Therefore, we believe that calcium and PKC signals are required for sufficient Nur77/Nor-1 mitochondrial localization and reversal of Bcl-2 pro-survival function. In this study, we report the biological activity of a synthetic DAG-lactone, HK434, in thymocytes. HK434, like the other synthesized DAG analogs, binds with Forskolin high potency to the phorbol ester/DAG binding site within the C1 domain of PKC 52. Using the crystal structure of the PKCδ C1b domain with pharmacophore and receptor-guided approaches, structurally

primitive DAG-lactone ligands were designed with binding affinities for PKCα in the low nanomolar range 39. These DAG-lactones exhibit 3–4 orders of magnitude higher affinity for PKC isozymes than natural DAG and phorbol esters. They have been characterized in other cell types and have phorbol ester-like effects 39, 53–56. Here, we report that DAG-lactone, HK434 and ionomycin signals are sufficient to induce Nur77/Nor-1 mitochondrial targeting in thymocytes. Furthermore, HK434, like phorbol esters can induce apoptosis in thymocytes. An interesting finding is that HK434 and PMA exert their regulation of Nur77 and their apoptotic activities through activation of different subsets of PKC isoforms (Fig. 6B). While the classical PKC isoform inhibitor

Gö6976 is sufficient in blocking HK434/ionomycin-induced Nur77 mitochondrial targeting and thymocyte apoptosis, no effect was observed with PMA/ionomycin-stimulated thymocytes. A correlation was found between Kinase Inhibitor Library cell assay PKC activation, induction of thymocyte apoptosis, either Nur77/Nor-1 phosphorylation, mitochondria translocation and exposure of the Bcl-2 BH3 epitope in stimulated thymocytes, further confirming the important role of Nur77/Nor-1 mitochondria translocation in TCR-induced thymocyte apoptosis. It is not clear if PKC acts directly or indirectly on Nur77/Nor-1. An interaction between PKC and Nur77 has been reported

before 57. Ser350 within the DNA binding domain of Nur77 was previously shown to be phosphorylated by protein kinase A and PKC in an in vitro kinase assay of stimulated PC12 neuronal cells 49. However, in another study, the association of Nur77 and PKCθ in T-cell hybridomas did not induce Nur77 phosphorylation 57. It is possible that a direct PKC regulation of Nur77 might be unique to immature T cells. Alternatively, phosphorylation of Nur77 may be indirectly regulated by PKC proteins. PKCθ has been initially suggested to be the PKC isoform crucial for negative selection. This notion was based on findings that during negative selection, PKCθ, but not other PKC isoenzymes, is recruited to the site of TCR aggregation 35. However, PKCθ−/− mice show no defects in negative selection 58. This suggests some functional redundancy among PKC family members and that a PKC isoenzyme distinct from PKCθ is involved in TCR signaling events in thymocytes.