Hybridoma growth limitations: The roles of energy metabolism and ammonia production

Energy metabolism and the production of ammonia in hybridoma cell culture and its inhibitory effects on cell growth are reviewed. The interactive roles of glucose and glutamine metabolism affect the rate of production of ammonia, and these interactions are described. It is shown that growth inhibition usually occurs between 2–4 mM ammonia although some cell lines have been shown to adapt to much higher concentrations, particularly in continuous culture. In batch cultures cell growth appears to be particularly susceptible to increased ammonia concentrations during the early stages of growth; ammonia increased the rate of cell death in the late stage of batch growth. The specific productivity of monoclonal antibodies is much less sensitive to the released ammonia than is growth; lower volumetric productivities relate to the lower viable cell concentrations which are achieved at the high ammonia levels. Techniques to prevent ammonia accumulation or remove ammonia selectively have been relatively unsuccessful to date.     

The real face of HIF1α in the tumor process

It is well known that the hypoxia-inducible factor 1 α (HIF1α) is detectable as adaptive metabolic response to hypoxia. However, HIF1/HIF1α is detectable even under normoxic conditions, if the metabolism is altered, e.g., high proliferation index. Importantly, both hypoxic metabolism and the Warburg effect have in common a decrease of the intracellular pH value. In our interpretation, HIF1α is not directly accumulated by hypoxia, but by a process which occurs always under hypoxic conditions, a decrease of the intracellular pH value because of metabolic imbalances. We assume that HIF1α is a sensitive controller of the intracellular pH value independently of the oxygen concentration. Moreover, HIF1α has its major role in activating genes to eliminate toxic metabolic waste products (e.g., NH₃/NH₄+) generated by the tumor-specific metabolism called glutaminolysis, which occur during hypoxia, or the Warburg effect. For that reason, HIF1α appears as a potential target for tumor therapy to disturb the pH balance and to inhibit the elimination of toxic metabolic waste products in the tumor cells.