Protein kinase C in the immune system: from signalling to chromatin regulation (2024)

Abbreviations

BAF60c

Brg1/Brm‐associated factor 60c

Bcl10

B‐cell leukemia/lymphoma 10

BCR

B‐cell receptor

Btk

Bruton's tyrosine kinase

CARMA1

caspase recruitment domain family (CARD)‐containing membrane‐associated guanylate kinase (MAGUK) protein 1

CIITA

class II transactivator

CREB

cAMP response element‐binding protein

DAG

diacylglycerol

ERα

oestrogen receptor α

GLK

germinal centre kinase (GCK)‐like kinase (MAP4K3)

H1

histone H1

H2B

histone H2B

H3

histone H3

HEXIM1

hexamethylene‐bis‐acetamide‐induced mRNA‐encoded proteins 1

IFN

interferon

IKK

inhibitor of κB (IκB) kinase

IL

interleukin

IκB

inhibitor of κB

K

lysine

Ki‐1/57

57‐000 MW human protein antigen recognized by the CD30 antibody Ki‐1

MALT1

mucosa‐associated lymphoid tissue 1

MyD88

myeloid differentiation primary‐response protein 88

NF‐κB

nuclear factor κB

NLS

nuclear localization signal

P

proline

PCAF

p300/CREB‐binding protein‐associated factor

PKA

protein kinase A

PKC

protein kinase C

PKG

protein kinase G

S

serine

S/T‐P‐S/T

SPT

SATB1

special AT‐rich binding protein 1

STAT

signal transducer and activator of transcription

T

threonine

TAK1

transforming‐growth‐factor–activated kinase 1

TCR

T‐cell receptor

Th

T helper

TIR

Toll–IL‐1 receptor

TIRAP

Toll–IL‐1 receptor domain‐containing adaptor protein

TLR

Toll‐like receptor

TRAF6

tumour necrosis factor receptor‐associated factor 6

TRAM

TRIF‐related adaptor molecule

TRIF

TIR domain‐containing adaptor inducing interferon‐β

Y

Tyrosine

Table 1

Protein kinase C (PKC) isoform cell‐specific expression and defects in knockout mice

PKCε in TLR4 signalling

Toll‐like receptors play a key role in the innate immune system by defending the host against microbial infection.¹⁰ TLRs are a family of pattern recognition receptors that are present in many cell types, with most of the expression by macrophages, neutrophils and dendritic cells.¹¹ The best‐studied TLR ligand is lipopolysaccharide, a component of the Gram‐negative bacteria. It activates the innate immune system through binding with TLR4 to initiate two intracellular pathways: MyD88 (myeloid differentiation primary‐response protein 88)‐dependent and MyD88‐independent pathways (Fig. ​(Fig.22a).¹² The MyD88‐dependent pathway requires TIRAP [Toll–interleukin‐1 (IL‐1) receptor (TIR) domain‐containing adaptor protein] to link MyD88 to TLR4 receptor, leading to the expression of inflammatory cytokines such as IL‐1, IL‐6 and tumour necrosis factor α. In contrast, the MyD88‐independent pathway is mediated by TRIF [TIR domain‐containing adaptor inducing interferon‐β (IFN‐β)] and TRAM (TRIF‐related adaptor molecule) to induce expression of type I IFNs.¹³ It is beyond the scope of this review to cover these pathways in detail hence we refer readers to the multiple reviews that explain the TLR4 signalling pathway in depth.¹⁰, ¹¹, ¹⁴, ¹⁵

Although PKC isoforms are involved at many levels of the TLR signalling cascade, we will focus on the involvement of PKCε in TLR4 signalling as PKCε is implicated as an important player in the TLR4 signalling pathway during macrophage activation.⁷ The role of PKCε in host defence against bacterial infection was revealed through studies in PKCε knockout mice, where mice lacking PKCε have a diminished response to lipopolysaccharide stimulation, characterized by low levels of several cytokines, namely tumour necrosis factor‐α and IL‐1β.¹⁶ Other studies show PKCε playing a role in both the MyD88‐dependent and MyD88‐independent pathways of TLR4 signalling (Fig. ​(Fig.22a).¹⁷, ¹⁸ In the MyD88‐dependent pathway, lipopolysaccharide stimulation leads to PKCε recruitment to TLR4 and phosphorylation on S346 and S368 via MyD88.¹⁷ These phosphorylations lead to binding with 14‐3‐3β, which is also MyD88 dependent. The phosphorylation event is important for downstream signalling as cells expressing mutant PKCε S346A/S368A were unable to activate nuclear factor‐κB (NF‐κB) upon TLR induction. This suggests that PKCε not only needs to be phosphorylated for its ability to bind to 14‐3‐3β, but it also exists as a complex with TLR, MyD88 and 14‐3‐3β to regulate gene expression. In the MyD88‐independent pathway, PKCε is required for TLR4 activation via the TRAM substrate as phosphorylation of TRAM is disrupted in PKCε‐deficient cells.¹⁸ TRAM is localized to the plasma membrane in the unstimulated state but upon lipopolysaccharide stimulation, it is phosphorylated by PKCε on a serine residue near the N‐terminal end.¹⁴ This phosphorylation dissociates TRAM from the membrane, allowing it to then link TLR4 with TRIF.⁷ The TLR4–TRAM–TRIF complex is necessary for activation of further downstream NF‐κB and IFN regulatory factor‐3/7 signalling pathways.¹⁹

The innate immunity provided by the TLR pathway is also required for the adaptive response by T‐cell activation against antigens.²⁰, ²¹, ²² Cross‐talk in signalling pathways is not uncommon but the specificity of the response depends on the stimulus provided and the cell type engaged to generate the appropriate immune response.

PKCθ in TCR signalling

The cells of the adaptive immune system form the immunological synapse with antigen‐presenting cells to activate signal transduction pathways that induce gene expression programmes for lymphocyte function. In T cells, activation of T cells occurs when the TCR recognizes an antigen presented by the antigen‐presenting cells. This leads to T‐cell differentiation and is a process involving the activation of multiple pathways including PKC signalling (Fig. ​(Fig.2b).2b). Following TCR–antigen‐presenting cell complex formation, PKC localizes to the immunological synapse and subsequently stimulates the recruitment and activation of nuclear transcription factors (such as NF‐κB, activator protein 1 and nuclear factor of activated T cells) required for induction of immune effector genes.⁸ These effector genes are then expressed in a rapid and transient manner to produce cytokines, chemokines, cell surface molecules and growth factors, important for T‐cell proliferation and differentiation. The details of the T‐cell activation pathways have been reviewed extensively elsewhere.²³, ²⁴, ²⁵, ²⁶, ²⁷, ²⁸ Although other members of the PKC family can also be found in the immunological synapse of different T‐cell subsets,²⁹ PKCθ is the most prominently studied PKC in TCR signalling since the discovery of its selective recruitment to the immunological synapse in effector T cells.³⁰ Furthermore, it is selectively expressed in T cells within the haematopoietic cell population.³¹ Upon TCR activation, PKCθ localizes to the central supramolecular activation cluster of the immunological synapse at the plasma membrane.³² The membrane translocation of PKCθ requires association with co‐stimulatory molecule CD28 with the V3 domain of PKCθ.³³, ³⁴, ³⁵ The ability of PKCθ to segregate correctly to the central supramolecular activation cluster is dependent on the presence of CD28 as activated T cells from CD28‐deficient mice were unable to form the mature immunological synapse with PKCθ, forming a diffuse pattern throughout the synapse instead.³³ The specific localization of PKCθ to the immunological synapse is critical for an effective T‐cell activation and this translocated PKCθ is also enzymatically active.³⁰ Interestingly, the activity of PKCθ is also regulated by the intracellular redox state, in which the oxidized inactive form of PKCθ is recruited to the plasma membrane in naive T cells.³⁶

The role of PKCθ in regulating T‐cell function was initially characterized using PKCθ‐knockout mice (Table 1).³⁷, ³⁸, ³⁹ Further studies on PKCθ‐deficient T cells reveal that PKCθ performs different functions depending on the T‐cell subpopulations. For example, PKCθ is required for a T helper type 2 (Th2) cell but not Th1 cell in vivo immune response against helminth infection and allergic airway inflammation.⁴⁰ However, contrasting studies in mouse experimental autoimmune encephalomyelitis show impaired Th1 responses in PKCθ‐deficient mice suggesting that PKCθ is important for regulating both Th1 and Th2 responses but in an antigen‐dependent and organ‐specific manner.⁴¹, ⁴² These studies have also shown that PKCθ is required for the Th17‐dependent development of experimental autoimmune encephalomyelitis, implicating PKCθ in controlling Th17 differentiation.⁴¹, ⁴² According to Kwon et al.,⁴³ PKCθ up‐regulates signal transducer and activator of transcription 3 (STAT3) under Th17 priming conditions upon TCR stimulation with PMA. PKCθ promotes the activation of STAT3 by regulating the association of activator protein 1 and NF‐κB transcription factors to STAT3 promoter.⁴³ More recently, PKCθ was found to be essential in suppressing Th1‐typical genes such as Stat4, Tbet and Ifng during Th17 immune activation, as a way to stabilize the Th17 cell phenotype.⁴⁴ PKCθ is also involved in regulating immune memory. In a study on antiviral responses by CD8⁺ T‐cell responses, PKCθ is required for antigen recall responses upon in vitro infection by lymphocytic choriomeningitis virus and influenza virus.⁴⁵, ⁴⁶ Furthermore, the efficient and timely recruitment of PKCθ to the immunological synapse is critical for memory T‐cell development.⁴⁷

Interestingly, PKCθ also localizes to the immunological synapse in effector T cells to positively regulate cell function but activation of regulatory T cells sequesters PKCθ away from the immunological synapse, leading to negative regulation of induced regulatory T cells.⁴⁸, ⁴⁹ This negative regulation by PKCθ involves inhibiting differentiation of induced regulatory T cells through the AKT/Foxo1/3a pathway.⁵⁰ Hence, PKCθ plays a role in regulating the T‐cell immune response through maintaining the equilibrium of T‐cell subpopulations. However, the precise mechanism by which it deciphers the signals received within each cell subset to perform the cell‐type‐specific function is yet to be elucidated.

PKCβ in BCR signalling

Like T cells, B‐cell activation leads to production of regulatory cytokines and chemokines to eliminate pathogens. However, B cells also present antigen to T cells and specialize in producing high‐affinity antibodies and long‐lived memory cells, generating rapid and long‐lasting protection against secondary exposure to the same pathogen.⁵¹ Activation of B cells occurs when an antigen binds to the BCR, initiating phosphorylation events by Src‐family kinases as well as Syk and Bruton's tyrosine kinase (Btk)/Tec family kinases. This signals the organized assembly kinases and adaptor proteins forming the signalosome and activating multiple signalling cascades (Fig. ​(Fig.22c).⁵² Secondary messengers such as DAG and inositol‐1,4,5‐triphosphate are generated as part of the BCR activation signalling cascade to initiate Ca²⁺ and PKC downstream signalling pathways, respectively, leading to activation of transcription factors (Myc, nuclear factor of activated T cells, NF‐κB, activator protein 1) critical for B‐cell function.⁵³ While B cells express multiple isoforms of PKC (α, β, δ, ε, η, ζ, and λ), PKCβ is the key PKC isoform that is important in BCR signalling.⁵⁴, ⁵⁵, ⁵⁶, ⁵⁷, ⁵⁸

The role for PKCβ in regulating B‐cell functions was first discovered through PKCβ gene knockout mice, which were shown to have impaired B‐cell activation, inability to proliferate upon BCR stimulation and defects in T‐cell‐independent immune responses (Table 1).⁵⁸ The immunodeficiency traits exhibited by these PKCβ knockout mice are similar to those seen in Btk‐deficient or X‐linked immunodeficient mice, suggesting that Btk and PKCβ may be linked in BCR signalling.⁵⁹ Indeed, Btk has been shown to be important for NF‐κB activation upon BCR engagement and is regulated by PKCβ in a negative feedback mechanism.⁶⁰, ⁶¹, ⁶² Specifically, PKCβ directly phosphorylates Btk to down‐regulate Btk kinase activity and alter its membrane localization in BCR signalling.⁶² This result was also confirmed using a PKCβ‐selective inhibitor, where inhibiting PKCβ kinase activity leads to an increase in Btk kinase activity to enhance calcium mobilization, so implicating PKCβ in the regulation of calcium release upon BCR activation.⁶³

Another study used N‐ethyl‐N‐nitrosurea‐induced mutagenesis to generate PKCβ mutant mice (Tilcara) that have heterozygous mis‐sense mutations.⁶⁴ The Tilcara mutant mice have single amino acid substitutions in conserved residues within the kinase domain of PKCβ. This results in an S552P mutation that is close to a docking site for the pseudosubstrate domain. These mice show impaired T‐cell independent antibody responses, as seen in the PKCβ gene knockout mice.⁵⁸ Another effect of the Tilcara mutation is loss of active PKCβI but not PKCβII in B‐cell protein lysates in hom*ozygous mutants, implying that the region of mutation is important for PKCβI function. While the Tilcara mutant mice do not show as significant an effect phenotypically compared to PKCβ knockout mice, the S552P substitution occurs at an evolutionarily conserved residue, hence there could be changes occurring at the transcriptional level. It would be of great interest to examine how mutation at this conserved residue can affect BCR signalling on a genotypic level.

PKCβ is also involved in forming the BCR signalosome upon BCR engagement (Fig. ​(Fig.2c).2c). PKCβ phosphorylates CARMA1 [caspase recruitment domain family (CARD)‐containing membrane‐associated guanylate kinase (MAGUK) protein 1] on S668 to lead to the formation and recruitment of the CARMA1, B‐cell leukaemia/lymphoma 10 (Bcl10), and mucosa‐associated lymphoid tissue 1 (MALT1) complex to lipid rafts to form part of the BCR signalosome.⁶⁵ PKCβ is also required in recruiting inhibitor of κB (IκB) kinase (IKK) to the CARMA1–Bcl10–MALT1 complex.⁵⁶ For IKK activation to occur, the adaptor protein CARMA1 is directly phosphorylated by PKCβ, which then brings IKK and another protein kinase transforming‐growth factor‐activated kinase 1 (TAK1) close together.⁶⁶, ⁶⁷ This allows TAK1 to phosphorylate IKK leading to its activation.⁶⁶ As a result, IκB is phosphorylated by IKK to lead to activation of NF‐κB, hence demonstrating the critical role that PKCβ plays in BCR‐dependent NF‐κB signalling.

Hence, from all the examples shown in the different immune cells, PKC signalling pathways converge with other signalling pathways in the nucleus to regulate inducible gene transcription. Furthermore, only a specific isozyme has been discussed in this review but there are other isozymes that participate in each of the signalling pathways, in which crosstalk between the different isozymes could occur. Undoubtedly, the PKC signalling pathway is important in transducing signals in the cytoplasm upon immune cell activation but PKCs are also found in the nucleus, suggesting a dual role by PKC as nuclear signal transducers as well as cytoplasmic signal transducers.

Nuclear translocation signals for nucleocytoplasmic shuttling

Proteins can be transported between the cytoplasm and the nucleus through nuclear pores located within the nuclear envelope. The ability of proteins to translocate into or out of the cell nucleus involves nuclear translocation signals such as nuclear localization signals (NLS) or nuclear export signals, respectively.⁸³ It can also occur through binding with proteins that have NLS as a way to enter the nucleus. The best characterized NLSs are classical NLS motifs, which can have a monopartite or bipartite motif.⁸⁴ PKC isoforms do not have the canonical NLS but contain a nuclear targeting motif that is similar to the classical bipartite NLS, forming a putative NLS that is conserved across the different PKC isoforms (Fig. ​(Fig.11).⁸⁵, ⁸⁶ In line with this, a putative NLS motif was proven to be functional for nuclear import of PKCδ to initiate apoptosis.⁸⁶ Upon induction with apoptotic agents in ParC5 cells, PKCδ is phosphorylated at Y64 and Y155, causing a conformational change to expose the NLS and allow the nuclear import factor, importin‐α, to bind and facilitate nuclear import of PKCδ.⁸⁷ Atypical PKC was also shown to contain both a functional NLS and nuclear export signals within the zinc‐finger domain, allowing for rapid shuttling between the nucleus and cytoplasm.⁸⁸ Similarly, aPKC is phosphorylated at Y256 upon nerve growth factor stimulation in PC12 cells, and bound by importin‐α for nuclear translocation.⁸⁹ Hence, there seem to be similarities in the mechanism of translocation between PKC isoforms but this is quite likely to occur in a stimulus‐specific and cell context‐dependent manner.

Translocation of PKC into the nucleus occurs through mechanisms that are different from the proteins with canonical NLS.⁹⁰, ⁹¹ For example, PKCα requires an intact cytoskeleton to translocate into the nucleus but a protein with canonical NLS is unaffected when the cytoskeleton is disrupted.⁹⁰ In addition, PKCα does not require nuclear import factors p97/importin/karyopherin β or GTP to transport into the nucleus, unlike proteins with canonical NLS.⁹¹ Interestingly, aPKCs have isoform‐specific regulation of nucleocytoplasmic shuttling. Even though both PKCζ and PKCι contain the same NLS, only PKCζ require the NLS for nuclear translocation while the ability of PKCι to translocate is independent of the NLS.⁹² In contrast, using chimeric proteins, Seidl et al.⁹² showed that PKCι requires the hinge region for nuclear localization but the hinge region of PKCζ excludes it from the nucleus. Undoubtedly, the requirement of the PKC NLS and the associated protein domains as well as nuclear import factors plays a role in nucleocytoplasmic shuttling of PKC. What is interesting is how different isoforms each have specific requirements for each of those features, which leads to isoform‐specific localization of PKC, and hence isoform‐specific regulation of nuclear function.

In addition to the NLS, another motif, the S/T‐P‐S/T (SPT‐like) motif, was suggested to perform like a general NLS for signalling proteins that do not have the canonical NLS, implying that PKC could potentially be regulated by the SPT‐like motif.⁹³ Using protein sequence alignment, Sutcliffe et al.⁹⁴ showed that novel and conventional PKC isoforms were predicted to have SPT‐like motif present. The SPT‐like motif appears to reside close to where the regulatory and kinase/catalytic domains are and deletions in the regulatory or kinase domains of PKC are shown to affect nuclear translocation.⁹⁵, ⁹⁶ Mutation studies substituting the SPT motif in PKCθ with an alternative motif that cannot be phosphorylated impaired the localization of PKCθ to the nucleus, suggesting that phosphorylation of SPT is required for nuclear translocation.⁹⁴ Interestingly, mutating the putative NLS motif has no effect on the distribution of PKC in the cell, suggesting that the SPT‐like motif may be an alternative NLS motif used by signalling kinases that do not have the canonical NLS for nuclear transport.⁹⁴ Hence, it would be of interest to examine how PKC activation can affect the configuration of the protein folding/structure and as a consequence the exposure of the nuclear localization motifs for regulation.

The fact that there are numerous substrates described for nuclear PKC suggests that PKC could potentially regulate nuclear functions like gene transcription (reviewed in refs. ⁹⁷, ⁹⁸). Indeed, some of the nuclear PKC substrates include chromatin factors such as histone (H1, H2B, H3), RNA polymerase II, transcription factors Fos and cAMP response element‐binding protein (CREB), as well as architectural proteins such as high mobility group proteins, implicating PKC in directly regulating gene transcription in the nucleus.

Table 2

Chromatin‐associated protein kinase C (PKC) substrates

PKCδ as a histone kinase in immune‐mediated apoptosis

Protein kinase Cδ regulates the immune system through the events of mitochondrial‐dependent apoptosis by phosphorylating histones at apoptotic histone residues such as H2BS14, H3S10 and H3T45 (Fig. ​(Fig.33).¹⁰⁶, ¹⁰⁷, ¹⁰⁸, ¹⁰⁹ In B cells, the nuclear localization of a catalytically active PKCδ leads to phosphorylation of H2BS14 to result in apoptosis of resting B cells.¹⁰⁷ Similarly, in Sjögren's syndrome, interaction of B cells with human salivary gland cells causes translocation and activation of PKCδ into the nucleus to phosphorylate H2BS14 leading to a B‐cell‐mediated apoptosis of epithelial cells.¹⁰⁶ Using PKCδ small interfering RNAs in Jurkat T cells, Park et al.¹⁰⁹ showed that phosphorylation of H3S10 by PKCδ is required for DNA damage‐induced apoptosis. In vitro studies in neutrophils showed that phosphorylation of H3T45 increases significantly in apoptotic cells and that this phosphorylation is mediated by PKCδ.¹⁰⁸ These systems focused on the pro‐apoptotic function of PKCδ as a result of cellular stress but PKCδ can also be anti‐apoptotic depending on the cell type. Specifically, PKCδ promotes the survival of cancer cells through established anti‐apoptotic pathways including NF‐κB.¹¹⁰ However, whether the anti‐apoptotic activity by PKCδ is regulated through histone phosphorylation is yet to be studied.

PKCθ association with histone modifiers in T‐cell activation

In addition to phosphorylating histones, PKC can also interact with and phosphorylate histone modifiers (Table 2). Specifically in the immune system, PKCθ not only has a cytoplasmic signalling role during TCR activation, it is also present in the nucleus of T cells to regulate transcription of inducible immune genes.⁹⁴, ¹¹¹, ¹¹² In stimulated human Jurkat T cells, PKCθ associates with the N‐terminal transcriptional activation domain of p300 and phosphorylates S384 on p300 leading to its transcriptional activation.¹¹¹ In a study by Sutcliffe et al.,¹¹² PKCθ does not appear to phosphorylate histones directly but instead forms a complex with RNA polymerase II, histone kinase mitogen and stress‐activated protein kinase 1, adaptor protein 14‐3‐3ζ and histone demethylase lysine‐specific demethylase 1 at the proximal promoter of inducible genes in human Jurkat T cells (Fig. ​(Fig.3).3). The assembly of this complex is dependent on NF‐κB as inhibiting NF‐κB impaired PKCθ, RNA polymerase II and lysine‐specific demethylase 1 recruitment to immune gene promoters upon TCR activation.⁹⁴ This formation of an active transcriptional complex on key inducible genes can also be seen in breast cancer stem cells, where PKCθ acts as a molecular switch that regulates epithelial to mesenchymal transition.¹¹², ¹¹³ Interestingly, inhibiting PKCθ in T cells leads to increased expression of microRNAs, small non‐coding RNAs responsible for post‐transcriptional repression of gene expression.¹¹² This negative regulation of microRNA genes by PKCθ involves NF‐κB, both of which may be part of a repressive complex on microRNA genes to repress its transcription.⁹⁴ This nuclear activity further strengthens the notion that PKCθ is a key enzyme in the regulation of T‐cell activation. So far only PKCθ is shown to associate and phosphorylate histone modifiers in the immune system. Given the key role that histone modifiers play in regulating inducible gene expression, it can be postulated that other PKC isoforms could potentially regulate histone modifiers through their kinase activity in the immune context.

Direct regulation of transcription factors by PKCs

Protein kinase C signalling in the cytoplasm activates downstream signalling cascades leading to transcription factor activation, and hence gene transcription (Fig. ​(Fig.2).2). In the nucleus, PKC plays a more direct role in regulating transcription factors through phosphorylating the factors directly or regulating interactions of transcriptional regulatory factors to lead to transcriptional activation of genes (Table 2). For example, monocyte induction by macrophage colony‐stimulating factor leads to phosphorylation of NF‐κB p65 at S276 by PKCα, which is important for transcriptional activation of NF‐κB.¹¹⁴ In dendritic cells, PKCζ phosphorylation of p65 at S311 prevents binding of histone methyltransferase glucagon‐like peptide to p65 at monomethylated K310, allowing expression of p65 target genes.¹¹⁵ Similarly, IL‐32β mediates PKCδ phosphorylation of CCAAT/enhancer‐binding protein α at S21 to prevent binding of the CCAAT/enhancer‐binding protein α to IL‐10 promoter, hence activating the gene in human myeloid cells.¹¹⁶

Upon heat‐shock induction, PKCε phosphorylates STAT1 at S727 in Jurkat T cells to activate the heat‐shock protein 90β (hsp90β) gene.¹¹⁷ While in macrophages, induction by IFN‐γ leads to STAT1 phosphorylation by PKCδ at S727 to promote class II transactivator (CIITA) gene expression.¹¹⁸ A similar study shows that CIITA gene expression in B cells is controlled by CREB phosphorylation by PKCδ.¹¹⁹ The phosphorylation of CREB by PKCδ occurs at S133 upon B‐cell activation.¹²⁰ Whereas in T cells, PKCθ is involved in the phosphorylation and binding of CREB to the IL‐2 promoter.¹²¹ In human Jurkat T cells, phosphorylation by PKC can regulate the interaction of special AT‐rich binding protein 1 (SATB1) with either histone deacetylase 1 or p300/CREB‐binding protein‐associated factor (PCAF).¹²² In the basal state, PKC‐phosphorylated SATB1 at S185 associates with histone deacetylase 1, so acting as a repressor, but upon activation, dephosphorylated SATB1 associates with PCAF, leading to activation of the IL‐2 gene. These studies show that nuclear PKCs directly regulate transcription factors through phosphorylation for transcriptional activation in the immune system. However, phosphorylation of transcription factors by PKCs can also lead to gene repression¹²³, ¹²⁴ but this has not been shown in the immune context.

Other PKC isoforms as chromatin regulators in non‐immune systems

At present, there are no reported epigenetic activities by PKCγ and PKCη. Although not published to have epigenetic roles in immune cells, PKCα and PKCβ have been shown to act as histone kinases in other studies (Table 2).¹²⁵, ¹²⁶ A screen with 97 kinases using HeLa cells showed that PKCα, PKCβI and PKCβII can phosphorylate H3T6 but not H3T3, H3S10 or H3T11.¹²⁶ This is in line with a study using high‐resolution nuclear magnetic resonance spectroscopy, where PKCα and both PKCβ isoforms could phosphorylate H3T6 and H3S10 but in a mutually exclusive manner (Fig. ​(Fig.33).¹²⁵ Using prostate cancer cells, Metzger et al.¹²⁶ showed that PKCβI phosphorylates H3T6, which blocks H3K4 demethylation by lysine‐specific demethylase 1 during androgen receptor‐dependent gene activation. In breast cancer cells, PKCβI is recruited to the oestrogen receptor α (ERα) promoter B as part of a regulatory complex with RanBP‐type and C3HC4‐type zinc finger containing 1 and ERα to regulate ERα gene expression (Fig. ​(Fig.33).¹²⁷ This recruitment of PKCβI is associated with the presence of PKCβI‐dependent H3K4me2 and H3T6 phosphorylation though a direct causal effect has not been demonstrated in this model.¹²⁷ Given that PKCβ is specifically expressed in B cells, it can be postulated that PKCβ could potentially regulate immune gene expression as an epigenetic enzyme.

We have discussed examples of PKC isoforms functioning as epigenetic enzymes in neutrophils (PKCδ), macrophages (PKCα, PKCδ), myeloid cells (PKCδ), dendritic cells (PKCζ), B cells (PKCδ) and T cells (PKCδ, PKCε, PKCθ). Given the defects seen in immune cell homeostasis, activation and function in various PKC knockout studies (Table 1), it is highly possible that PKC isoforms that are not known to have an epigenetic function could potentially regulate the expression of key immune genes as epigenetic enzymes.

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