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Hypoxia‐associated proteins in human cells cultivated in vitro: lack of association with hypoxia‐induced cell cycle regulation

The synthesis of proteins expressed in human NHIK 3025 cells following exposure to extremely hypoxic conditions (<4 ppm O2) has been studied. Populations of cells, either in exponential growth or synchronized by the method of detaching mitotic cells, were exposed to extremely hypoxic conditions for up to 20 h. The rate of total protein synthesis was measured at various time points after reoxygenation, and it appeared to be relatively constant and similar to the control level. The protein expression in cells was studied by pulse labelling for 1 h with [35S]‐methionine, and subsequently visualized by SDS‐PAGE and autoradiography. Six proteins appeared to have a changed expression after exposure to extreme hypoxia as compared to control cells; four of them (45, 80, 100 and 150 kD) showed increased, while two (46 and 90 kD) showed decreased expression. The response of these proteins to extreme hypoxia seems to be relatively slow, i.e. with half‐times of several hours. Since extreme hypoxia influences cell cycle progression by instantaneous blockage at the G1/S border as well as halting DNA synthesis in S cells, these proteins can hardly cause these effects. Neither is the altered expression of these proteins due to the accumulation of G1 cells caused by hypoxia.