Diacylglycerol Signaling

7 Regulation and Function of Protein Kinase D Signaling
in PANC-1 cells increases DNA synthesis, cell proliferation and anchorage-independent
proliferation (K. Kisvalvi and E. Rozengurt, unpublished results).
A number reports using transgenic or mutant mice have raised the possibility
that the PKC/PKD axis plays an important role in the regulation of intestinal epithelial
cell proliferation in vivo. For example, mice with transgenic overexpression
PKCbII in the intestinal epithelium exhibit hyperproliferation, increased Wnt signaling
and an increased susceptibility to azoxymethane-induced preneoplastic
lesions in the colon (Murray et al. 1999; Yu et al. 2003). Since PKCb overexpression
stimulates PKD catalytic activation (Zugaza et al. 1996), it is conceivable that
PKD mediates, at least in part, the growth-promoting effects of this PKC isoform.
Furthermore, in the APC min mouse model of colon cancer, PKD (but not any of the
PKCs) exhibited dysplasia-specific nuclear localization, suggesting that this
enzyme is activated during adenomatous transformation (Klein et al. 2000). Indeed,
our studies demonstrated that active PKD accumulates in the nucleus in response
to GPCR stimulation (see above). To clarify the role of PKD in intestinal epithelial
cell proliferation in vivo, we generated transgenic mice that express elevated PKD
protein in the distal small intestinal and proximal colonic epithelium. We found a
significant increase in DNA synthesizing cells in the crypts of the PKD transgenic
mice as compared with non-transgenic littermates (our unpublished results). In
view of recent results showing that PKD can phosphorylate Par1 (Watkins et al.
2008), it is plausible that PKD plays a role in modulating the polarity and proliferation
of epithelial cells in the gut.
Invasive breast cancer cells have the ability to extend membrane protrusions,
invadopodia, into the extracellular matrix. These structures are associated with
sites of active matrix degradation. The amount of matrix degradation associated
with the activity of these membrane protrusions has been shown to directly correlate
with invasive potential. PKD has been implicated in the regulation of
these structures (Bowden et al. 1999). Indeed, PKD, cortactin and paxillin were
co-immunoprecipitated as a complex from invadopodia-enriched membranes.
This complex of proteins was not detected in lysates from non-invasive cells that
do not form invadopodia (Bowden et al. 1999). These data suggested that the
formation of this PKD-containing complex correlates with cellular invasiveness.
Increased cellular adhesion to extracellular matrix proteins, such as collagen
type IV, often increases metastatic potential. Stimulation of adhesion of human
metastatic breast carcinoma cells to collagen type IV in response to arachidonic
acid is associated with the activation and translocation of PKD from the cytoplasm
to the membrane (Kennett et al. 2004). Additional results indicate that
PKD is necessary for the increased adhesion promoted by arachidonic acid.
These studies suggest that PKD is an important element in breast tumor cell
adhesion and metastasis. However, a recent study using breast cancer tissues as
well as cell lines reached a different conclusion (Eiseler et al. 2009). Specifically,
loss of PKD expression appears to increase the malignant potential of breast
cancer cells. This may be due to the function of PKD as a negative regulator of
matrix-metalloproteinases expression (Eiseler et al. 2009). These results suggest
that decreased PKD expression may be a marker for invasive breast cancer.
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