The gene encompasses 36 kb of DNA and contains 18 exons.

2.2 kb mRNA.

The protein kinase C (PKC) is a family of serine/threonine kinases that plays key roles in cell proliferation and apoptosis (Berridge 1984; Nishizuka 1992). The mammalian PKC family consists of 11 different isoforms named PKCα, PKCβI, PKCβII, PKCγ, PKCδ, PKCε, PKCζ, PKCη, PKCθ, PKCι and PKCλ. Based on their requirements the PKC family is divided in 3 subfamilies: the conventional (cPKC) are regulated by diacylglycerol (DAG) and Ca²⁺. They are composed by the α, βI, βII, γ isoforms; the novel PKCs (nPKC) composed by the δ, ε, η, θ isoforms are regulated only by DAG and the atypical PKC (aPKC), composed by the ζ and ι/λ isoforms are activated by phorbol esther and they are independent of DAG and Ca²⁺(Mellor and Parker 1998; Gschwendt 1999; Basu 2003).

- Structure
PKCδ, a member of the novel PKC group, is a 78 kDa protein composed of a regulatory and a catalytic domain. The N-terminal regulatory domain contains an auto-inhibitory region named the pseudosubstrate and four conserved domains, the C1 and C2 in the regulatory region and the C3 and C4 motifs in the catalytic region. PKCδ contains also five variable regions (V). The variable region 3 (V3), called the hinge region, separates the catalytic and regulatory domains (Cho 2001; Basu 2003) (Figure 1).
The C1 motif contains DAG/PMA (Phorbol 12-myristate 13-acetate) binding sequences that allow the interaction to a hydrophilic cleft located at a hydrophobic surface of this domain. The binding to the hydrophobic cleft forms a contiguous hydrophobic surface that promotes PKC binding to membranes. PKCδ has a C2-like region located at the N terminal domain. It has the same core residues of the C2 domain, but it lacks the essential calcium coordinating acidic residues that allows classical PKCs to bind Ca²⁺ (Pappa, Murray-Rust et al. 1998). The C3 and C4 are required for ATP/substrate binding and catalytic activity of the enzyme. The pseudosubstrate domain, located between the C1 and C2 motifs, maintains PKCδ in an inactive conformation by blocking the access to the substrate binding pocket. The proteolytic activation of PKCδ generates a 40 kDa fragment that can translocate to the mitochondria and/or nucleus (Hurley and Misra 2000; Cho 2001; Steinberg 2004) (Table 1).
Table 1. Important domains of PKCδ.

Domain	Amino acid sequence	Function
PKCδ translocation inhibitor	S8FNSYELGSL17;	Prevents translocation to cell membrane (Inagaki, Chen et al. 2003)
PKCδ translocation activator	M74RAAEDPM81;	Binding of cell membrane (Jaken and Parker 2000)
Pseudosubstrate domain	P140TMNRRGAIKQAKIHY155IKN158	Prevents PKCδ activation by blocking the substrate binding pocket (Dempsey, Newton et al. 2000)
Caspase cleavage site	Y311QGFEKKTAVSGNDIPDNNGTY332GKI	Sequence cleaved by caspase-3 (Blake, Garcia-Paramio et al. 1999)
ATP binding sequence	G361KGSFGKVLLAELKGK376	Binding site for ATP. Promotes catalytic activity (Hurley and Misra 2000)
Activation loop	RAST505FCGTPDY512IAPEILQGLKY523	Contains important phosphorylation sites necessary for the catalytic activation (Thr505, Thr512, Thr523) (Rybin, Sabri et al. 2003)
Nuclear localization signal (NLS)	K611RKVEPPFKPKVKSPSDY628S	Targets PKCδ to the nucleus (DeVries, Neville et al. 2002)
Turn motif	S643	Auto-phosphorylation site important for PKCδ maturation (Rybin, Sabri et al. 2003)
Hydrophobic motif	S662	Facilitate PKCδ down-regulation by releasing it from the cell membrane (Feng, Becker et al. 2000; Rybin, Sabri et al. 2003)

- Isotypes
So far eight PKCδ isotypes, generated by alternative splicing, have been identified in different species. PKCδ isotypes show different characteristics. PKCδI is the only isotype expressed in all species and is a target of caspase-3 (Sakurai, Onishi et al. 2001). In contrast, the PKCδII isotype is present in mouse and is insensitive to caspase-3 cleavage. PKCδIII is expressed in rats and shows weak translocation to the cell membrane upon stimulation by phorbol esther (Ueyama, Ren et al. 2000). PKCδIV, PKCδV, PKCδVI, and PKCδVII are expressed exclusively in mouse testis and they lack the V1 and C2-like domains. PKCδIV and PKCδV are expressed in spermatids, during mices sperm maturation, while PKCδVI and PKCδVII are expressed in spermatigonia and spermatocytes (Kawaguchi, Niino et al. 2006). PKCδVIII is expressed in humans upon retinoic acid treatment. This isotype is resistant to caspase-3 cleavage and its expression rescues NT2 cells from etoposide-induced apoptosis. This suggest an antiapoptotic role for the PKCδVIII isotype in NT2 cells (Jiang, Apostolatos et al. 2008).

- Phosphorylation sites
There are several phosphorylable sites that depending on the stimuli and/or cell type have a different contribution on the activation of PKCδ. Three phosphorylation sites are conserved among all PKC isotypes: Thr⁵⁰⁵ (activation loop), Ser⁶⁴³ (turn motif), and Ser⁶⁶² (hydrophobic motif) (Steinberg 2004). In PKCδ, these three sites were shown to be substantially phosphorylated in vivo (Konishi, Yamauchi et al. 2001). However, unlike the other PKCs, mutations of Thr⁵⁰⁵ to Ala in PKCδ doesnt affect its kinase activity, but seems to regulate PKCδ stability (Stempka, Girod et al. 1997). The phosphorylation of Ser⁶⁴³ and Ser⁶⁶² seems to be important for PKCδs catalytic maturation. Ser⁶⁴³ is auto-phosphorylated, however the phosphorylation of Ser⁶⁶² has been shown to be regulated by PKCζ and a pathway involving the mammalian target of rapamycin (mTOR) (Ziegler, Parekh et al. 1999). PKCδ has eight Tyr residues (located at position 52, 155, 187, 311, 332, 512, 523, and 565), that can be phosphorylated by tyrosine kinases. These phosphorylation sites are conserved among species, but only Tyr⁵¹² is conserved in other members of the PKC family. Phosphorylation at Tyr¹⁵⁵ has been involved in the inhibitory effect of PKCδ on cell proliferation whereas Tyr⁶⁴ and Thr¹⁸⁷ are the mayor sites for PMA-dependent phosphorylation in etoposide-induced apoptosis (Szallasi Z et al. 1995, Sun X et al. 2000). Phosphorylation at Tyr³¹¹, Tyr³³² and Tyr⁵¹² at the hinge and activation regions induce PKCδ activation and differential subcellular distribution onto the membranes (Konishi, Tanaka et al. 1997; Blake, Garcia-Paramio et al. 1999) (Table 1, Figure 3). In contrast, the phosphorylation of Tyr¹⁵⁵ and Tyr¹⁸⁷ are important for the anti-apoptotic effect of PKCδ. Mutation of this sites to phosphor-mimicking mutants results in an increase in cell proliferation in response to PMA (Kronfeld, Kazimirsky et al. 2000). The association of PKCδ and different tyrosine kinases results in different phosphorylation patterns and possibly differential activation of downstream targets in response to specific stimulus. Tyrosine kinases like Src (Sarcoma), Fyn, Lyn (v-yes-1 Yamaguchi sarcoma viral related oncogene homolog), PDK1 (Phosphoinoisitide dependent kinase 1), PYK2 (Protein tyrosine kinase 2) have been shown to phosphorylate PKCδ (Gschwendt, Kielbassa et al. 1994; Li, Mischak et al. 1994; Szallasi, Denning et al. 1995; Song, Swann et al. 1998; Yuan, Utsugisawa et al. 1998; Balendran, Hare et al. 2000; Sun, Wu et al. 2000; Wrenn 2001).
See figure 2.
- Activation
PKC family members exist in an immature inactive conformation that requires post-translational modifications to achieve catalytic maturity before activation by DAG/PMA. cPKC, nPKC and aPKC are subject to phosphorylation of the activation loop and this event acts as a priming step that allows the catalytic maturation of PKC (Dutil, Toker et al. 1998). The catalytic maturation of PKCδ involves the auto-phosphorylation of Ser⁶⁴³ and the phosphorylation of Thr⁵⁰⁵ and Ser⁶⁶². It has been proposed that the phosphorylation of Ser⁶⁴³ is the first step for PKCδ maturation, since Ser⁶⁶² is subject of dephosphorylation in the absence of Thr⁵⁰⁵ phosphorylation (Parekh, Ziegler et al. 1999). Phosphorylation of Ser⁶⁴³ can be mediated by PKCζ, in 293 cells or by the mammalian target of rapamicin (mTOR) (Le Good, Ziegler et al. 1998; Parekh, Ziegler et al. 1999; Ziegler, Parekh et al. 1999; Wood, Kelly et al. 2007) (Figure 2). PKCδ also interacts with the mTOR homolog in Saccharomyces cerevisiae, FRAP and its necessary for the FRAP-dependent phosphorylation of 4e-BP1 (4e-binding protein 1) (Kumar, Pandey et al. 2000).
PDK1 phosphorylates PKCδ in Thr⁵⁰⁵ stabilizing PKCδs structure promoting the alignment of these residues with the catalytic pocket (PKCδ immature inactive). In contrast with other members of the PKC family, the phosphorylation of this residue seems not to be necessary for the catalytic activity of PKCδ, since PKCδ exhibits full enzymatic activity in the absence of the Thr⁵⁰⁵ phosphorylation, in vitro (Stempka, Schnolzer et al. 1999). It is possible that the negative charge provided by the phosphate required for the catalytic activation of PKCδ is provided by the Glu⁵⁰⁰. Glu⁵⁰⁰, located in the activation loop, seems to play an important role on PKCδ activity since the mutation of this aminoacid to Val exhibits a reduction of 70% in auto-phosphorylation and substrate phosphorylation (Stempka, Schnolzer et al. 1999). After the phosphorylation of Thr⁵⁰⁵, autophosphorylion of Ser⁶⁶² occurs and this event allows the catalytic maturation of PKCδ. However, even after the auto-phosphorylation of this residue, PKCδ remains in an inactive conformation due to the interaction of the pseudosubstrate domain with the substrate-binding region of the catalytic site.
Upon stimulation by DAG or PMA, PKCδ is also auto-phosphorylated at the conserved residues: S²⁹⁹, S³⁰² and S³⁰⁴, located in the V3 region. The phosphorylated PKCδ translocates to the plasma and nuclear membrane (Durgan, Michael et al. 2007). Phosphorylation of Ser²⁹⁹ has been proposed to be a marker for PKCδ catalytic activation, since it only occurs after its translocation to the plasma and nuclear membrane (Durgan, Michael et al. 2007). PKCδ can bind membranes through its C1/C2 domains. The binding induces a conformational change that allows the release of the pseudosubstrate domain from the substrate-binding site. This event allows PKCδ phosphorylation of membrane substrates (Steinberg 2004). Tyrosine kinases like: Src, Lyn, have been shown to phosphorylate residues on the T-loop region of the C4 domain in response to different apoptotic inducers and non-apoptotic stimuli (Denning, Dlugosz et al. 1996; Shanmugam, Krett et al. 1998; Kikkawa, Matsuzaki et al. 2002). The phosphorylation of PKCδ in specific tyrosine residues regulates PKCδ localization and activity depending on the cell type and stimuli (Song, Swann et al. 1998; Yuan, Utsugisawa et al. 1998; Konishi, Yamauchi et al. 2001; Blass, Kronfeld et al. 2002). PKCδ is proteolytically cleaved by caspase-3 at the hinge region, separating the regulatory domain from the catalytic domain (Figure 2). The cleavage creates a 40 kDa catalytically active fragment (CF). The proteolytic activation of PKCδ can be modulated by tyrosine phosphorylation. Overexpression of Y52F, Y155F and Y565F mutants in glial cells enhances the apoptotic response to etoposide induced apoptosis; however the overexpression of Y64F and Y187F reduces caspase-3-dependent PKCδ activation and reduced etoposide-induced apoptosis (Blass, Kronfeld et al. 2002).
Notably, unlike other PKC-family members, PKCδ can act as a lipid-independent enzyme and can be entirely activated without translocation to the cell membrane (Steinberg 2004). PKCδ can localize to the mitochondria after activation by PMA in U937 cells causing the release of cytochrome C and activation of caspase-3 (Majumder, Pandey et al. 2000). The catalytic fragment of PKCδ translocates to the nucleus utilizing its NLS (nuclear localization signal) and triggers apoptosis (Emoto, Manome et al. 1995; Ghayur, Hugunin et al. 1996). Consistent with this, the expression of PKCδ caspase-3-uncleavable-mutant reduces its nuclear accumulation. Furthermore, mutations within the NLS of CF reduce PKCδ nuclear accumulation and apoptosis in salivary acinar cells (Figure 2) (DeVries, Neville et al. 2002). However, cell death induction of LNCap cells treated with phorbol esthers, CHO cells treated with H2O2 and HaCaT cells UV irradiated doesnt induce caspase-3-deppendent cleavage of PKCδ (Fujii, Garcia-Bermejo et al. 2000; Fukunaga, Oka et al. 2001; Konishi, Yamauchi et al. 2001).
After activation by 12-O-tetradecanoylphorbol-13-acetate (TPA), PKCδ is ubiquitinated and targeted to the proteasome for degradation. Treatment with the proteasome inhibitors, MG101 and MG132, prevents TPA-induced depletion of PKCδ in rat fibroblasts (Lu, Liu et al. 1998).
See figure 3
- Substrates
Several proteins with diverse biological activity have been shown to be substrates of PKCδ (Yoshida 2007). PKCδ modulates translational elongation factors, such as eEF-1α (eukaryotic elongation factor 1-α) which upon treatment with TPA is activated by phosphorylation at Thr⁴³¹ (Kielbassa, Muller et al. 1995). PKCδ interacts and regulates the activity of mTOR (mammalian target of rapamycin), an important regulator of the 4E-BPs, required to control translation modulating the translation initiation factor eIF4E (Kumar, Pandey et al. 2000). In addition, PKCδ regulates the function of several transcription factors such as Sp1, NF-κB, p300, Stat1, Stat3, among others (Novotny-Diermayr, Zhang et al. 2002; Yuan, Soh et al. 2002; Liu, Yang et al. 2006; Gorelik, Fang et al. 2007; Kim, Lim et al. 2007; Kwon, Yao et al. 2007). The role of each transcription factor will be further explained in the transcriptional regulation section.
During mast cell activation, PKCδ phosphorylates the IgE (immunoglobulin E) receptor causing its endocytosis (Germano, Gomez et al. 1994). In addition, PKCδ maintains homeostasis by phosphorylating the calcium efflux regulator PMCA (plasma membrane calcium ATPase) regulating Ca²⁺ levels in the skin (Garcia and Strehler 1999; Ahn, Jeong et al. 2007). PKCδ phosphorylates GIRK channels to normalize K⁺ levels after membrane depolarization 2 (Breitwieser 2005; Brown, Thomas et al. 2005; Xie, John et al. 2007). Moreover, during hypertrophy of vascular smooth muscle cells, PKCδ mediates the transactivation of the EGF receptor and activates ERK1, ERK2, PI3K and ATF-1 signalling pathways, leading to the up-regulation of NOX1 (Fan, Katsuyama et al. 2005).
PKCδ has an important role as a modulator of cell death in different cell types and the interaction of this protein kinase with multiple of its substrates induces apoptosis. Exposure of keratinocytes to UV light induces the PKCδ-dependent phosphorylation of Mcl-1 (the myeloid cell leukemia protein 1) causing the release of cytochrome c and subsequently activation of apoptosis (DCosta and Denning 2005; Sitailo, Tibudan et al. 2006). Phosphorylation of the checkpoint protein Rad9 by PKCδ also positively regulates apoptosis by promoting the binding of Rad9 to Bcl-2 (Yoshida, Wang et al. 2003). Most recently caspase-3, a key mediator of apoptosis, was shown to associate and be phosphorylated by PKCδ in human primary monocytes. The phosphorylation of caspase-3 promotes its apoptotic activity in vivo and in vitro (Voss, Kim et al. 2005). Moreover, silencing of PKCδ reduced etoposide-induced apoptosis and consistently overexpression of a PKCδ-cat increased caspase-3-dependent apoptosis (Voss, Kim et al. 2005). PKCδ phosphorylates also the tumor suppressor p53 at Ser⁴⁶, inducing cell death (Yoshida, Liu et al. 2006).
Oxidative stress induces the formation of a PKCδ-p52Shc-p66Shc complex allowing PKCδ phosphorylation of p52 at Ser²⁹, p66 at Ser¹³⁸. The phosphorylation of p66Shc at Ser¹³⁸ is crucial for H2O2 induced ERK activation (Hu, Kang et al. 2007). Furthermore, PKCδ interacts, phosphorylates Abl tyrosine kinase, in response to H2O2 treatment. As a consequence of this interaction, PKCδ is also phosphorylated by Abl showing an active interaction between the two kinases in response to oxidative stress (Sun, Wu et al. 2000). In addition, the release of nitric oxide in the cells causes the activation of PKCδ by tyrosine nitration, leading to the phosphorylation p53 at Ser¹⁵, which in turn increases the stability of p53 by destabilizing MDM2 (murine double minute), leading to cell death (Lee, Kim et al. 2006). Recently, it was shown that the interaction of the heat shock protein 25 (Hsp25, the mouse homolog of the human Hsp27) through the direct binding with the PKCδ V5 catalytic region inhibits apoptosis (Lee, Lee et al. 2005). Taken together, the diversity of PKCδ substrates demonstrates the central role of this kinase in diverse biological processes.
See figure 4
Sequence (Fasta Format): NP 997704
MAPFLRIAFNSYELGSLQAEDEANQPFCAVKMKEALSTERGKTLVQKKPTMYPEWKSTFDAHIYEGRVIQIVLMRAAEEPVS
EVTVGVSVLAERCKKNNGKAEFWLDLQPQAKVLMSVQYFLEDVDCKQSMRSEDEAKFPTMNRRGAIKQAKIHYIKNHEFI
ATFFGQPTFCSVCKDFVWGLNKQGYKCRQCNAAIHKKCIDKIIGRCTGTAANSRDTIFQKERFNIDMPHRFKVHNYMSPT
FCDHCGSLLWGLVKQGLKCEDCGMNVHHKCREKVANLCGINQKLLAEALNQVTQRASRRSDSASSEPVGIYQGFEKKTG
VAGEDMQDNSGTYGKIWEGSSKCNINNFIFHKVLGKGSFGKVLL
PKCδ-Phosphorylable Consensus Sequence
S/TXXR/K (X represents any amino acid) (Yoshida, Wang et al. 2003).

The localization of PKCδ seems to play an important role in determining its activity. PMA-induced activation of PKCδ causes its translocation from the cytoplasm to the plasma membrane. This event is followed by a slower nuclear membrane translocation in hamster ovary cells (Wang, Bhattacharyya et al. 1999). Induction of apoptosis by TNF and Fas mediates the release of ceramide, causing PKCδ translocation from the plasma membrane to the cytosol in leukemia cells (Sawai, Okazaki et al. 1997). The released ceramide accumulates at the Golgi causing the translocation of PKCδ to this cellular compartment. Ceramide induces the phosphorylation of PKCδ at Tyr³¹¹ and Tyr³³² via Src kinase (Sarcoma), inducing its activation. Treatment with TPA causes PKCδ translocation from the Golgi to the plasma membrane suggesting that PKCδ moves continuously from the Golgi complex to the cytoplasm (Kajimoto, Ohmori et al. 2001). Moreover, in glioma cells, PKCδ was found to induce cell death when targeted to the mitochondria, cytoplasm, and nucleus. However, in the case of glioma cells, localization of PKCδ to the endoplasmic reticulum was suggested to protect glioma cells from etoposide and TNF-ligand induced cell death (Gomel, Xiang et al. 2007). It has been suggested recently that nuclear localization of PKCδ in response to Fas ligand, etoposide, ionizing radiation and growth factor deprivation is required for its ability to induce apoptosis (Yuan, Utsugisawa et al. 1998; Scheel-Toellner, Pilling et al. 1999; Blass, Kronfeld et al. 2002). Transfection of the PKCδ catalytic fragment (CF) in parotid salivary acinar cells induces nuclear localization of PKCδ and apoptosis. Mutations in the NLS motif of CF were show to inhibit cell death, demonstrating the importance of PKCδs CF nuclear localization in apoptosis. However, the translocation of full-length PKCδ to the nucleus has also been suggested (DeVries, Neville et al. 2002).

- Apoptosis
PKCδ has been emerging as an important regulator of apoptosis. This apoptotic function is achieved by its interaction and phosphorylation of several proteins that are involved in the cell death (Basu 2003). For example, PKCδ associates and phosphorylates caspase-3 promoting the apoptotic activity of the cysteine caspase during etoposide-induced apoptosis and also in spontaneous apoptosis of monocytes (Voss, Kim et al. 2005). Apoptosis induced by diverse agents including: TNF-α or Fas ligation, etoposide, mitomycin, cytosine arabinoside, etoposide, UV, and ionizing radiation induces the cleavage of PKCδ, freeing the catalytically from the regulatory domain (Emoto, Manome et al. 1995; Datta, Banach et al. 1996; Ghayur, Hugunin et al. 1996; Park, Park et al. 2000; Fukunaga, Oka et al. 2001; Blass, Kronfeld et al. 2002). The cleavage of PKCδ occurs at a specific amino-acid site (see Table 1) and is mediated by caspase-3. It was suggested that the PKCδ catalytic fragment may serve to amplify downstream events in the apoptotic pathway, for example by allowing the relocalization of the catalytic domain into the nucleus where is able then to phosphorylate an additional repertoire of substrates (Brodie and Blumberg 2003). Consistent with this hypothesis, overexpression of the catalytic fragment in the absence of an apoptotic stimulus was sufficient to induce apoptosis in a variety of cell types (Leverrier, Vallentin et al. 2002). Hence, it has been suggested that PKCδ operates in a feedback loop to regulate apoptosis. PKCδ participates in the early events of apoptosis by modulating the activation of caspase-3 and later, downstream of the caspase activation, by phosphorylating proteins of diverse biological function that in turn regulate the execution of cell death (Basu 2003).
Consistent with a central role of PKCδ in apoptosis, PKCδ-/- mice were found to be protected against γ-irradiation induced apoptosis (Humphries, Limesand et al. 2006). Moreover, glioma cells treated with etoposide in the presence of rottlerin, a PKCδ inhibitor, showed lack of PKCδ activation and absence of the caspase-3-dependent PKCδ cleaved catalytic domain, suggesting an essential team-work of PKCδ and capase-3 in the induction of apoptosis (Blass, Kronfeld et al. 2002). The apoptotic activity of PKCδ is controlled, at least in part, by a member of the heat shock family. The heat shock protein 25 (Hsp25, the mouse homolog of the human Hsp27) was shown to inhibit apoptosis through the direct binding PKCδs V5 catalytic region. Consistent with this mechanisms, Hsp25 inhibited PKCδ kinase activity and membrane translocation reducing cell death in mouse fibroblasts (Lee, Lee et al. 2005) (Figure 4).
- Homeostasis and membrane exitability
PKCδ regulates membrane excitability by modulating different ion channels and pumps such as the calcium efflux regulator PMCA (plasma membrane calcium ATPase) (Crotty, Cai et al. 2006; Ahn, Jeong et al. 2007). In the skin, if the permeability barrier is disrupted by physical or mechanical damage, there is an increase on trans-epidermal water loss, followed by a decrease of the extracellular Ca²⁺ levels (Garcia and Strehler 1999). The decrease in Ca²⁺ levels causes the opening of calcium channels allowing calcium influx to restore basal levels of Ca²⁺. Accumulation of Ca²⁺ in the cell causes the activation of phospholipase C (PLCγ). The activated phospholipase cleaves phosphatidylinositol 4,5-biphosphate (PI(4,5)P2) generating DAG and inositol 1,4,5-triphosphate which in turn activate PKCδ (Figure 4) (Berridge 1984). Thus, PKCδ is fundamental to maintain Ca²⁺ levels and the homeostatic balance in the skin.
In muscle cells, PKCδ plays an important role restoring K⁺ homeostasis after membrane depolarization of the action potential. Membrane depolarization caused by an excitatory stimulus activates Na⁺ pumps; this causes an inward movement of Na⁺ ions. The entrance of Na⁺ ions causes a decrease of membrane negative charge and a change to a positive inner membrane potential. The inner positive electrical gradient favors the activation of the G protein-activated Inwardly Rectifying K⁺ channels (GIRK channels) to open and release K⁺, this allows the transmission of a second impulse. Activation of the G-coupled protein causes the generation of PI(4,5)P2 and DAG through PLCγ activation (Berridge 1984). After PKCδ is recruited to the membrane, is able to phosphorylate the GIRK channels leading to the alteration of the channels conformation and/or interactions with Gβγ-subunits, preventing a channel activation by PI(4,5)P 2. This allows the cell to maintain the homeostasis by normalizing potassium basal levels (Breitwieser 2005; Brown, Thomas et al. 2005; Xie, John et al. 2007).
- Transcriptional regulation
Multiple transcription factors are phosphorylated by PKCδ (Kim, Seo et al. 2006; Liu, Yang et al. 2006; Kim, Choi et al. 2007; Kim, Lim et al. 2007). In this context, phosphorylation of the Sp1 transcription factor, promoted cyclin D3 expression in cells treated with the histone deacetylase apicidin (Kim, Lim et al. 2007). Moreover, the expression of NF-κB was modulated by apicidin, through the inhibition of Sp1. PI3K and PKC are required for the NFκB activation by apicidin. In addition NF-κB plays an important role as a modulator determining cell fate in response to HDAC inhibitors (Kim, Seo et al. 2006; Liu, Yang et al. 2006). Also, it has been observed that RelA / NF-κB upregulate PKCδ expression, in UV treated cells. PKCδ is necessary and sufficient to mediate JNK activation by RelA, in response to UV treatment promoting apoptosis in mouse fibroblasts (Liu, Yang et al. 2006) (Figure 4). In addition, PKCδ can modulate DNA methylation through induced-phosphorylation of transcription factors, and through the modulation of the ERK signal pathway (Yuan, Soh et al. 2002; Gorelik, Fang et al. 2007). PKCδ also phosphorylates the acetyl transferase p300 at Ser⁸⁹ inhibiting its activity in vitro and in vivo. This event inhibits p300 histone acetyl transferase (HAT) activity causing a reduction in the nucleosome histone acetylation and reduction of gene expression on HeLa cells (Yuan, Soh et al. 2002) (Figure 4). Moreover, members of the Stat transcription factor family are regulated by PKCδ. PKCδ phosphorylates Stat1 (signal transducer transactivator-1) at Ser⁷²⁷, allowing the transcription of the CIITA (class II transactivator) promoter promoting the activity of the MHC class II receptor (Kwon, Yao et al. 2007). PKCδ also phosphorylates and associates with Stat-3 enhancing the interaction between Stat3 and IL-6 receptor subunit glycoprotein (gp) 130, the initial step for Stat3 activation (Novotny-Diermayr, Zhang et al. 2002).
- Redox-sensitive kinase
PKCδ functions as a redox-sensitive kinase in various cell types (Sun, Wu et al. 2000; Kanthasamy, Kitazawa et al. 2003). Exposure to reactive oxygen species (ROS) induces the release of DAG, activation and membrane translocation (Cummings, Parinandi et al. 2002). ROS can induce the activation of the NF-κB signaling pathway through the activation of the IκB kinases (IKKs), in a PKCδ signal pathway dependent manner (Yamaguchi, Miki et al. 2007). Thus, PKCδ may regulate the role of NF-κB as a redox-sensitive factor (Figure 4).
- Immunological response
The regulation of the antigen presentation is a key to generate an effective immune response. CD1 family members are antigenic peptides presented by MHC class I or class II molecules (Jackman, Moody et al. 1999; Brutkiewicz, Lin et al. 2003). Inhibition of PKCδ by rottlerin results in a substantial reduction in CD1d-mediated antigen presentation and alters the intracellular localization of CD1d to the lipid membrane. Also, PKCδ can modulate the expression levels of the trans-activator MHC class II, CD28, and CD24 , mediating a negative feedback signals in lymphocytes though the phosphorylation of MHC class I and II (Brutkiewicz, Lin et al. 2003). PKCδ activation also promotes the mobilization of the MHC class II receptors to the cell surface. Inhibition of PKCδ activation by rottlerin prevents dendritic cell activation of T lymphoctes (Majewski, Bose et al. 2007). In addition, PKCδ can regulate the expression of MHC class II receptor controlling the expression of the class II transactivator (CIITA). The expression of CIITA is required for MHC class II expression (Reith, LeibundGut-Landmann et al. 2005). Upon inflammation macrophages release IFNγ causing the activation of PKCδ. Once PKCδ is activated, induces the phosphorylation of Stat-1 at Ser⁷²⁷ allowing it to interact with the HATs, CBP/p300, promoting acetylation of the CIITA promoter. The PKCδ-dependent transcription of CIITA allows MHC class II expression in murine macrophages (Kwon, Yao et al. 2007) (Figure 4).