Induction characteristics of oxygen regulated proteins

Abstract

Extreme hypoxia induces many changes in the biology of cells, including the enhanced synthesis of oxygen regulated proteins (ORPs). We investigated the conditions required for the induction of ORPs and by modifying culture conditions, eliminated variables other than oxygen concentration. Several exponentially growing rodent and human cell lines were examined before, during, and after various periods of extreme hypoxia. The following responses were analyzed: cell growth, clonogenic survival, glucose consumption, lactate production, media pH, total protein synthesis, and specific protein synthesis. EMT6/Ro cells did not increase in cell number or progress through the cell cycle after initiation of extreme hypoxia. Cell morphology and cell survival were nearly normal for up to 12 hr of hypoxia. During this period, media pH remained constant, with the concentrations of glucose and lactate being virtually indistinguishable from aerobic cultures or initial values. Associated with these conditions, a marked inhibition of total protein synthesis was observed for EMT6/Ro cells, such that the hypoxic protein synthesis rate was about 60% of the aerobic rate. However, enhanced synthesis of a set of proteins, designated as ORPs, was preferentially induced in less than 6 hr. The molecular weights of the five major ORPs are 260, 150, 100, 80 and 33 kD. Under these conditions, the primary inducing agent was a low concentration of oxygen. This set of ORPs was distinctly different from the set of heat induced (heat-shock) proteins, but included the major 100 kD and 80 kD glucose regulated proteins. Although the functions of ORPs are unknown, their induction under conditions that are known to modify the sensitivity of cancer cells to therapeutic agents suggests that the presence of ORPs should be further investigated to determine their possible value in diagnosis and predicting treatment response.