The nutrient factor queuine protects HeLa cells from hypoxic stress and improves metabolic adaptation to oxygen availability

Abstract

Queuine (q), a cyclopentendiol derivative of 7‐aminomethyl‐7‐deazaguanine, is a nutrient factor for lower and higher eukaryotes, except yeast; it is synthesized in eubacteria partly at the level of tRNA. In eukaryotes q is preferentially inserted into the wobble position of specific tRNAs in differentiated and adult tissues, but occurs mainly free in embryonic and fast proliferating cells. HeLa cells grow to a higher cell density under aerobic than under hypoxic conditions only when supplemented with q. Here we show that in hypoxically grown HeLa cells, sufficiently supplied with q, free q accumulated when serum factors become limiting while the respective tRNAs remained completely q deficient. In these cells the levels of lactate dehydrogenase A (LDH A) mRNA and of LDH A protein were at least twofold higher than in aerobically grown cells, independent of the absence or presence of q. In response to q the LDH A₄ isoenzyme was further activated by a post‐translational mechanism. In q‐deficient HeLa cells the activity of the major anoxic stress protein, LDH_k, increased as a result of hypoxia; this increase was suppressed by q. In aerobically grown, q‐deficient cells significant activities of LDH A₄ and LDH_k were present; both activities were markedly lowered by q, while the mitochondrial electron flow was imporved. The results show that free q is essential for relieving hypoxic stress in HeLa cells that results from oxygen limitation.