New perspectives on PKCtheta, a member of the novel subfamily of protein kinase C

Inhibition of the c-Jun N-terminal kinase/AP-1 and NF-kappaB pathways by PICOT, a novel protein kinase C-interacting protein with a thioredoxin homology domain

Protein kinase C-theta (PKCtheta) is a Ca(2+)-independent PKC isoform that is selectively expressed in T lymphocytes (and muscle), and is thought to play an important role in T cell receptor-induced activation. To gain a better understanding of the function and regulation of PKCtheta, we have employed the yeast two-hybrid system to identify PKCtheta-interacting proteins. We report the isolation and characterization of a cDNA encoding a novel 335-amino acid (37. 5-kDa) PKCtheta-interacting protein termed PICOT (for PKC-interacting cousin of thioredoxin). PICOT is expressed in various tissues, including in T cells, where it colocalizes with PKCtheta. PICOT displays an N-terminal thioredoxin homology domain, which is required for the interaction with PKC. Comparison of the unique C-terminal region of PICOT with expressed sequence tag data bases revealed two tandem repeats of a novel domain that is highly conserved from plants to mammals. Transient overexpression of full-length PICOT (but not its N- or C-terminal fragments) in T cells inhibited the activation of c-Jun N-terminal kinase (but not extracellular signal-regulated kinase), and the transcription factors AP-1 or NF-kappaB. These findings suggest that PICOT and its evolutionary conserved homologues may interact with PKC-related kinases in multiple organisms and, second, that it plays a role in regulating the function of the thioredoxin system.

Differential expression of protein kinase C isoform transcripts in human hematopoietic progenitors undergoing differentiation

Protein kinase C (PKC), a key component of the signaling pathways leading to proliferation and differentiation, consists of a family closely related serine/threonine protein kinases. The mRNA expression of these PKC isoforms has been characterized during hematopoietic differentiation. Using the reverse-transcriptase polymerase chain reaction technique, we have analyzed the levels of isoform transcripts in bone marrow CD34(+) hematopoietic progenitors and their progeny differentiated along erythroid, megakaryocyte, or granulocyte/monocyte lineages, upon exposure to growth factors. In contrast with isoforms alpha, beta(I), beta(II), delta, and epsilon, ubiquitously expressed, isoforms theta, eta/L, zeta, and iota/lambda exhibited a lineage-restricted expression. These qualitative changes, which allow to distinguish the erythroid and megakaryocyte phenotypes from the granulocyte/monocyte phenotype, include zeta exclusively upregulated in granulocytes/monocytes and theta, eta/L, and iota/lambda exclusively expressed in megakaryocytes and erythroblasts. In contrast, erythroblasts and megakaryocytes, which supposedly share a common bipotential progenitor, displayed only quantitative changes. These results evidence the selective expression of PKC isoforms at transcriptional and/or posttranscriptional levels in hematopoietic progenitors induced to differentiate, which may suggest a differential contribution of individual isoforms to cellular signaling.

Direct interaction in T-cells between thetaPKC and the tyrosine kinase p59fyn

The protein kinase C (PKC) family has been clearly implicated in T-cell activation as have several nonreceptor protein-tyrosine kinases associated with the T-cell receptor, including p59fyn. This report demonstrates that thetaPKC and p59fyn specifically interact in vitro, in the yeast two-hybrid system, and in T-cells. Further indications of direct interaction are that p59fyn potentiates thetaPKC catalytic activity and that thetaPKC is a substrate for tyrosine phosphorylation by p59fyn. This interaction may account for the localization of thetaPKC following T-cell activation, pharmacological disruption of which results in specific cell-signaling defects. The demonstration of a physical interaction between a PKC and a protein-tyrosine kinase expands the class of PKC-anchoring proteins (receptors for activated C kinases (RACKs)) and demonstrates a direct connection between these two major T-cell-signaling pathways.

Protein kinase C-theta (PKCtheta) distribution analysis in hematopoietic cells: proliferating T cells exhibit high proportions of PKCtheta in the particulate fraction

A comparative analysis of protein kinase C-theta (PKCtheta) protein expression was performed in various mouse organs and tissues, freshly isolated populations of mouse and human hematopoietic cells, primary leukemias, and established cell lines of different histological origins. Results demonstrated a predominant expression of PKCtheta in lymphoid tissues and skeletal muscle. Expression levels of PKCtheta, as well as PKCalpha, delta, epsilon, zeta, and eta in the thymus, were not markedly changed during postnatal development. High levels of expression were observed in CD4(+) and CD8(+) single-positive T cells and CD4(+)CD8(+) double-positive thymocytes, while B cells were completely devoid of PKCtheta. PKCtheta was found also in platelets, but relatively low levels or no detection of PKCtheta expression were observed in neutrophils, monocytes, and macrophages. Highly proliferating leukemic T cells of established lines or primary tumors, but not freshly isolated resting peripheral blood T cells, exhibited high levels of membrane-bound PKCtheta. Increased proportions of PKCtheta in the particulate fraction was not restricted to malignant cells but correlated with the extent of proliferation of the T cells. Thus, human peripheral blood T cells that were induced to proliferate by exposure to mitogen and IL-2 expressed increased levels of PKCtheta in the particulate fraction. Significantly lower proportions of membrane-bound PKC were observed for five other isoenzymes expressed in T cells. The occurrence of PKCtheta in T, but not B, cells and its subcellular distribution in proliferating cells implicate PKCtheta in cellular mechanisms regulating the sustained proliferation of T cells.