Diacylglycerol Signaling

120 E. Rozengurt
The initial description of PKD as an atypical isoform of PKC (Johannes et al.
1994) and the inclusion of PKD/PKCm in reviews concerning the PKC family,
which belongs to the AGC group (named for PKA, PKG and PKC) (Newton 1997;
Mellor and Parker 1998), contributed to a perception that PKD belongs to the PKC
family. However, it was noted from the outset that the catalytic domain of PKD
has highest sequence homology with myosin light chain kinase and CaMKs
(Valverde et al. 1994). Indeed, the three isoforms of PKD are now classified as a
new protein kinase subfamily within the CaMK group, separate from the AGC
group (Hanks 2003). This scheme reflects the notion that the evolutionary relationship
between protein kinases is most appropriately linked to their respective
catalytic domain structures.
Full-length PKD isolated from multiple cell types or tissues exhibits very low
catalytic activity (Van Lint et al. 1995), which can be stimulated by phosphatidylserine
micelles and either DAG or phorbol esters (Van Lint et al. 1995; Johannes
et al. 1995; Matthews et al. 1997). These early studies demonstrated that PKD is a
phospholipid/DAG-stimulated serine/threonine protein kinase and implied that
PKD represents a novel component of the signal transduction initiated by DAG
production in their target cells (Rozengurt et al. 1997).
7.2 Regulation of PKD Activation
7.2.1 Rapid PKD Activation in Intact Cells: A PKC/PKD
Phosphorylation Cascade
Subsequent studies, aimed to define the regulatory properties of PKD within intact
cells, produced multiple lines of evidence that elucidated a mechanism of PKD
activation distinct from the direct stimulation of enzyme activity by DAG/phorbol
ester plus phospholipids obtained in vitro. Treatment of intact cells with phorbol
esters, cell-permeable DAGs or bryostatin induced a dramatic conversion of PKD
from an inactive to an active form, as shown by in vitro kinase assays performed in
the absence of lipid co-activators (Matthews et al. 1997; Zugaza et al. 1996). In all
these cases, PKD activation was selectively and potently blocked by cell treatment
with PKC inhibitors (e.g., GFI, Ro31-8220 and Go6983) that did not directly
inhibit PKD catalytic activity (Matthews et al. 1997; Zugaza et al. 1996), suggesting
that PKD activation in intact cells is mediated, directly or indirectly, through
PKCs. In line with this conclusion, co-transfection of PKD with active mutant
forms of “novel” PKCs (PKCs d, e, h, q) resulted in robust PKD activation in the
absence of cell stimulation (Waldron et al. 1999a; Zugaza et al. 1996; Yuan et al.
2002; Storz et al. 2004a).
A variety of regulatory peptides, including bombesin, bradykinin, endothelin
and vasopressin, or growth factors (e.g., PDGF) also induced PKD activation via a
PKC-dependent pathway in intact fibroblasts (Zugaza et al. 1997). These results