pamiteplase production during the cell cycle, synchronous cultures of MTX-resistant clones were grown and mitotic selection was applied using various concentrations of growth factor (Daigo's GF21). The production rate of pamiteplase showed an immediate response to the differences of GF21 concentrations during their cell cycle transition. The production rate peaked at G2/M phase at a lower level with clone PM200 and at a higher level with its derivative clone PM400, which are resistant to 200 and 400 nM MTX concentration, respectively. Clone PM400 had two peaks of pamiteplase production, a smaller one at early S phase and a larger one at G2/M phase, and the heights of these peaks were proportional to the concentration of GF21. Pamiteplase production varied more dramatically than cell cycle time depending on the medium conditions, and each parameter was affected independently. The cell cycle time became saturated at a lower concentration of GF21; however, the pamiteplase production rate kept increasing even at higher concentrations.