Intracellular responses of productive hybridomas subjected to high osmotic pressure

Abstract

It has previously been found that hybridoma cells under hyperosmotic stress produce higher amounts of antibody. This study identified the cellular processes and mechanisms that occur during this event. In studies of hybridomas adapted to osmolarities ranging between 300 and 450 mOsm (using NaCl), antibody production increased to a saturation level while cell growth decreased progressively. At 500 mOsm, lower cell numbers and markedly decreased productivity resulted. Sucrose and KCl were found to induce similar trends, except to different extents. Several important changes in cellular responses were observed. Elevation of osmolarity with NaCl from 300 to 350 mOsm causes an increase of zwitterionic amino acid uptake, which occurred via Na+-dependent transport systems. In particular, system A was enhanced by 1.86-fold, but no enhancement was observed for Na+-independent transport systems. In addition, amino acids reactive with Na+-dependent transport systems were observed to be abundant within osmotically stressed hybridomas in the middle and late exponential stages. Sucrose and KCl caused similar uptake effects, but to a lesser degree, as long as sodium ions were present in solution. Specific consumption rates of glucose and glutamine increase by 19% and 20%, respectively, under high osmolarity treatment. These increases were confirmed by the 5% to 10% increase in cellular metabolic activity. At 350 mOsm, growth rate was slower compared with the 300-mOsm culture, which was reflected by the lower DNA concentration. Stressed cultures contained enhanced levels of total RNA, of which ~80% is ribosomal RNA (rRNA). Higher rRNA content could in turn increase the translation rates of proteins. This was reflected in the accumulation of both dry cell weight and total cellular protein at linear rates of 0.42 μg/106 cells/mOsm and 0.21 μg/106 cells/mOsm, respectively, with increasing osmolarity between 300 and 450 mOsm. Overall, hybridomas increased their metabolic activities and amino acids uptake via the Na+-dependent symports to compensate for the osmotically elevated external environment. These effects contribute directly and indirectly to the increased cell mass consisting of a larger pool of amino acids, RNA, cellular proteins, and secreted antibody product.