Elevation of protein kinase C in thyrocytes isolated from a Lewis rat model of autoimmune thyroiditis prevents assembly of immunodetectable connexin43 gap junctions and reduces intercellular communication

Altered expression of the gap junction protein connexin43 is associated with papillary thyroid carcinomas when compared with other noncancer pathologies of the thyroid

BACKGROUND

Gap junctions are membrane structures composed of connexins (Cx) that allow diffusion of small molecules between cells. They are involved in tissue homeostasis, and various organ dysfunctions have been associated with gap junction defects. To verify their possible involvement in thyroid pathologies, the expression of connexin43 (Cx43), the major Cx in the human thyroid, was evaluated in a variety of diseases including cancer.

METHODS

There were 122 samples from various thyroid pathologies that were collected to analyze the presence of Cx43 by immunofluorescence. Through confocal microscopy, different patterns of Cx43 localization were identified as normal (membrane) or abnormal (cytoplasmic or lack of detection). The analysis of Cx43 expression was further performed by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry in a subset of 25 papillary carcinomas and compared with nontumoral thyroid tissues.

RESULTS

The presence of Cx43 was commonly altered in thyroid cancer, as abnormal Cx43 staining was detected in 94.1% of cancer, 47.4% of adenomas, 45.7% of multinodular goiter, 16.7% of Graves' disease, and 25% of thyroiditis. In papillary carcinoma samples, the deregulation of Cx43 expression was mostly the consequence of a decrease of Cx43 mRNA (68% of cases) when compared with normal tissue. When Cx43 mRNA was not downregulated (32% of cases), both loss of membrane staining and aberrant cytoplasmic distribution of the protein were observed.

CONCLUSIONS

These results show that aberrations of Cx43 expression are associated with thyroid papillary carcinoma.

Rottlerin inhibits migration of follicular thyroid carcinoma cells by PKCdelta-independent destabilization of the focal adhesion complex

This study examined the effect of rottlerin on the focal adhesion-mediated cell migration of CGTH W-2 human follicular thyroid carcinoma cells. Rottlerin (10 microM) resulted in decreased adhesion of CGTH W-2 cells to matrix substance, which was correlated with metastatic potential. Rottlerin treatment also resulted in a marked reduction in the migration of CGTH W-2 cells. Protein levels of integrin beta1, FAK, and paxillin were decreased by rottlerin. Consistent with this, immunostaining of FAK, vinculin, and paxillin revealed disassembly of the focal adhesions. Disruption of actin stress fibers was noted, which was compatible with reduced expression levels and activities of Rac-1 and Rho. The effect of rottlerin on cell migration was not attributable to inhibition of PKCdelta activity since siRNA knockdown of PKCdelta did not recapitulate the effects of rottlerin on cell adhesion and migration. Furthermore, activation of PKCdelta by phorbol esters failed to restore the rottlerin-inhibited migratory ability. The mitochondrial uncoupler, carbonylcyanide-4-(trifluoromethoxy)-phenylhydrazone, was able to mimic several rottlerin's effects. In summary, we demonstrated that rottlerin inhibits the migration of CGTH W-2 cells by disassembly of focal adhesion complexes in a PKCdelta-independent manner, and might play as a mitochondrial uncoupler role in these events.

Differential expression of connexin 43 in human autoimmune thyroid disease

Gap junctions provide a pathway for cell-to-cell communication. Reduced thyroid epithelial cell-cell communication has been reported in some animal models of autoimmune thyroid disease. In order to assess whether this change was similar to human autoimmune thyroid disease, we identified some connexin proteins and their corresponding mRNA in human thyroid gland. The aim of our study was to explore the expression of connexin 43 (Cx43) in the thyroid gland from normal and diseased human thyroid tissue by immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR). The expression levels of Cx43 in Grave's disease were significantly increased in comparison with those of normal thyroid tissue. There was a significant decrease in expression of Cx43 in Hashimoto's thyroiditis, compared with normal thyroid tissue. These data indicate that changes of Cx43 expression in human autoimmune thyroid disease were associated with variations in thyroid function and hormone secretion.

Thyrocytes isolated from autoimmune-diseased thyroids secrete soluble tumor necrosis factor-R1 that is related to their elevated protein kinase C activity

Soluble tumor necrosis factor (TNF)-alpha receptors have the potential to modulate TNF-alpha activity during autoimmune thyroiditis. In this study we examined cell-surface TNF-alpha receptors and soluble TNF-alpha receptor production by thyrocytes from normal and MRL-lpr(-/-) (diseased) mice, which spontaneously develop autoimmune thyroiditis. We found that murine thyrocytes possess the 55-kd receptor (TNF-R1). Examination of soluble TNF-R1 production revealed that diseased thyrocytes produced sevenfold more soluble TNF-R1 than normal thyrocytes. Furthermore, basal protein kinase C (pKC) activity in diseased thyrocytes was 67% higher than that found in normal murine thyrocytes. The elevated basal pKC activity in diseased thyrocytes was related to their enhanced production of soluble TNF-R1 because inhibition of pKC activity with calphostin C caused soluble TNF-R1 production to decrease significantly. Additionally, soluble TNF-R1 production by murine thyrocytes was not a result of cell-surface receptor shedding but through secretion of a truncated version of TNF-R1. This was evident when cell-surface TNF-R1 levels were unchanged after treatment of diseased thyrocytes with calphostin C. Also, the 28-kd form of TNF-R1, which corresponds to the soluble receptor, was present in the intracellular membranes of the diseased thyrocytes.

Formation of three-dimensional thyroid follicle-like structures by polarized FRT cells made communication competent by transfection and stable expression of the connexin-32 gene

Pig thyrocytes, either in the intact gland or cultured under conditions leading to thyroid follicle reconstitution, coexpress two gap junction proteins, connexin-32 (Cx32) and connexin-43 (Cx43). As thyrocytes cultured in the form of a monolayer only express Cx43, we hypothesized that Cx32 could play a role in thyroid folliculogenesis. In the present work, we analyzed the ability of polarized FRT cells (that are gap junction deficient) to form follicle-like structures after stable transfection with either Cx32 or Cx43 genes. Wild-type and transfected FRT cells, while growing, showed the capacity to form three-dimensional structures corresponding to domes that result from the accumulation of fluid underneath limited areas of the cell layer. The number of domes formed by FRT cells expressing Cx32 (FRT-Cx32) was 2- to 3-fold higher than that obtained with either wild-type or Cx43-transfected FRT cells (FRT-Cx43). Domes generated by FRT-Cx32 cells were stable (beyond 3 weeks of culture), whereas those formed from wild-type or FRT-Cx43 cells were transient, disappearing when cells reached confluence. Inspection of the cell organization within domes formed from FRT-Cx32 cells by phase contrast and confocal microscopy revealed a progressive transition from domes toward closed structures with a lumen. The tightness of the lumen was demonstrated by the retention of a fluorescent probe, lucifer yellow, introduced by microinjection. Electron microscope examinations showed that the neoformed follicle-like structures had an inside-out polarity. Analyses of cell motion and division with time, by fluorescence video microscopy, indicated that the transformation of domes into inside-out follicles brings into play the migration of cells and, to a lesser extent, cell multiplication underneath the domes. In conclusion, FRT cells forced to express Cx32 give rise to domes that transform into closed inside-out follicles. This gain of function appears Cx specific, as FRT-Cx43 cells did not form similar structures. Our data suggest that the formation and/or functioning of Cx32 gap junctions might represent a key event in thyroid epithelium morphogenesis, i.e. formation of a lumen from a tight epithelial cell layer.