Protein kinase C as a second messenger target.

Protein kinase C (PKC) is a multifunctional serine/threonine specific protein kinase, first described by Nishizuka and colleagues (Takai et d., 1977), that is dependent upon Ca2+, phospholipid and diacylglycerol (recently reviewed Nishizuka 1986; Woodgett et af., 1987). The enzyme is widely distributed and has been purified to apparent homogeneity from a number of sources (see Woodgett et al., 1987). The properties of PKC have suggested that in intact cells, activation will occur in response to diacylglycerol production (Takai et al., 1979); this neutral lipid permitting activation of PKC within the physiological range of Ca2 + concentrations. Circumstantial evidence that diacylglycerol can have such a second messenger role in the activation of PKC has been provided by studies employing membrane-'permeable' diacylglycerols such as 1-oleyl-2-acetyl glycerol (Kaibuchi et al., 1983) and 1,2-dioctanoyl glycerol (Davis et al., 1985) or biologically active phorbol esters which have been shown to mimic the effects of diacylglycerol in vitro (Castagna et al., 1982). Diacylglycerol can be produced physiologically through the action of phospholipase C enzymes on membrane phospholipids. In particular, the breakdown of phosphoinositides has received much attention with respect to PKC, in view of the rapid agonist-induced activation of phosphoinositide-specific phospholipase C and some circumstantial evidence that PKC is indeed involved downstream of such responses (see, e.g. Ieyasu et al., 1982). The polyphosphoinositide phosphatidylinositol 4,5-bisphosphate appears to be a major target for hydrolysis and the water-soluble product inositol 1,4,5-trisphosphate, that is liberated coincidentally with the diacylglycerol, is involved in the regulation of cytosolic Ca2+ (Streb et al., 1984). Thus it has been suggested that these two second messengers can act synergistically in the activation of PKC. The cloning of PKC cDNAs from four mammals has now been reported (bovine, Parker et a[., 1986; human, Coussens et al., 1986, 1987; rat, Knopf et al., 1986; Ono et al., 1986; rabbit, Ohno et al., 1987), as well as from Drosophila (Rosenthal et al., 1987). For the mammals these studies have revealed the presence of at least four highly related polyeptides which we have designated PKC-a, -@,, -@, and y P see Coussens et a[., 1986, 1987). These polypeptides are derived from three genes (for man: a-chl7; @-chl6; y-chl9; Coussens et al., 1986) with human PKC-PI and -P2 being derived from alternative splicing of exons at the 3'-end of the @-gene (Coussens et al., 1987). One further related gene product has been partially defined through cDNA cloning, and this covers the kinase domain of a close relative or novel member of the PKC family (Housey et al., 1987). The mRNAs for these polypeptides are differentially expressed (Knopf et al., 1986; Coussens et al., 1987; Brandt et al., 1987, Ohno et al., 1987) and the high degree of conservation of these multiple subtypes in primates, rabbit, rodents and cattle suggests that there are specific functional differences between these polypeptides that provide an element of selectivity with respect to this signal transduction pathway. An understanding of how much selectivity might work is likely to come from a detailed biochemical analysis of the polypeptides. Studies along these lines in our laboratory have