The stressful condition as a nutritionally dependent adaptive dichotomy.
The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.
D000222:Adaptation, Physiological; D004702:Endocrine Glands / Q000503:physiopathology; D006801:Humans; D006854:Hydrocortisone / Q000502:physiology; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000502:physiology; D009752:Nutritional Status; D011228:Prealbumin / Q000502:physiology; D012176:Retinoids / Q000378:metabolism; D012177:Retinol-Binding Proteins / Q000502:physiology; D054840:Retinol-Binding Proteins, Cellular; D054839:Retinol-Binding Proteins, Plasma; D013312:Stress, Physiological / Q000503:physiopathology; D013961:Thyroid Gland / Q000503:physiopathology; D014156:Transcortin / Q000502:physiology