Diacylglycerol Signaling

366 J. Kim and M.G. Kazanietz
Signaling analysis revealed that PMA induces the activation of mitogenactivated
protein kinase (MAPK) cascades in prostate cancer cells. Phorbol esters
strongly activate p38 MAPK and JNK in LNCaP cells, and inhibition of these pathways
impairs PMA-induced apoptosis. On the other hand, inhibition of the ERK
cascade with a MEK inhibitor enhances PMA-induced apoptosis. This suggests
opposite roles for MAPK cascades in apoptosis of androgen-dependent prostate
cancer cells (Tanaka et al. 2003).
PTEN loss occurs in a large percentage of human prostate tumors, and Akt is
a dominant survival pathway in prostate cancer. The PI3K-Akt pathway is hyperactivated
in LNCaP cells due to loss of PTEN function (Majumder and Sellers
2005). Remarkably, PKC activators promote a rapid dephosphorylation of Akt.
The reduction in Akt activity does not involve an inhibition of upstream inputs,
as phosphorylation of the PI3K effector PDK1 is not altered by PMA treatment
(Tanaka et al. 2003). On the other hand, okadaic acid prevents Akt dephosphorylation,
suggesting that PKC activation leads to the dephosphorylation of Akt by
activation of the phosphatase PP2A (Li et al. 2003). Interestingly, Akt dephosphorylation
seems to be dependent on PKCa rather than PKCd, suggesting the
contribution of at least two PKC isozymes to apoptotic cell death by phorbol
esters (Tanaka et al. 2003).
PKC activation is known to promote the release of factors from cells and to trigger
autocrine and paracrine loops (Gonzalez-Guerrico and Kazanietz 2005). It is
interesting that phorbol ester-induced apoptosis is mediated by the autocrine release
of death factors. Indeed, conditioned medium from PMA-treated LNCaP, DU145,
or PC-3 cells has apoptotic activity when added to LNCaP cells (Gonzalez-Guerrico
and Kazanietz 2005; Xiao et al. 2009). PKCd RNAi depletion in LNCaP cells
treated by PMA impairs the secretion of death factors, and therefore conditioned
medium from these cells lost its apoptotic activity. It became clear that the autocrine
effect is mediated primarily by tumor-necrosis factor a (TNFa) and TRAIL
(TNF-related apoptosis-inducing ligand) but not by Fas. This has been demonstrated
through numerous approaches (Gonzalez-Guerrico and Kazanietz 2005).
For example, the apoptotic effect of PMA in LNCaP cells was lost in the presence
of TAPI-2, an inhibitor of TNFa converting enzyme (TACE), the enzyme involved
in TNFa shedding, or after TACE RNAi depletion. TNFa or TRAIL neutralizing
antibodies, as well as blockade or RNAi depletion of death receptors, inhibited the
apoptotic effect of PMA. PMA caused a marked TNFa mRNA induction as well as
TNFa release from prostate cancer cells, effects that were prevented by PKCd or
TACE RNAi (Gonzalez-Guerrico and Kazanietz 2005). Signaling analysis revealed
that conditioned medium from PMA-treated LNCaP cells promotes the activation
of p38, JNK, and caspase-8, suggesting that PKCd-mediated apoptosis involves the
activation of the extrinsic apoptotic cascade (Xiao et al. 2009). Moreover, RNAi
depletion of caspase-8 or a dominant-negative mutant of the death receptor adaptor
Fas-associated death domain (FADD), impaired of the apoptotic effect of PMA in
LNCaP cells (Gonzalez-Guerrico and Kazanietz 2005; Gavrielides et al. 2006).
As expected from the opposite effects of PKCd and PKCe in many cell types,
PKCe is a prosurvival kinase in prostate cancer cells. Overexpression of PKCe
conferred resistance to phorbol ester-induced apoptosis, an effect associated with