Protein kinase C

Discovery of a phosphorylating activity independent cAMP

The enzymes that we call protein kinase C (PKC) became known during the late 1970s as a cyclic nucleotide-independent protein kinase present in bovine cerebellum.36 As first described, this activity appeared to be the product of limited proteolysis by a calcium-dependent protease,37 but shortly afterwards it was found to be activated by Ca2 (50 mol L 1) and phospholipids, with no need for proteolysis.38 More significantly, the activity of this protein kinase could also be stimulated by the addition of a small quantity of diacylglycerol (DAG, product of the phospholipase-C reaction, together with phospholipids (generally phosphatidylserine) and Ca2 , now at concentrations not far above the physiological range (2–

6 mol L 1).39

The new enzyme became a focus of interest when it was found that it constitutes the predominant intracellular target for the active principles of croton oil, long recognized as skin irritants with tumour-promoting capacities.40 Such phorbol esters41 activate PKC, substituting for, and

competing with, the physiological activator DAG.42,43 Phorbol ester (Figure 9.7) has since been applied to every cell and system imaginable. It is apparent that PKC is involved in an enormous number of cellular processes. These range from tumour formation, host defence, embryological development, pain perception, neurite outgrowth, and the development of long-term memory. We consider the roles of protein kinase C in tumour formation and cell polarity in Chapter 19.

FIG 9.7  Structure of phorbol myristate acetate (PMA/TPA) and diacylglycerol.

Note the similar orientation of the three critical oxygens (red) which are essential for activation of PKC.

Croton oil is a poisonous, viscous liquid obtained from the seeds of a small Asiatic tree, Croton tiglium, a member of the spurge family (Euphorbiaceae). The tree is native to

India and the Indonesian archipelago. Croton oil is pale yellow to brown and is transparent, with an acrid persistent taste and disagreeable odour. It is a violent irritant. The use of croton oil as a drug was introduced to the West from China by the Dutch in the 16th century. The 19th century physician (and pioneer in the field of signal transduction) Sidney Ringer (see page 185) describes its use

as a purgative and its topical application in the treatment of ringworm.44 It is now considered

too dangerous for medicinal use.

The phorbol ester most often used in the laboratory is phorbol 12-myristate 13-acetate or 12-O-tetradecanoyl phorbol-13-acetate. The alternative abbreviations PMA and TPA appear widely in the literature. In general, we have preferred the term

PMA but TPA cannot be avoided as it gives rise to the TRE, the TPA

responsive element (see page 578).