Diacylglycerol Signaling

13 PKCs as Mediators of the Hedgehog and Wnt Signaling Pathways
A model was proposed in that DAG activates PKCd on the membrane, and
PKCd phosphorylates Dishevelled directly. However, Dishevelled is known to
interact with other kinases, such as CK-1 and -2, Par-1, and PAK1/MuSK
(Willert et al. 1997; Sun et al. 2001; Luo et al. 2002). PKCd may regulate such
protein kinases and thus indirectly regulate Dishevelled phosphorylation. It
would be interesting to examine whether PKCd phosphorylates Dishevelled
directly, and to elucidate the role of Dishevelled phosphorylation in its localization
and in the activation of downstream signaling. Identification of the phosphorylation
sites in Dishevelled upon Fz7 stimulation will shed light onto this
complex topic.
13.4.6 Heterotrimeric G Proteins in PkC Activation by Wnts
The Frizzled family of Wnt receptors belong structurally to the 7-TM (also known
as GPCR) superfamily, and together with SMO they constitute a separate group.
Indeed, several evidences suggest that Frizzled signals not only through Dishevelled,
but also independently through engagement of heterotrimeric G proteins (Gabg),
like SMO.
Early on, Slusarski et al. reported that Wnt5a, through activation of Fz2, induces
calcium release through the Gbg subunit of Gi proteins, since pertussis toxin, GDPb-S
and Gat blocked the calcium increase (Slusarski et al. 1997a). In another elegant
study, a chimeric receptor between the b-adrenergic receptor ligand binding
domain and Fz2 (a non-canonical Fz that is the Wnt5a receptor) was used to control
calcium release with an adrenergic agonist (isoproterenol). Expression and activation
of this chimera in F9 teratocarcinoma cells induced the formation of primitive
endoderm and calcium transients (Liu et al. 1999a). Primitive endoderm formation
was blocked by interference with Gi proteins by pertussis toxin, by depletion of
Gao (a Gi member) or Gb2 with antisense oligodeoxynucleotides, and by inhibitors
of PKC (bisindolylmaleimide I) and MEK1/2 (PD98059). In comparison, the
same group performed studies with overexpressed Fz1 (a canonical Fz that responds
to Wnt8) in F9 cells. The findings indicate that primitive endoderm formation
induced by canonical Wnt8/Fz1 is sensitive not only to Gao, but also to Gaq, and
further downstream, to PKC and MEK (Liu et al. 1999b). Moreover, a chimeric
receptor containing the intracellular loops of canonical Fz1 on a b-adrenergic backbone,
induced b-catenin stabilization, primitive endoderm formation, and transcriptional
activation of a Tcf/LEF reporter construct when stimulated with isoproterenol.
These effects were abolished by pertussis toxin, indicating Gi proteins involvement,
and by depletion of Gao (a Gi family member) and Gaq (Liu et al. 2001). These
important observations suggest that G proteins are integral part of the Wnt pathway,
both canonical and non-canonical, as we have found to be the case for the Hedgehog
signaling pathway. Indeed, the role of Gao is evolutionary conserved since it is
immediately downstream of Frizzled in Drosophila Wnt /b-catenin and planar cell
polarity pathways (Katanaev et al. 2005).
281