Phosphorylation of MARCKS (80-kDa) protein, a major substrate for protein kinase C in oligodendroglial progenitors

CircTRIM1 encodes TRIM1-269aa to promote chemoresistance and metastasis of TNBC via enhancing CaM-dependent MARCKS translocation and PI3K/AKT/mTOR activation

Peptides and proteins encoded by noncanonical open reading frames (ORFs) of circRNAs have recently been recognized to play important roles in disease progression, but the biological functions and mechanisms of these peptides and proteins are largely unknown. Here, we identified a potential coding circular RNA, circTRIM1, that was upregulated in doxorubicin-resistant TNBC cells by intersecting transcriptome and translatome RNA-seq data, and its expression was correlated with clinicopathological characteristics and poor prognosis in patients with TNBC. CircTRIM1 possesses a functional IRES element along with an 810 nt ORF that can be translated into a novel endogenously expressed protein termed TRIM1-269aa. Functionally, we demonstrated that TRIM1-269aa, which is involved in the biological functions of circTRIM1, promoted chemoresistance and metastasis in TNBC cells both in vitro and in vivo. In addition, we found that TRIM1-269aa can be packaged into exosomes and transmitted between TNBC cells. Mechanistically, TRIM1-269aa enhanced the interaction between MARCKS and calmodulin, thus promoting the calmodulin-dependent translocation of MARCKS, which further initiated the activation of the PI3K/AKT/mTOR pathway. Overall, circTRIM1, which encodes TRIM1-269aa, promoted TNBC chemoresistance and metastasis by enhancing MARCKS translocation and PI3K/AKT/mTOR activation. Our investigation has yielded novel insights into the roles of protein-coding circRNAs and supported circTRIM1/TRIM1-269aa as a novel promising prognostic and therapeutic target for patients with TNBC.

The myristoylated alanine-rich C-kinase substrates (MARCKS): A membrane-anchored mediator of the cell function

The myristoylated alanine-rich C-kinase substrate (MARCKS) and the MARCKS-related protein (MARCKSL1) are ubiquitous, highly conserved membrane-associated proteins involved in the structural modulation of the actin cytoskeleton, chemotaxis, motility, cell adhesion, phagocytosis, and exocytosis. MARCKS includes an N-terminal myristoylated domain for membrane binding, a highly conserved MARCKS Homology 2 (MH2) domain, and an effector domain (which is the phosphorylation site). MARCKS can sequester phosphatidylinositol-4, 5-diphosphate (PIP2) at lipid rafts in the plasma membrane of quiescent cells, an action reversed by protein kinase C (PKC), ultimately modulating the immune function. Being expressed mostly in innate immune cells, MARCKS promotes the inflammation-driven migration and adhesion of cells and the secretion of cytokines such as tumor necrosis factor (TNF). From a clinical point of view, MARCKS is overexpressed in patients with schizophrenia and bipolar disorders, while the brain level of MARCKS phosphorylation is associated with Alzheimer's disease. Furthermore, MARCKS is associated with the development and progression of numerous types of cancers. Data in autoimmune diseases are limited to rheumatoid arthritis models in which a connection between MARCKS and the JAK-STAT pathway is mediated by miRNAs. We provide a comprehensive overview of the structure of MARCKS, its molecular characteristics and functions from a biological and pathogenetic standpoint, and will discuss the clinical implications of this pathway.

A novel effect of MARCKS phosphorylation by activated PKC: the dephosphorylation of its serine 25 in chick neuroblasts

MARCKS (Myristoylated Alanine-Rich C Kinase Substrate) is a peripheral membrane protein, especially abundant in the nervous system, and functionally related to actin organization and Ca-calmodulin regulation depending on its phosphorylation by PKC. However, MARCKS is susceptible to be phosphorylated by several different kinases and the possible interactions between these phosphorylations have not been fully studied in intact cells. In differentiating neuroblasts, as well as some neurons, there is at least one cell-type specific phosphorylation site: serine 25 (S25) in the chick. We demonstrate here that S25 is included in a highly conserved protein sequence which is a Cdk phosphorylatable region, located far away from the PKC phosphorylation domain. S25 phosphorylation was inhibited by olomoucine and roscovitine in neuroblasts undergoing various states of cell differentiation in vitro. These results, considered in the known context of Cdks activity in neuroblasts, suggest that Cdk5 is the enzyme responsible for this phosphorylation. We find that the phosphorylation by PKC at the effector domain does not occur in the same molecules that are phosphorylated at serine 25. The in situ analysis of the subcellular distribution of these two phosphorylated MARCKS variants revealed that they are also segregated in different protein clusters. In addition, we find that a sustained stimulation of PKC by phorbol-12-myristate-13-acetate (PMA) provokes the progressive disappearance of phosphorylation at serine 25. Cells treated with PMA, but in the presence of several Ser/Thr phosphatase (PP1, PP2A and PP2B) inhibitors indicated that this dephosphorylation is achieved via a phosphatase 2A (PP2A) form. These results provide new evidence regarding the existence of a novel consequence of PKC stimulation upon the phosphorylated state of MARCKS in neural cells, and propose a link between PKC and PP2A activity on MARCKS.

Coronin 3 and its role in murine brain morphogenesis

Coronins belong to the fundamental WD40-repeat proteins. They are mainly found at the submembraneous area, they bind F-actin in vitro, and most of the seven mammalian coronins have unclear roles. Coronin 3 is abundantly expressed in the adult CNS. All murine brain areas express coronin 3 during embryogenesis and the first postnatal stages. Expression in grey matter decreases postnatally, except for hippocampal pyramidal and dentate gyrus neurons, and cerebellar Purkinje cells, while levels in white matter increase in the course of myelination. Consistently, coronin 3 is abundant in differentiating neuro-2a and PC-12 cells and in primary oligodendrocytes. Treatment with PKC activator PMA reduced coronin 3 protein levels. To address its functions, neuro-2a and PC-12 cells were transfected with GFP-tagged coronin 3 versions. Full-length coronin 3 among other areas localized to outgrowing neurites, whereas truncated proteins efficiently suppressed neurite formation. Our results favour a role for coronin 3 in neuron morphogenesis and possibly migration.

Protein kinase C and mitogen-activated protein kinase signalling in oligodendrocytes

Oligodendrocytes (OL) play a significant physiological role in the central nervous system by creating the myelin sheath that allows for the efficient conduction of nerve impulses. Therefore, it is important to understand which signalling cascades define the proliferation, differentiation, survival, and myelin formation potential of these cells. Currently, much of the knowledge in this field has focused on two sets of protein kinase signalling molecules: Protein kinase C (PKC) and the mitogen-activated protein kinases (MAPKs). The roles of these kinases in OL are complex, and appear to be highly dependent on the developmental stage of the OL. Even so, some broad conclusions can be drawn from the multitude of experiments conducted on the roles of PKC and MAPKs in OL. For instance, PKC appears to have a proliferative effect on immature OL, while at the same time having an inhibitory effect on OL differentiation. In mature OL, the effects of PKC include increased process extension and myelin formation. The extracellular signal-regulated (ERK) members of the MAPK family also appear to increase process extensions in mature OL. On the other hand, the c-Jun N-terminal kinase (JNK) and p38 kinase members of the MAPK family appear to regulate apoptotic events in OL.

PDGF and FGF-2 signaling in oligodendrocyte progenitor cells: regulation of proliferation and differentiation by multiple intracellular signaling pathways

In this paper we address the linking of platelet-derived growth factor (PDGF) and basic fibroblast growth factor (FGF-2) to intracellular signaling molecules in oligodendrocyte progenitors. It is demonstrated that both growth factors activate downstream targets similar to those shown for protein kinase C (PKC) activation. Yet, neither the arrest of terminal oligodendrocyte differentiation nor the proliferation induced by PDGF or FGF-2 can be antagonized by inhibition of PKC. Rather, p42/p44 mitogen-activated protein kinase (MAPK), p38 MAPK, and pp70 S6 kinase were found to be necessary for the mitogenic activity of PDGF and FGF-2. Paradoxically, these kinases were also necessary for the onset of oligodendrocyte differentiation in control cells. In addition, cAMP-dependent kinase A (PKA) activation inhibited the mitogenic response of oligodendrocyte progenitors to FGF-2. Taken together, the molecular mechanism that controls oligodendrocyte lineage progression is operated by at least two signal pathways, which interfere either with proliferation and/or differentiation of oligodendrocyte progenitors.

Protein kinase C prevents oligodendrocyte differentiation: modulation of actin cytoskeleton and cognate polarized membrane traffic

In a previous study, we showed that activation of protein kinase C (PKC) prevents oligodendrocyte differentiation at the pro-oligodendrocyte stage. The present study was undertaken to identify downstream targets of PKC action in oligodendrocyte progenitor cells. Activation of PKC induced the predominant phosphorylation of an 80-kD protein, identified as myristoylated alanine-rich C-kinase substrate (MARCKS). Upon phosphorylation, MARCKS is translocated from the plasma membrane to the cytosol. Furthermore, PKC activation perturbed the organization of the actin cytoskeleton, causing a redistribution of actin filaments to the submembranous or cortical actin cytoskeleton. As a consequence, transport of a protein traffic marker, the vesicular stomatitis virus glycoprotein, from the trans-Golgi network to the plasma membrane becomes perturbed. The effect of disruption of the actin filament network by cytochalasin D perfectly matched the effect of PKC. These data thus favor the existence of a causal relationship between actin rearrangement and docking and/or fusion of proteins to the plasma membrane. Interestingly, neither in control cells nor in PKC-activated cells did another protein traffic marker, influenza hemagglutinin (HA), reach the cell surface. However, an eminent and specific accumulation of HA just underneath the plasma membrane became apparent upon PKC activation. Yet, this effect could not be simulated by cytochalasin D treatment. Therefore, these observations imply that although MARCKS represents a prominent PKC target site in regulating differentiation, another target involves the differential control of cognate polarized trafficking pathways, which are apparently operating in oligodendrocyte progenitor cells.

Regulation of oligodendrocyte differentiation: protein kinase C activation prevents differentiation of O2A progenitor cells toward oligodendrocytes

Oligodendrocytes differentiate on a specific schedule in vivo in order to myelinate axons at the precise time and at the appropriate position. The current study was undertaken to obtain further insight as to how this timed appearance is regulated intracellularly. We observed that exposure of O2A progenitor cells in culture to phorbol 12-myristate 13-acetate (PMA; an activator of protein kinase C, PKC) inhibited their differentiation to oligodendrocytes by suppressing the expression of specific myelin markers at the O4-stage. To positively identify a role of PKC per se in differentiation, the use of a minimal medium with low serum content turned out to be essential. This was demonstrated by showing that the inhibitory effect of PMA on oligodendrocyte differentiation could be completely abolished by a combined action of insulin, triiodothyronine (T3), hydrocortisone and other components of a chemically defined medium (CDM). Furthermore, the PMA-mediated inhibition of oligodendrocyte differentiation could be partially restored by activation of the cAMP signal transduction pathway. The results indicate that PKC plays a crucial role in the differentiation of O2A progenitor cells toward oligodendrocytes: PKC activation prevents differentiation of O2A progenitor cells, whereas differentiation toward oligodendrocytes is dependent on other signaling compounds which may counteract the PKC signal transduction route.

Carbachol stimulates c-fos expression and proliferation in oligodendrocyte progenitors

To determine if muscarinic receptor-activation plays a role in oligodendrocyte development, the effect of carbachol a stable acetylcholine analog, on gene expression and proliferation was investigated. Using Northern blot analysis we showed that carbachol caused a time and concentration-dependent increase in c-fos mRNA. This effect was blocked by atropine, a non-selective muscarinic antagonist. In addition, the muscarinic-stimulated c-fos increase was inhibited by 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), but not by N-2-(p-bromocinnamylamino)-ethyl-5-isoquinoline-sulfonamide (H-89), a potent inhibitor of protein kinase A, suggesting the involvement of PKC in mediating the response. Down-regulation of PKC by overnight pre-treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) blocked only the phorbol ester-stimulated c-fos accumulation while no effect was observed in the carbachol-induced response. These results suggested that carbachol stimulated an H-7 sensitive PKC pathway which may be different than that activated by TPA. Further evidence for two separate mechanisms of proto-oncogene induction was provided by the additive effect of carbachol and TPA. Induction of c-fos mRNA by carbachol was dependent on both influx of extracellular Ca2+ and release from intracellular stores, as both EDTA and BAPTA blocked the response. Since activation of muscarinic receptors can affect cell division in other cellular systems, the effect of carbachol on [3H]thymidine and bromodeoxyuridine incorporation into oligodendrocyte DNA was measured. Carbachol stimulated DNA synthesis in oligodendrocyte progenitors. This effect was mediated by muscarinic receptors as [3H]thymidine incorporation was prevented or significantly reduced by the addition of atropine. In conclusion, the present findings suggest that, the neurotransmitter, acetylcholine may act as a trophic factor in developing oligodendrocytes, regulating their growth and development in the central nervous system.

Developmental expression of protein kinase C isozymes in oligodendrocytes and their differential modulation by 4 beta-phorbol-12,13-dibutyrate

Myelin gene expression in normal oligodendrocytes (OLG) depends on developmentally regulated protein kinase C (PKC) enzyme activity (Asotra and Macklin: J Neurosci Res 34:571-588, 1993). We studied the developmental expression of the Ca(++)-dependent PKC-alpha, -beta 1, -beta II and -gamma isozymes, and the Ca(++)-independent PKC-delta, -epsilon, -zeta and -eta isozymes in enriched rat brain OLG cultures. In A2B5+ O-2A progenitors, only PKC-delta, PKC-epsilon and PKC-zeta were detected immunocytochemically. In 04+ proligondendrocytes, PKC-beta I, -delta and -zeta were expressed moderately and low levels of PKC-alpha and -epsilon were detected. GD3+ OLG, GC+ OLG and MBP+ OLG showed increased levels of PKC-alpha, -beta I, -delta and -zeta isozymes. PKC-beta II, -gamma and -eta were poorly expressed in OLG. On immunoblots, PKC-alpha was present early and increased continually up to 18 days but PKC-beta I increased until 12 days in cultured OLG. High levels of PKC-delta, PKC-epsilon and PKC-zeta, the most abundant PKC isozymes in OLG, were maintained up to 12 days and were then slightly reduced. Interestingly, relatively high levels of PKC-alpha, PKC-beta I, PKC-beta II, PKC-gamma and PKC-epsilon isozymes were detected in purified myelin membrane although greater levels of PKC-delta were found in OLG than in purified myelin. Thus, most of the PKC isozymes found in cultured OLG were also present in myelin, although at different levels. Treatment with 50 nM 4 beta-phorbol-12,13-dibutyrate (PDB) caused a delayed downregulation of PKC-delta levels after 8 hr without modulating the expression of other PKC isozymes in 1-day OLG; in the 3-day-old and 6-day-old OLG, PDB downmodulated PKC-beta I, -delta and epsilon isozymes with only a minor effect on PKC-alpha and no reduction in PKC-zeta. Induction or downmodulation of individual PKC isozymes by phorbol esters appears to depend on the differentiation state of OLG. These data suggest that PKC-beta I, -delta and -epsilon isozymes have an important function in different cellular events of OLG differentiation. We conclude that the PKC-dependent modulation of myelin gene expression in OLG results predominantly from the Ca(++)-dependent PKC-beta I isozyme activity and the CA(++)-independent PKC-delta and PKC-epsilon activitives in a cell differentiation state-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha-tocopherol inhibits agonist-induced monocytic cell adhesion to cultured human endothelial cells

Antioxidants have been proposed to be anti-atherosclerotic agents; however, the mechanisms underlying their beneficial effects are poorly understood. We have examined the effect of alpha-tocopherol (alpha-tcp) on one cellular event in atherosclerotic plaque development, monocyte adhesion to stimulated endothelial cells (ECs). Human umbilical vein ECs were pretreated with alpha-tcp before stimulation with known agonists of monocyte adhesion: IL-1 (10 ng/ml), LPS (10 ng/ml), thrombin (30 U/ml), or PMA (10 nM). Agonist-induced monocytic cell adhesion, but not basal adhesion, was inhibited in a time- and concentration-dependent manner by alpha-tcp. The IC50 of alpha-tcp on an IL-1-induced response was 45 microM. The inhibition correlated with a decrease in steady state levels of E-selectin mRNA and cell surface expression of E-selectin which is consistent with the ability of a monoclonal antibody to E-selectin to inhibit monocytic cell adhesion in this system. Probucol (50 microM) and N-acetylcysteine (20 mM) also inhibited agonist-induced monocytic cell adhesion; whereas, several other antioxidants had no significant effect. Protein kinase C (PKC) does not appear to play a role in the alpha-tcp effect since no suppression of phosphorylation of PKC substrates was observed. Activation of the transcription factor NF-kappa B is reported to be necessary but not sufficient for E-selectin expression in EC. Electrophoretic mobility shift assays failed to show an alpha-tcp-induced decrease in activation of this transcription factor after cytokine stimulation. It has been hypothesized that alpha-tcp acts as an anti-atherosclerotic molecule by inhibiting generation of oxidized LDL--a putative triggering molecule in the atherosclerotic process. Our results point to a novel alternative mechanism of action of alpha-tcp.

Protein kinases mediate basic fibroblast growth factor's stimulation of proliferation and c-fos induction in oligodendrocyte progenitors

Primary culture was used to examine the effects of basic fibroblast growth factor (bFGF) on oligodendrocyte progenitor proliferation and c-fos expression. Basic FGF induced proliferation approximately six fold. This increased DNA synthesis could be blocked both with genistein, a tyrosine kinase inhibitor, and H-7, a protein kinase C (PKC) inhibitor. These results indicate that protein tyrosine kinase activity and protein kinase C are involved in mediating oligodendrocyte progenitor proliferation. The protooncogene c-fos was investigated as a likely proliferation mediator. Firstly, optimal conditions for bFGF-induced c-fos expression were determined. The oncogene responded maximally between 30 and 60 min of bFGF stimulation. Induction in response to bFGF occurred at 1 ng/ml, increased in a concentration-dependent manner and was maximal at 50 ng/ml. H-7 (50 microM) and genistein (100 microM) blocked c-fos induction as did PKC down-regulation with chronic treatment of phorbol 12-myristate 13-acetate. These results indicate that bFGF induces c-fos expression through receptor tyrosine phosphorylation and PKC activation. Thus similar early signals lead to bFGF-driven proliferation and c-fos induction suggesting a link between these two processes.

Involvement of protein kinase C in cAMP regulation of myelin basic protein gene expression

Since synthesis of myelin components has been seen to be stimulated by cAMP in both oligodendrocytes and Schwann cells we have begun investigating the specific sequence(s) in the 5' flanking region of the myelin basic protein (MBP) gene that are responsible for the induction of MBP transcription by cAMP. Using stably transfected cell lines containing various deletions of the MBP promoter directing the bacterial chloramphenicol acetyltransferase (CAT) gene we have identified a region of the MBP gene that is inhibitory to stimulation by increased cAMP levels. This inhibition can be overcome by pretreating the cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) for 48 hr. The effects on MBP gene expression modulated by TPA and cAMP involve altered DNA-protein interactions in the 5' end of the MBP promoter. The effect of TPA also appears to be mediated by down-regulation of protein kinase C.

Okadaic acid-sensitive protein phosphatases dephosphorylate MARCKS, a major protein kinase C substrate

The myristoylated alanine-rich C kinase substrate (MARCKS) undergoes a rapid and, in certain circumstances, transient increase in phosphorylation in response to stimuli that activate protein kinase C. We have investigated the protein-serine/threonine phosphatase activity responsible for reversing the phosphorylation of MARCKS. In cell-free assays, protein phosphatases 1, 2A and 2C (PP1, PP2A and PP2C) all dephosphorylate recombinant MARCKS or a synthetic peptide based on its phosphorylation site domain. In intact Swiss 3T3 cells, okadaic acid, a specific inhibitor of PP1 and PP2A, had little effect on MARCKS phosphorylation on its own, but largely prevented the dephosphorylation of MARCKS that occurred following activation of protein kinase C by bombesin with subsequent receptor blockade. These results indicate that although the dephosphorylation of MARCKS can be mediated by PP2C in vitro, this protein is dephosphorylated by okadaic acid-sensitive phosphatases in the intact cell.

Protein kinase C activity modulates myelin gene expression in enriched oligodendrocytes

Protein kinase C (PKC) and its potential role in myelin gene expression were investigated in primary cultured rat oligodendrocytes. The major myelin genes were expressed in a developmentally coordinated manner in cultured oligodendrocytes. PKC activity in these cells was similarly regulated with differential expression of PKC isozyme mRNAs. PKC-gamma mRNA was expressed transiently and was most abundant in 9-day cells in vitro. PKC-alpha and PKC-beta mRNAs were present at low levels throughout development in these cells, and their expression increased in 18-25 day cells. Immunocytochemical colocalization of PKC with oligodendrocyte-specific markers--O4, galactosyl cerebroside, MBP, and PLP--in enriched oligodendrocyte cultures suggested that the PKC enzyme activities assayed in these cultures were predominantly contributed by oligodendrocytes. PKC inhibition resulting from long-term exposure to 4 beta-phorbol-12,13-dibutyrate (4 beta-PDB) reduced steady-state levels of MBP, PLP, MAG, CNP, and PKC-alpha mRNAs, as detected by slot blots or in situ hybridization, and downregulated the oligodendrocyte-specific markers O4, galactosyl cerebroside, and the major constituent proteins MBP and PLP, as detected by immunocytochemistry. PKC-mediated downmodulation of myelin gene expression was most profound in normally differentiating oligodendrocytes at or before the onset of myelin protein synthesis. Six-day oligodendrocytes were most susceptible to such modulation. To elucidate the mechanism of reduction in various myelin gene messages upon modulation of PKC, we analyzed mRNA levels in oligodendrocytes, which were pretreated with either the transcriptional inhibitor actinomycin D or the protein synthesis blocker cycloheximide before exposure to 4 beta-PDB. Our results demonstrate that the PKC inhibition-mediated loss in myelin mRNA levels did not require the transcription of any genes, but appeared to be at least partially dependent on continuous protein synthesis.

Transient reversion of O4+ GalC- oligodendrocyte progenitor development in response to the phorbol ester TPA

The physiological importance of protein kinase C during oligodendrocyte progenitor maturation was investigated by analyzing the effects of the protein kinase C activator phorbol 12-myristate 13-acetate (TPA) on the morphology, proliferation, and differentiation of oligodendrocytes at sequential stages of development. Monoclonal antibodies A2B5 and O4 were used to identify the A2B5+O4- and the A2B5+O4+ galactocerebroside- progenitor stages. Anti-galactocerebroside and anti-myelin basic protein were used to identify mature, post-mitotic oligodendrocytes. Proliferation was measured by bromodeoxyuridine incorporation. Within 24 hr after addition, TPA induced a down-regulation of the O4 antigen in OL progenitors, and an increase of expression of the intermediate filament protein vimentin, leading to a phenotypic reversion from the vimentin-A2B5+O4+ phenotype to the less mature vimentin+A2B5+O4- stage. Concomitantly, TPA induced an increase in the number of bromodeoxyuridine-labeled oligodendrocyte progenitors and extensive process elongation. The response of O4+ progenitors was transient. Even with continued exposure to TPA, by 4 days after TPA addition the reverted cells ceased proliferation, reacquired O4 immunoreactivity, became vimentin-negative, and began to express galactocerebroside and myelin basic protein, and to display the complex, highly branched morphology characteristic of terminally differentiated oligodendrocytes. These results indicate that modulation of protein kinase C activity by TPA induces a transient reversion of O4+ progenitors to less mature O4- cells, causing a transient inhibition of terminal differentiation. The relationship of these data to similar responses of the OL lineage to specific growth factors and implications for remyelination after pathologic injury are discussed.

Cell proliferation and protooncogene induction in oligodendroglial progenitors

Cell proliferation and the expression of the protooncogenes c-fos and c-jun have been examined in the primary cultures of oligodendroglial (OL) progenitor cells in response to phorbol 12-myristate 13-acetate (PMA), serum, insulin, insulin-like growth factor-I (IGF-I), platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF). Combined [3H]thymidine autoradiography and immunocytochemistry was used to assess the mitogenic response of O4 (an oligodendrocyte-specific marker)-positive OL progenitors. In addition, the rate of DNA synthesis was measured by the incorporation of [3H]thymidine into acid-precipitable material. It was found that all of the agents tested stimulated DNA synthesis in OL progenitors and induced a rapid increase in c-fos and c-jun protooncogene expression. The induction of c-fos gene expression and DNA synthesis in response to PMA was completely blocked by 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), thereby suggesting a role for PKC in the control of c-fos expression and cell proliferation in OL progenitors.