Cholesterol-rich microdomains in rat and porcine thyroid membranes involved in TSH-induced endocytotic processes

A Novel Role for Flotillin-Containing Lipid Rafts in Negative-Feedback Regulation of Thyroid-Specific Gene Expression by Thyroglobulin

BACKGROUND

Thyroglobulin (Tg) stored in thyroid follicles regulates follicular function in thyroid hormone (TH) synthesis by suppressing thyroid-specific gene expression in a concentration-dependent manner. Thus, Tg is an intrinsic negative-feedback regulator that can restrain the effect of thyrotropin (TSH) in the follicle. However, the underlying mechanisms by which Tg exerts its prominent autoregulatory effect following recognition by thyrocytes remains unclear.

METHODS

In order to identify potential proteins that recognize and interact with Tg, mass spectrometry was used to analyze immunoprecipitated Tg-bound proteins derived from Tg-treated rat thyroid FRTL-5 cells.

RESULTS

Flotillin 1 and flotillin 2, two homologs that are integral membrane proteins in lipid rafts, were identified as novel Tg-binding proteins with high confidence. Further studies revealed that flotillins physically interact with endocytosed Tg, and together these proteins redistribute from the cell membrane to cytoplasmic vesicles. Treatment with the lipid raft disrupter methyl-β-cyclodextrin abolished both the endocytosis and the negative-feedback effect of Tg on thyroid-specific gene expression. Meanwhile, siRNA-mediated knockdown of flotillin 1 or flotillin 2 also significantly inhibited Tg effects on gene expression.

CONCLUSION

Together these results indicate that flotillin-containing lipid rafts are essential for follicular Tg to be recognized by thyrocytes and exert its negative-feedback effects in the thyroid.

Intrinsic regulation of thyroid function by thyroglobulin

BACKGROUND

The established paradigm for thyroglobulin (Tg) function is that of a high molecular weight precursor of the much smaller thyroid hormones, triiodothyronine (T3) and thyroxine (T4). However, speculation regarding the cause of the functional and morphologic heterogeneity of the follicles that make up the thyroid gland has given rise to the proposition that Tg is not only a precursor of thyroid hormones, but that it also functions as an important signal molecule in regulating thyroid hormone biosynthesis.

SUMMARY

Evidence supporting this alternative paradigm of Tg function, including the up- or downregulation by colloidal Tg of the transcription of Tg, iodide transporters, and enzymes employed in Tg iodination, and also the effects of Tg on the proliferation of thyroid and nonthyroid cells, is examined in the present review. Also discussed in detail are potential mechanisms of Tg signaling in follicular cells.

CONCLUSIONS

Finally, we propose a mechanism, based on experimental observations of Tg effects on thyroid cell behavior, that could account for the phenomenon of follicular heterogeneity as a highly regulated cycle of increasing and decreasing colloidal Tg concentration that functions to optimize thyroid hormone production through the transcriptional activation or suppression of specific genes.

It's a lipid's world: bioactive lipid metabolism and signaling in neural stem cell differentiation

Lipids are often considered membrane components whose function is to embed proteins into cell membranes. In the last two decades, studies on brain lipids have unequivocally demonstrated that many lipids have critical cell signaling functions; they are called "bioactive lipids". Pioneering work in Dr. Robert Ledeen's laboratory has shown that two bioactive brain sphingolipids, sphingomyelin and the ganglioside GM1 are major signaling lipids in the nuclear envelope. In addition to derivatives of the sphingolipid ceramide, the bioactive lipids discussed here belong to the classes of terpenoids and steroids, eicosanoids, and lysophospholipids. These lipids act mainly through two mechanisms: (1) direct interaction between the bioactive lipid and a specific protein binding partner such as a lipid receptor, protein kinase or phosphatase, ion exchanger, or other cell signaling protein; and (2) formation of lipid microdomains or rafts that regulate the activity of a group of raft-associated cell signaling proteins. In recent years, a third mechanism has emerged, which invokes lipid second messengers as a regulator for the energy and redox balance of differentiating neural stem cells (NSCs). Interestingly, developmental niches such as the stem cell niche for adult NSC differentiation may also be metabolic compartments that respond to a distinct combination of bioactive lipids. The biological function of these lipids as regulators of NSC differentiation will be reviewed and their application in stem cell therapy discussed.

Role of caveolin-1 in thyroid phenotype, cell homeostasis, and hormone synthesis: in vivo study of caveolin-1 knockout mice

In human thyroid, caveolin-1 is localized at the apex of thyrocytes, but its role there remains unknown. Using immunohistochemistry, (127)I imaging, transmission electron microscopy, immunogold electron microscopy, and quantification of H(2)O(2), we found that in caveolin-1 knockout mice thyroid cell homeostasis was disrupted, with evidence of oxidative stress, cell damage, and apoptosis. An even more striking phenotype was the absence of thyroglobulin and iodine in one-half of the follicular lumina and their presence in the cytosol, suggesting that the iodide organification and binding to thyroglobulin were intracellular rather than at the apical membrane/extracellular colloid interface. The latter abnormality may be secondary to the observed mislocalization of the thyroid hormone synthesis machinery (dual oxidases, thyroperoxidase) in the cytosol. Nevertheless, the overall uptake of radioiodide, its organification, and secretion as thyroid hormones were comparable to those of wild-type mice, suggesting adequate compensation by the normal TSH retrocontrol. Accordingly, the levels of free thyroxine and TSH were normal. Only the levels of free triiodothyronine showed a slight decrease in caveolin-1 knockout mice. However, when TSH levels were increased through low-iodine chow and sodium perchlorate, the induced goiter was more prominent in caveolin-1 knockout mice. We conclude that caveolin-1 plays a role in proper thyroid hormone synthesis as well as in cell number homeostasis. Our study demonstrates for the first time a physiological function of caveolin-1 in the thyroid gland. Because the expression and subcellular localization of caveolin-1 were similar between normal human and murine thyroids, our findings in caveolin-1 knockout mice may have direct relevance to the human counterpart.

Sphingolipid-cholesterol domains (lipid rafts) in normal human and dog thyroid follicular cells are not involved in thyrotropin receptor signaling

Partition of signaling molecules in sphingolipid-cholesterol-enriched membrane domains, among which are the caveolae, may contribute to signal transduction efficiency. In normal thyroid, nothing is known about a putative TSH/cAMP cascade compartmentation in caveolae or other sphingolipid-cholesterol-enriched membrane domains. In this study we show for the first time that caveolae are present in the apical membrane of dog and human thyrocytes: caveolin-1 mRNA presence is demonstrated by Northern blotting in primary cultures and that of the caveolin-1 protein by immunohistochemistry performed on human thyroid tissue. The TSH receptor located in the basal membrane can therefore not be located in caveolae. We demonstrate for the first time by biochemical methods the existence of sphingolipid-cholesterol-enriched domains in human and dog thyroid follicular cells that contain caveolin, flotillin-2, and the insulin receptor. We assessed a possible sphingolipid-cholesterol-enriched domains compartmentation of the TSH receptor and the alpha- subunit of the heterotrimeric G(s) and G(q) proteins using two approaches: Western blotting on detergent-resistant membranes isolated from thyrocytes in primary cultures and the influence of 10 mm methyl-beta-cyclodextrin, a cholesterol chelator, on basal and stimulated cAMP accumulation in intact thyrocytes. The results from both types of experiments strongly suggest that the TSH/cAMP cascade in thyroid cells is not associated with sphingolipid-cholesterol-enriched membrane domains.

Cholesterol and non-cardiovascular disease: basic science

Cholesterol metabolism is of fundamental biological importance. This review examines the role of cholesterol metabolism in relation to non-cardiovascular disease (non-CVD). Particular attention is paid to the question of whether or not low levels of cholesterol may have harmful effects on cell function or lead to pathological processes. Many in vitro phenomena have been demonstrated at levels of cholesterol which are very low in comparison to physiological conditions. Nevertheless, low cholesterol is more favourable than high cholesterol for most aspects of cell function. There is no evidence that any catastrophic cellular response or pathological process occurs due to exposure of organisms to low cholesterol. On the other hand, the inflammatory process is a powerful and consistent cause of decreased cholesterol levels. This, together with other confounding factors, appears to explain a major component of the association between low cholesterol levels and non-CVD.