Discussion
NF-κB transcriptional activity is important in regulating intracellular signaling, stress response, proliferation, survival, differentiation and inflammation [8]. To regulate gene transcription, NF-κB p50/p65 heterodimers translocate to the nucleus from the cytoplasm, however, their translocation is not sufficient to activate gene transcription. Post-translational modification of NF-κB heterodimers such as phosphorylation or acetylation also contributes to its transcriptional activity [41], [42], [43]. Because NF-κB constitutively translocates to the nucleus in monocytic lineage [13] and activates gene transcription and survival in murine macrophages [11], we sought to determine how M-CSF affected NF-κB transcriptional activity in human MDMs and whether this activation regulated their survival. The present study demonstrated that M-CSF stimulated PKCα kinase upstream of NF-κB transcriptional activity in primary human MDMs and RAW 264.7 cells. We showed inhibition of conventional PKCs by PKC inhibitors, kinase deficient PKCα or PKCα siRNA blocked M-CSF-induced cell survival and NF-κB-regulated gene expression. This block correlated with a reduction in M-CSF-induced phosphorylation of NF-κB p65 at Ser276. Consistent with these findings the dominant negative PKCα constructs also inhibited NF-κB p65 phosphorylation at Ser276 but not at Ser536 resulting in reduced NF-κB transcriptional activation and M-CSF-induced MDM survival. A simplified proposed cartoon model in Figure 9 demonstrates that M-CSF induced monocytes survival is regulated by activating NF-κB p65 phosphorylation at Ser276 via PKC. Finally, in a NF-κB p65−/− fibroblast cell line, we confirmed that the Ser276 residue of NF-κB p65 is important and essential for PKC modulation of NF-κB activity. More compelling is that we observed a similar regulation between two different cell lineages in our study.
10.1371/journal.pone.0028081.g009 Figure 9  Proposed model for M-CSF-induced monocyte survival via PKC regulation by activating NF-κB p65 phosphorylation at Ser276.   Post-translational modification of NF-κB p65 regulates its activating or repressing effects on gene expression. Among the seven reported putative NF-κB p65 phosphorylation sites, five increase nuclear translocation, DNA binding and NF-κB transcriptional activity [17]. Similar to our finding, Zhong et al. found that in response to LPS, NF-κB p65 is phosphorylated at the highly conserved Ser276 residue by the catalytic subunit of PKA [16]. In addition to PKA, Ser276 can be phosphorylated by MSK1 in the nucleus upon TNFα treatment [18]. Phosphorylation of NF-κB p65 at Ser536 occurs in response to many inflammatory stimuli as well as kinases, IKKα, IKKβ and RSK1 [17], [18], [19], [20], [21]. Notably, in addition to phosphorylating Ser276 of NF-κB, our study revealed that M-CSF also modulated the phosphorylation of the NF-κB p65 subunit at Ser536, however, PKC inhibitors or kinase deficient PKCα constructs did not affect this phosphorylation event. Moreover, mutant constructs containing a point mutation at Ser536 of NF-κB p65 did not reduce NF-κB activity in cells lacking endogenous NF-κB p65, while the NF-κB p65 276S/A construct did reduce NF-κB activity in these cells. Therefore, specific post-translational modification of NF-κB p65 is likely to regulate transcriptional activation in response to specific stimuli [15], [43].
It is notable that M-CSF activated NF-κB p65 Ser276 phosphorylation and transcriptional activation in a PKCα-dependent manner. PKCα is a member of the conventional PKC family of protein kinases that are critical for cell growth, differentiation and cell death. PKCα primarily modulates anti-apoptotic and proliferation signals following cytokine treatment in various cell types [42]. Expression of PKCα attenuates apoptosis in many different cell types, while PKCα inhibition generally potentiates apoptosis [42]. In our studies using conventional PKC inhibitors and dominant negative PKCα constructs, PKCα was critical in M-CSF-induced macrophage survival by inducing Ser276 phosphorylation of NF-κB p65 in both primary human MDMs and Raw 264.7 cells. We also demonstrated PKC inhibition downregulated NF-κB-induction of the anti-apoptotic gene BCL-xL. Consistent with our finding, others reported NF-κB regulated the gene expression of many anti-apoptotic proteins including other BCL-2 family members and inhibitor of apoptosis (IAP) proteins which are regulators of apoptosis and function upstream of caspases [44]. It has been shown that phosphorylation of p65 at Ser276 prevents its degradation by ubiquitin-mediated proteolysis and promotes cell survival in HeLa cells [24]. In addition to BCL-xL expression as examined in this study, we are evaluating the possibility that PKCα-regulated NF-κB activity upregulates additional anti-apoptotic genes in an M-CSF-dependent fashion to modulate cell survival.
The use of inhibitors and siRNA in our study has not ruled out that other conventional PKCs might also be important in M-CSF-induced NF-κB activation and macrophage survival. In contrast to the actions of PKCα activation of other conventional PKC isoforms, like PKCβI/βII, appear necessary for macrophage apoptosis. The PKCβI/βII isoforms are expressed in apoptotic U937 myelomonocytic cells [45]. The human myeloid cell line HL-525 which is devoid of endogenous PKCβII is resistant to TNFα-induced apoptosis [46]. Re-expression of PKCβII in HL-525 cells restores their susceptibility to TNFα-induced apoptosis implying that PKCβII is pro-apoptotic and may be required to induce macrophage apoptosis.
Recent studies demonstrated that NF-κB transcriptional activity can be regulated by phosphorylation of the NF-κB p65 subunit in the absence of IκBα degradation [47]. By this pathway, phosphorylation of NF-κB regulates its interaction with other components of the basal transcriptional machinery without affecting its capability to bind DNA [21]. Since NF-κB can bind the promoters of numerous gene targets, post-translational modification of NF-κB p65 may affect its interaction with other proteins, refining the expression of gene targets. We did not find that PKC was involved in IκBα degradation or NF-κB p65 nuclear translocation induced by M-CSF, but that M-CSF-induced phosphorylation of NF-κB p65 at Ser276 was dramatically reduced by the conventional PKC inhibitor and kinase deficient PKCα and by siRNA towards PKCα. Current studies are underway to delineate the protein complexes formed by activated NF-κB and other transcriptional co-activators in response to M-CSF and PKC activation. Recently studies indicated that the non-canonical pathway was activated during human monocyte-macrophage differentiation [48]. During this process, expression of IKKα among other proteins, is elevated leading to increased p52 NF-κB (relB) expression through partial proteolytic degradation of the p100 NF-κB protein. Thus, it is possible that M-CSF regulates monocyte survival through both the canonical and non-canonical NF-κB pathways, which will be a focus in future research.
We previously showed that M-CSF reduced caspase-3 and -9 activity and prevented monocyte apoptosis [3]. This study reveals that treatment of monocytes with conventional PKC inhibitors, or overexpression of kinase-deficient PKCα blocked M-CSF-induced cell survival. Our data revealed that conventional PKCs are upstream of NF-κB activation in response to M-CSF and mediate post-translational activation of NF-κB p65 by phosphorylating Ser276 to regulate gene transcription and cell survival. Thus, our observation may provide insight in potential therapeutic targets for inflammatory diseases.