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

In the Lewis rat model of experimental autoimmune thyroiditis (EAT), decreased immunodetectable connexin assembly into gap junctions and diminished intercellular communication are associated with the loss of thyroid function (hypothyroidism) that occurs prior to significant tissue destruction. The current study explores the hypothesis that the loss of connexin 43 (Cx43)-mediated intercellular communication in these cells is caused by upregulation of protein kinase C (pKC) activity. Thyrocytes isolated from EAT rats exhibited a 78% increase in basal pKC activity; whereas, basal protein kinase A (pKA) activity was unchanged. Increased pKC activity was a result of increased isozyme protein levels. Thyroid cells expressed pKC isozymes γ and λ and had elevated levels of α (40%), β (30%), δ (31%), and ∈ (25%) as quantified by western blot analyses. Furthermore, modulation of pKC activity inversely altered Cx43 assembly and function in monolayer thyrocytes. For example, octoacetyl glycerol (OAG) treatment of normal thyrocyte monolayers to increase pKC activity resulted in deficient Cx43 gap junction assembly and reduced intercellular communication indistinguishable from the deficits in EAT thyrocytes. Conversely, calphostin C inhibition of pKC activity in EAT thyrocyte monolayers restored these parameters to normal. Thus, pharmacological modulations of pKC activity in cultured thyrocytes support a causal relation between the changes in pKC activity and Cx43-mediated intercellular communication. Abnormalities in autoimmune diseased thyroid tissue (eg, increased pKC) appear to contribute to reduced intercellular coordination of thyroid follicles and thereby can affect subsequent thyroid function. The persistence of target cell abnormalities in the absence of infiltrating lymphocytes and their products supports an alternative mechanism by which thyroid function can be affected that does not depend on the loss of thyroid glandular epithelium.