Available online at www.sciencedirect.com Protein kinase C intervention—the state of play Jon Roffey 1 , Carine Rosse 2 , Mark Linch 2 , Andrew Hibbert 2 , Neil Q McDonald 3 and Peter J Parker 2,4 Intervention in protein kinase C (PKC) has a chequered history, partly because of the poor selectivity of many inhibitors and partly a reflection of the sometimes antagonistic action of related PKC isoforms. Recent advances in targeting PKC isoforms have come from structural work on isolated kinase domains that have provided opportunities to drive selectivity through structure-based avenues. The promise of isoform selective inhibitors and the rationale for their development are discussed in the broader context of the PKC inhibitor arsenal. Addresses 1 Discovery Laboratory, Cancer Research Technology Limited, Wolfson Institute for Biomedical Research, Gower Street, London WC1E 6BT, UK 2 Protein Phosphorylation Laboratory, London Research Institute, Lincoln’s Inn Fields Laboratories, 44 Lincoln’s Inn Fields, London WC2A 3PX, UK 3 Structural Biology Laboratory, London Research Institute, Lincoln’s Inn Fields Laboratories, 44 Lincoln’s Inn Fields, London WC2A 3PX, UK 4 Division of Cancer Studies, Kings College London, Section of Cancer Cell Biology and Imaging, New Hunt’s House, Guy’s Hospital, St Thomas Street, London SE1 1UL, UK Corresponding author: Parker, Peter J (Peter.Parker@cancer.org.uk) Current Opinion in Cell Biology 2009, 21:268–279 This review comes from a themed issue on Cell regulation Edited by Brian Hemmings and Nikolas Tonks Available online 23rd February 2009 0955-0674/$ – see front matter # 2009 Elsevier Ltd. All rights reserved. DOI 10.1016/j.ceb.2009.01.019 Introduction Over the past decade or so the emerging understanding of the roles played by members of the protein kinase C (PKC) gene family in the control of cell and tissue functions has refocused efforts on the development of therapeutic interventions directed at specific members of the family. The history of PKC inhibitors is mixed with many of the first identified, potent inhibitors, being of low selectivity, including the widely employed ‘pan-kinase’ inhibitor staurosporine. Screening efforts and recent insights into structure have provided routes to more se- lective catalytic inhibitors, alongside peptide(-mimetic) and other approaches that have taken distinctive, rational inhibitor development routes. Here we will discuss the context for some of the PKC inhibitor developments specifically in relation to our current state of knowledge on PKC behaviour and function. We will then address the state of play of PKC interventions, embracing distinct approaches and including the clinical trials direct at PKC. The PKC family—a functional overview To provide a framework for understanding the spectrum of approaches to PKC inhibition and activation it is instructive to provide an overview of the properties of these protein kinases. While details vary for each isoform and additional layers of complexity exist, there is a useful underlying set of properties defined largely by the pio- neering work of Yasutomi Nishizuka [1] that provide a framework for principles of intervention. This framework in essence involves membrane recruitment of PKC that for classical forms (cPKC) is triggered by elevated Ca 2+ followed by interaction with the PKC effector diacylgly- cerol (DAG) typically derived from phosphatidylinositol 4,5 bisphosphate hydrolysis by phospholipase C [1]. The structural organisation of PKC gene family members is conserved throughout the classical (a,b,g), novel (d,e,h,u) and atypical (z,i) isoforms. They all comprise a C-terminal serine/threonine protein kinase domain (AGC class) linked through a variable ‘V3’ domain (see Figure 1) to a regulatory domain. The latter comprises three functional elements (i) an inhibitory region (pseu- dosubstrate site), (ii) a C1 domain (one copy or as a tandem repeat—C1A, C1B) and (iii) a C2 or PB1 domain. The distinctiveness of the regulatory domains contributes substantially to the particular roles played by individual isoforms. All members of the PKC family are activated through allosteric inputs. These comprise lipids, proteins and the combination of the two (Figure 1). Inactivity is determined by the interaction of the regulatory domain with the cat- alytic domain and this is partly driven by the interaction of the inhibitory pseudosubstrate site in the regulatory domain with the substrate binding pocket in the catalytic domain. No full-length structures yet exist to corroborate this working model, which is based principally on muta- tional analysis [2]. It is also likely that further regulatory- kinase domain contacts are made on the basis of behaviour in vivo that is consistent with a ‘buried’ regulatory C1 domain effector binding pocket [3]. The extent to which these additional regulatory-kinase domain interactions determine properties is unclear given the retained allosteric activation behaviour of chimeric PKCs (see [4]). However, the particularity of the closed conformers of PKC provides a basis for intervention, whether to inactivate by locking this Current Opinion in Cell Biology 2009, 21:268–279 www.sciencedirect.com