In this review we outline the contributions of thyroid hormones to different aspects of innate and adaptive immune responses. The relationship between thyroid hormones and immune cells is complex and T(3) and T(4) may modulate immune responses through both genomic and nongenomic mechanisms. Future studies of the molecular signaling mechanisms involved in this cross-talk between thyroid hormones and the immune system may support development of new strategies to improve clinical immune responses.

The two snake venom myotoxins ammodytin L and myotoxin II, purified respectively from Vipera ammodytes ammodytes and Bothrops asper, have phospholipase-like structures but lack an Asp-49 in the active site and are without normal phospholipase activity. The interaction of these proteins with different types of liposomes indicated that the myotoxins were able to provoke rapid and extensive release of the aqueous content of liposomes. Leakage was measured by two different methods: fluorescence dequenching of liposome-entrapped carboxyfluorescein and ESR measurement of intravesicular TEM-POcholine reduction by external ascorbate. The process was independent of Ca2+ and took place without any detectable phospholipid hydrolysis. Nonmyotoxic phospholipases tested under the same conditions were unable to induce liposome leakage, which could be detected only when Ca2+ was added to the medium and with the concomitant hydrolysis of phospholipids. The kinetics of Ca(2+)-dependent and Ca(2+)-independent leakage were completely different, indicating two different mechanisms of interaction with the lipid bilayer. Studies using diphenylhexatriene as a probe of lipid membrane organization indicated that the myotoxins gave rise to a profound perturbation of the arrangement of the lipid chains in the membrane interior, whereas interaction of Naja naja phospholipase A2 with the membrane surface did not affect lipid organization. On the basis of these results we suggest that a new type of cytolytic reaction mechanism is responsible for the effects of phospholipase-like myotoxins in vivo.

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Natural polyphenol compounds are often good antioxidants, but they also cause damage to cells through more or less specific interactions with proteins. To distinguish antioxidant activity from cytotoxic effects we have tested four structurally related hydroxyflavones (baicalein, mosloflavone, negletein, and 5,6-dihydroxyflavone) at very low and physiologically relevant levels, using two different cell lines, L-6 myoblasts and THP-1 monocytes. Measurements using intracellular fluorescent probes and electron paramagnetic resonance spectroscopy in combination with cytotoxicity assays showed strong antioxidant activities for baicalein and 5,6-dihydroxyflavone at picomolar concentrations, while 10 nM partially protected monocytes against the strong oxidative stress induced by 200 µM cumene hydroperoxide. Wide range dose-dependence curves were introduced to characterize and distinguish the mechanism and targets of different flavone antioxidants, and identify cytotoxic effects which only became detectable at micromolar concentrations. Analysis of these dose-dependence curves made it possible to exclude a protein-mediated antioxidant response, as well as a mechanism based on the simple stoichiometric scavenging of radicals. The results demonstrate that these flavones do not act on the same radicals as the flavonol quercetin. Considering the normal concentrations of all the endogenous antioxidants in cells, the addition of picomolar or nanomolar levels of these flavones should not be expected to produce any detectable increase in the total cellular antioxidant capacity. The significant intracellular antioxidant activity observed with 1 pM baicalein means that it must be scavenging radicals that for some reason are not eliminated by the endogenous antioxidants. The strong antioxidant effects found suggest these flavones, as well as quercetin and similar polyphenolic antioxidants, at physiologically relevant concentrations act as redox mediators to enable endogenous antioxidants to reach and scavenge different pools of otherwise inaccessible radicals.

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