The lipid second messenger diacylglycerol (DAG) is transiently generated upon activation of tyrosine-kinase and G-protein-coupled receptors, and promotes the membrane translocation and activation of protein kinase C (PKC) isozymes. Classical and novel PKCs (cPKCs and nPKCs) sense DAG and DAG-mimetics such as the phorbol esters with high affinity through their C1 domains. PKC isozymes have been widely implicated in the positive and negative control of proliferation and survival. PKCd, a member of the nPKC family, inhibits cell cycle progression in various cell types by up-regulating p21 mRNA and protein, leading to Rb dephosphorylation and inhibition of cyclin A promoter activity. Recent studies in prostate cancer cells have determined that PKCd activation by phorbol esters leads to apoptosis via the autocrine release of death factors (TNFa, TRAIL) and activation of the extrinsic apoptotic cascade. Blockade of TACE, the enzyme responsible for TNFa shedding, or RNAi for TNFa receptors, impairs the apoptotic response of phorbol esters in prostate cancer cells. Likewise, RNAi depletion or inhibition of effectors of death receptors, including caspase-8, FADD, p38 MAPK, and JNK, significantly reduced the response of apoptogenic factors released upon PKCd activation. The requirement of autocrine mechanisms in the apoptotic effect of phorbol esters represents a novel paradigm in PKC signaling.

Another major advance in the DAG field has been the discovery of novel targets for DAG lacking kinase activity, such as the chimaerins (a1, a2, b1, and b2), a family of Rac GAPs. The C1 domain in chimaerins binds DAG and phorbol esters with similar affinity as PKC C1 domains and drives their translocation to membranes. This relocalization represents a mechanism that facilitates the association of chimaerins to their target (Rac), as revealed by FRET, and self-limits receptor-mediated activation of this small G-protein. b2-chimaerin indeed restricts EGF-mediated reorganization of the actin cytoskeleton, migration, proliferation and cyclin D1 expression. The crystal structure of b2-chimaerin revealed that major conformational changes are required to expose the Rac-GAP domain. As the C1 domain is not exposed, it is predictable that other events, such as post-translational modifications, would occur in response to receptor activation in order to facilitate DAG binding. Thus, b2-chimaerin represents the first example of a DAG/phorbol ester receptor unrelated to PKC that links DAG signaling to the inactivation of the Rac pathway. The discovery of intracellular DAG receptors without a kinase domain suggests a remarkable divergence in the signaling pathways that can be controlled by this lipid second messenger. The rationale development of drugs that can discriminate among the various DAG receptors is a key to dissect the role of PKC-dependent and PKC-independent pathways in signaling and cellular function.

 

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