It is well established that most TSH effects on the thyroid gland, including stimulation of proliferation, thyroid hormone synthesis and expression of thyroid-specific genes, are transmitted mainly by the adenylate cyclise pathway. However, in human follicular cells and in rat FRTL-5 cells, TSH can also stimulate the β-isoforms of PLC that catalyzes the hydrolysis of phosphatidyl-inositol 4,5-phosphate, yielding the second messengers DAG and inositol 1,4,5-phosphate facilitating an increase in intracellular Ca2+. In FRTL-5 cells TSH has been suggested to increase DAG via phospholipase D, which produces DAG from phosphatidylcholine hydrolysis, suggesting an alternative mechanism for TSH-dependent activation through protein kinase C (PKC).
In the present study, we characterize the PKCβ and PKCδ isoforms expression and function in human follicular carcinoma cells, FTC133, and in human transformed thyrocytes, Nthy-ori cells, in order to understand whether these PKC isoforms are involved in TSH-mediated follicular cell proliferation and apoptosis. We mainly focus on PKCβ and PKCδ isoenzymes which are the most abundantly expressed isoforms in several tissues, are the most extensively studied and have two opposing roles in regulating cell proliferation.
In the Nthy-ori cells TSH stimulated cell proliferation and protected from apoptosis with a PKC-mediated mechanism. At the contrary, TSH did not increase FTC-133 cell viability nor protected the cells from PKC-inhibitor induced apoptosis. However, in FTC-133 cells TSH induced PKC expression, as well as downstream PKC targets GSK3β and AKT phosphorylation through a PKC-mediated mechanism. Moreover, immunofluorescence showed PKCβ and PKCδ perinuclear and citosolic location. These data suggest that TSH plays different roles in normal vs neoplastic thyrocytes.
Further studies are needed to clarify the role of PKCβ and PKCδ in the TSH signaling pathway in thyroid cells.