A cancer-associated lactate dehydrogenase is expressed in normal retina

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

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, LDHk, increased as a result of hypoxia; this increase was suppressed by q. In aerobically grown, q-deficient cells significant activities of LDH A4 and LDHk were present; both activities were markedly lowered by q, while the mitochondrial electron flow was improved. The results show that free q is essential for relieving hypoxic stress in HeLa cells that results from oxygen limitation.

Cancer-associated lactate dehydrogenase is a tyrosylphosphorylated form of human LDH-A, skeletal muscle isoenzyme

Cancer-associated lactate dehydrogenase is a tyrosylphosphorylated form of human skeletal muscle isoenzyme, since the partial amino acid sequences of human liver LDH-K/A protein were found to be identical with the known primary structure of human LDH-A isoenzyme and the LDH-A isoenzymes from human placenta and bovine muscle were shown to be tyrosylphosphorylated. This tyrosylphosphorylated LDH-K/A protein was also found to be complexed with 21 kD, 30 kD, and 56 kD proteins.

Glucose metabolism in transdifferentiating and glucose-blocked cultures of chick embryo neuroretinal cells: an inverse relationship between glycogen and delta-crystallin accumulation

Chick embryo neuroretinal (NR) cells transdifferentiate extensively into lens when cultured for several weeks in low-glucose (FH) medium, but fail to do so when high levels of supplementary glucose (FHG) are present. We show here that most aspects of glucose metabolism are promoted in high-glucose cultures, including lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (G-6-PDH) activities, 2-deoxyglucose uptake, pentose shunt activity and lactate production. Continuous supplementation of high-glucose cultures with low levels of ouabain (FHGO) significantly lowers 2-deoxyglucose uptake, from FHG levels down towards FH levels, especially during the early stages of NR culture. Much later, extensive transdifferentiation into lentoids (with concomitant delta-crystallin accumulation) occurs in these FHGO cultures, which thus resemble FH rather than FHG controls. Another parameter strongly affected by ambient glucose levels is the accumulation of glycogen. Both glycogen itself and glycogen synthetase activity increase steadily in FHG cultures, but decrease slightly under FH conditions. Glycogen accumulation in FHG cultures is largely confined to glial-like cells, particularly those underlying clusters of neurones. Other studies have shown that glial differentiation in vitro is promoted by histotypic interactions with retinal neurones. Thus high glucose may act in concert with neuronal influences to stimulate or stabilize the normal differentiation of retinal glial cells, whose characteristic features in vivo include glycogen synthesis and storage. Furthermore, we show that supplementation of high-glucose cultures with forskolin or dibutyryl cyclic AMP (both of which promote glycogenolysis) results in a slower rate of glycogen accumulation and in enhanced transdifferentiation into lens. In both respects, the forskolin- and dibutyryl cAMP-supplemented FHG cultures are intermediate between FH and FHG controls. Thus the enhancement of normal glial differentiation in NR cultures by high glucose may inhibit or preclude subsequent transdifferentiation into lens.

Three diadenosine 5',5''-P1,P4-tetraphosphate hydrolytic enzymes from Physarum polycephalum with differential effects by calcium: a specific dinucleoside polyphosphate pyrophosphohydrolase, a nucleotide pyrophosphatase, and a phosphodiesterase

Two new enzymes that hydrolyze diadenosine tetraphosphate (Ap4A) have been isolated from the acellular slime mold Physarum polycephalum. Both enzymes are different from the Physarum Ap4A symmetrical pyrophosphohydrolase previously described on the basis of their substrate specificities, reaction products, molecular weights, and divalent cation requirements. One enzyme is a nucleotide pyrophosphatase that asymmetrically hydrolyzes Ap4A to AMP and ATP. This enzyme hydrolyzes several mono- and dinucleotides with the corresponding nucleotide monophosphate as one of the products. The percentage hydrolysis of NAD+, Ap4A, and Ap4G, each at 10 microM, was 100, 56, and 51, respectively. A divalent cation is required for activity, with Ca2+ yielding 20-30 times greater activity than Mg2+ or Mn2+. Values of Km for Ap4A and Vmax are similar to the corresponding values for Ap4A symmetrical pyrophosphohydrolase. The second enzyme is a phosphodiesterase I with broad substrate reactivity. This enzyme also asymmetrically hydrolyzes Ap4A, but it does not hydrolyze NAD+. Activity of the phosphodiesterase I is stimulated by divalent cations, with Ca2+ being 50-60 times more stimulatory than Mg2+ or Mn2+. The apparent molecular weights of the nucleotide pyrophosphatase and phosphodiesterase are 184,000 and 45,000, respectively. In contrast, the Ap4A pyrophosphohydrolase hydrolyzes Ap4A to ADP, is inhibited by Ca2+ and other divalent cations, and has an apparent molecular weight of 26,000 as previously reported.

LDHk in the retina of diverse vertebrate species: a possible link to the Warburg effect

LDHk is a cancer-associated lactate dehydrogenase which is also found at high levels in normal mammalian retina. Such retinas share with most cancer tissues a dependence on aerobic glycolysis, leading to high production of lactate. However, retinas of lower vertebrate species are significantly less dependent on aerobic glycolysis. We find that retinas of species less dependent on aerobic glycolysis express significantly lower levels of an LDHk-like activity, less than or equal to the low levels seen in brains. The enzymes from lower species differ from the mammalian retinal enzyme in their pH optima and responsiveness to oxygen; but share a similar degree of inhibition by 5'-5'-dinucleoside tetraphosphates. Therefore, the expression pattern of LDHk in brain and retina of diverse vertebrate species suggests a link with the Warburg effect.

Glycerol phosphate dehydrogenase in developing retina of normal and mutant mice

The specific activity of glycerol phosphate dehydrogenase (GPDH; E.C. 1.1.1.8) was determined in the retinas of C57BL/6J mice from 2 to 45 days after birth. Comparisons were made with age-matched mice congenic to C57BL/6J and homozygous for the autosomal recessive mutations for retinal degeneration (rd) and light ear (le). The total activity of GPDH in the developing retina was similar in each strain throughout development. However, following the loss of photoreceptor cells in the rdle mice, the specific activity of GPDH increased to approximately twice the level of that in the controls. These data suggest that GPDH is absent from the photoreceptor cells and that its expression in the remaining retinal cells is unaffected by the loss of the photoreceptor cells. Retinal GPDH does not exhibit the eight-to ten-fold increase seen during development of the brain, but instead remains essentially constant during this time period, and in adult animals the specific activity of GPDH in the retina is about 20-fold less than it is in the brain. Neither mouse retinal GPDH nor brain GPDH is affected by adrenalectomy, suggesting that they are not regulated by glucocorticoids. The major form of GPDH in the retina is distinct from the major form in mouse brain since immunotitration experiments demonstrated that 70-80% of retinal GPDH does not react with anti-GPDH IgG, whereas the reverse was true for the brain enzyme.

AppppA, heat-shock stress, and cell oxidation

Salmonella typhimurium LT2 induces a set of heat-shock proteins analogous to those found previously in Escherichia coli. These are virtually the only proteins synthesized after a temperature shift from 28 degrees C to 50 degrees C. Using a two-dimensional thin-layer chromatographic system developed to resolve adenylylated nucleotides, we have found that S. typhimurium and E. coli accumulate P1,P4-diadenosine-5'-tetraphosphate (AppppA), P1-(adenosine-5')-P3-(guanosine-3'-diphosphate-5')-triphosphate (ApppGpp), P1-(adenosine-5')-P4-(guanosine-5')-tetraphosphate (AppppG), P1-(adenosine-5')-P3-(guanosine-5')-triphosphate (ApppG), and P1,P3-diadenosine-5'-triphosphate (ApppA) after heat shock. These same adenylylated nucleotides accumulate after exposure to ethanol, an agent also known to induce the heat-shock response in a variety of cells. AppppA, ApppGpp, AppppG, ApppG, and ApppA were previously shown to accumulate under conditions of oxidation stress. We proposed that these adenylylated nucleotides may be alarmones--i.e., regulatory molecules, alerting cells to the onset of oxidation stress. The finding that these dinucleotides accumulate in response to heat shock suggests that oxidation and heat shock have a common physiological effect on cells. We hypothesize that these dinucleotides signal the onset of these stresses and trigger the "heat-shock response."