PKC delta mediates ionizing radiation-induced activation of c-Jun NH2-terminal kinase through MKK7 in human thyroid cells

The thyroid gland is one of the most sensitive organs in ionizing radiation (IR)-induced carcinogenesis. To determine, therefore, the specific cascade of IR-induced signal transduction in human thyroid cells, we investigated the functional role of protein kinase C (PKC), especially its interlocking activation of c-Jun NH2-terminal kinase (JNK) pathway. In the present study , using adenovirus expression vectors for diverse dominant-negative (DN) ty pes of PKC isoforms (alpha, beta2, delta, epsilon and zeta) expressed in pr imary cultured human thyroid cells, only DN/PKC delta suppressed IR-induced JNK activation. In addition, Rottlerin, a PKC delta specific inhibitor, in hibited IR-induced JNK activation. IR-induced activation of transcription f actor AP-1, downstream target of JNK, was also attenuated by DN/PKC delta. To examine the involvement of upstream kinases of JNK, we performed immune- complex kinase assays of mitogen-activated protein kinase kinase 4 (MKK4) a nd MKK7. IR activated MKK7 but not MKK4, and this activation was inhibited by Rottlerin. Furthermore, IR-induced JNK activation was suppressed by over expression of kinase-deficient MKK7. Our results indicate that IR selective ly activates the cascade of PKC delta -MKK7-JNK-AP-1 in human thyroid cells , suggesting a not apoptotic but radio-resistant role of PKC delta in human thyroid cells following IR.