Effects of NHE1 expression level on CHO cell responses to environmental stress

Ammonia, lactate and CO₂ inhibit animal cell growth. Accumulation of these metabolic byproducts also causes a decrease in intracellular pH (pH_i). Transport systems regulate pH_i in eukaryotic cells. Ion transporters have been cloned and overexpressed in cells but have not been examined for protection against the buildup of ammonia, lactate or CO₂. The Na⁺/H⁺ exchangers (NHE) transport H⁺ ions from cells during acidification to increase pH ⁱ. We examined whether overexpression of NHE1 would provide CHO cells with greater protection from elevated ammonia, lactate or CO₂. NHE1 CHO cells were compared to MT2-1-8 ("normal" levels of NHE) and AP-1 (devoid of any NHE activity) CHO cell lines. Expression of at least "normal" levels of NHE1 is necessary for CHO cell survival during exposure to 30 mM lactic acid without pH adjustment or to 20 mM NH₄Cl with pH adjustment. Resistance to an acute acid-load increased when NHE1 was overexpressed in CHO cells. Surprisingly, the inhibitory effect on cell growth at 195 mmHg pCO₂/435 mOsm/kg (normal levels are 40 mmHg PCO ₂/ 320 mOsm/kg) was not affected by the NHE1 level. Also, there was no further decrease in CHO cell growth in the absence of NHE1 expression during elevated osmolality alone (up to 575 mOsm/kg).