Protein kinase C as a mediator of TSH and thyroid autoantibody action

Although it is well-established that TSH activates a cAMP-dependent pathway in the thyroid follicular cell leading to thyroid hormone synthesis and release, the present review provides new evidence that TSH also activates a non-cAMP-dependent signal transduction system. This cascade involves phosphoinositide (PI) turnover, diacylglycerol accumulation and protein kinase C (PKC) activation. Activation of this pathway leads to an inhibition of differentiated thyroid function in vitro. Recent evidence suggests that TSH can activate both pathways via a single transcription unit. Unlike TSH, TSH-receptor antibodies may selectively activate cAMP with no effects on PI turnover. In contrast, preliminary studies suggest TSH-blocking antibodies may activate PKC. PKC may be an important mediator of TSH and, possibly, thyroid autoantibody action.