Inhibiting stromal Class I HDACs curbs pancreatic cancer progression

Oncogenic lesions in pancreatic ductal adenocarcinoma (PDAC) hijack the epigenetic machinery in stromal components to establish a desmoplastic and therapeutic resistant tumor microenvironment (TME). Here we identify Class I histone deacetylases (HDACs) as key epigenetic factors facilitating the induction of pro-desmoplastic and pro-tumorigenic transcriptional programs in pancreatic stromal fibroblasts. Mechanistically, HDAC-mediated changes in chromatin architecture enable the activation of pro-desmoplastic programs directed by serum response factor (SRF) and forkhead box M1 (FOXM1). HDACs also coordinate fibroblast pro-inflammatory programs inducing leukemia inhibitory factor (LIF) expression, supporting paracrine pro-tumorigenic crosstalk. HDAC depletion in cancer-associated fibroblasts (CAFs) and treatment with the HDAC inhibitor entinostat (Ent) in PDAC mouse models reduce stromal activation and curb tumor progression. Notably, HDAC inhibition (HDACi) enriches a lipogenic fibroblast subpopulation, a potential precursor for myofibroblasts in the PDAC stroma. Overall, our study reveals the stromal targeting potential of HDACi, highlighting the utility of this epigenetic modulating approach in PDAC therapeutics.

Inhibiting Stromal Class I HDACs Curbs Pancreatic Cancer Progression

Oncogenic lesions in pancreatic ductal adenocarcinoma (PDAC) hijack the epigenetic machinery in stromal components to establish a desmoplastic and therapeutic resistant tumor microenvironment (TME). Here we identify Class I histone deacetylases (HDACs) as key epigenetic factors facilitating the induction of pro-desmoplastic and pro-tumorigenic transcriptional programs in pancreatic stromal fibroblasts. Mechanistically, HDAC-mediated changes in chromatin architecture enable the activation of pro-desmoplastic programs directed by serum response factor (SRF) and forkhead box M1 (FOXM1). HDACs also coordinate fibroblast pro-inflammatory programs inducing leukemia inhibitory factor (LIF) expression, supporting paracrine pro-tumorigenic crosstalk. HDAC depletion in cancer-associated fibroblasts (CAFs) and treatment with the HDAC inhibitor entinostat (Ent) in PDAC mouse models reduce stromal activation and curb tumor progression. Notably, HDAC inhibition (HDACi) enriches a lipogenic fibroblast subpopulation, a potential precursor for myofibroblasts in the PDAC stroma. Overall, our study reveals the stromal targeting potential of HDACi, highlighting the utility of this epigenetic modulating approach in PDAC therapeutics.

A super-enhancer-regulated RNA-binding protein cascade drives pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a lethal malignancy in need of new therapeutic options. Using unbiased analyses of super-enhancers (SEs) as sentinels of core genes involved in cell-specific function, here we uncover a druggable SE-mediated RNA-binding protein (RBP) cascade that supports PDAC growth through enhanced mRNA translation. This cascade is driven by a SE associated with the RBP heterogeneous nuclear ribonucleoprotein F, which stabilizes protein arginine methyltransferase 1 (PRMT1) to, in turn, control the translational mediator ubiquitin-associated protein 2-like. All three of these genes and the regulatory SE are essential for PDAC growth and coordinately regulated by the Myc oncogene. In line with this, modulation of the RBP network by PRMT1 inhibition reveals a unique vulnerability in Myc-high PDAC patient organoids and markedly reduces tumor growth in male mice. Our study highlights a functional link between epigenetic regulation and mRNA translation and identifies components that comprise unexpected therapeutic targets for PDAC.

Protein kinase C fusion proteins are paradoxically loss of function in cancer

Within the AGC kinase superfamily, gene fusions resulting from chromosomal rearrangements have been most frequently described for protein kinase C (PKC), with gene fragments encoding either the C-terminal catalytic domain or the N-terminal regulatory moiety fused to other genes. Kinase fusions that eliminate regulatory domains are typically gain of function and often oncogenic. However, several quality control pathways prevent accumulation of aberrant PKC, suggesting that PKC fusions may paradoxically be loss of function. To explore this topic, we used biochemical, cellular, and genome editing approaches to investigate the function of fusions that retain the portion of the gene encoding either the catalytic domain or regulatory domain of PKC. Overexpression studies revealed that PKC catalytic domain fusions were constitutively active but vulnerable to degradation. Genome editing of endogenous genes to generate a cancer-associated PKC fusion resulted in cells with detectable levels of fusion transcript but no detectable protein. Hence, PKC catalytic domain fusions are paradoxically loss of function as a result of their instability, preventing appreciable accumulation of protein in cells. Overexpression of a PKC regulatory domain fusion suppressed both basal and agonist-induced endogenous PKC activity, acting in a dominant-negative manner by competing for diacylglycerol. For both catalytic and regulatory domain fusions, the PKC component of the fusion proteins mediated the effects of the full-length fusions on the parameters examined, suggesting that the partner protein is dispensable in these contexts. Taken together, our findings reveal that PKC gene fusions are distinct from oncogenic fusions and present a mechanism by which loss of PKC function occurs in cancer.

Triptolide targets super-enhancer networks in pancreatic cancer cells and cancer-associated fibroblasts

The tumor microenvironment in pancreatic ductal adenocarcinoma (PDAC) is highly heterogeneous, fibrotic, and hypovascular, marked by extensive desmoplasia and maintained by the tumor cells, cancer-associated fibroblasts (CAFs) and other stromal cells. There is an urgent need to identify and develop treatment strategies that not only target the tumor cells but can also modulate the stromal cells. A growing number of studies implicate the role of regulatory DNA elements called super-enhancers (SE) in maintaining cell-type-specific gene expression networks in both normal and cancer cells. Using chromatin activation marks, we first mapped SE networks in pancreatic CAFs and epithelial tumor cells and found them to have distinct SE profiles. Next, we explored the role of triptolide (TPL), a natural compound with antitumor activity, in the context of modulating cell-type-specific SE signatures in PDAC. We found that TPL, cytotoxic to both pancreatic tumor cells and CAFs, disrupted SEs in a manner that resulted in the downregulation of SE-associated genes (e.g., BRD4, MYC, RNA Pol II, and Collagen 1) in both cell types at mRNA and protein levels. Our observations suggest that TPL acts as a SE interactive agent and may elicit its antitumor activity through SE disruption to re-program cellular cross talk and signaling in PDAC. Based on our findings, epigenetic reprogramming of transcriptional regulation using SE modulating compounds such as TPL may provide means for effective treatment options for pancreatic cancer patients.

Genomic and Epigenomic Landscaping Defines New Therapeutic Targets for Adenosquamous Carcinoma of the Pancreas

Adenosquamous cancer of the pancreas (ASCP) is a subtype of pancreatic cancer that has a worse prognosis and greater metastatic potential than the more common pancreatic ductal adenocarcinoma (PDAC) subtype. To distinguish the genomic landscape of ASCP and identify actionable targets for this lethal cancer, we applied DNA content flow cytometry to a series of 15 tumor samples including five patient-derived xenografts (PDX). We interrogated purified sorted tumor fractions from these samples with whole-genome copy-number variant (CNV), whole-exome sequencing, and Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq) analyses. These identified a variety of somatic genomic lesions targeting chromatin regulators in ASCP genomes that were superimposed on well-characterized genomic lesions including mutations in TP53 (87%) and KRAS (73%), amplification of MYC (47%), and homozygous deletion of CDKN2A (40%) that are common in PDACs. Furthermore, a comparison of ATAC-seq profiles of three ASCP and three PDAC genomes using flow-sorted PDX models identified genes with accessible chromatin unique to the ASCP genomes, including the lysine methyltransferase SMYD2 and the pancreatic cancer stem cell regulator RORC in all three ASCPs, and a FGFR1-ERLIN2 fusion associated with focal CNVs in both genes in a single ASCP. Finally, we demonstrate significant activity of a pan FGFR inhibitor against organoids derived from the FGFR1-ERLIN2 fusion-positive ASCP PDX model. Our results suggest that the genomic and epigenomic landscape of ASCP provide new strategies for targeting this aggressive subtype of pancreatic cancer. SIGNIFICANCE: These data provide a unique description of the ASCP genomic and epigenomic landscape and identify candidate therapeutic targets for this dismal cancer.

Abstract A29: Bicompartmental regulation of disease-related gene networks by histone deacetylase inhibition curbs pancreatic cancer progression

A challenge for effective pancreatic ductal adenocarcinoma (PDAC) therapy is to target the aggressive neoplasm that is shrouded by the dense fibrotic stroma. Epigenetic modulation has the potential to regulate genes or gene networks causing disease phenotypes. Here we show that the HDAC inhibitor entinostat (Ent) provides therapeutic benefits by regulating the gene networks important for PDAC in both the stromal and epithelial compartments. While an HDAC inhibitor is anticipated to broadly open chromatin and activate genes, in stromal pancreatic stellate cells (PSCs), Ent unexpectedly restricts chromatin accessibility on and represses the gene networks essential for PSC activation, leading to proliferation arrest and reduced myofibroblast properties, including lower production of extracellular matrix and inflammatory and protumorigenic cytokines. Furthermore, Ent also attenuates the transcriptional responses of cancer-associated fibroblasts (CAFs) to injury-related cytokines from the tumor microenvironment including TGF-beta and TNF-alpha. In the neoplastic epithelium, Ent induces cytostasis and de novo vulnerability in DNA repair pathways by systematically suppressing cell cycle and DNA repair genes. Notably, the bicompartmental effects of Ent were found to arrest cancer progression, prolong survival, and synergize with DNA damage-inducing agent cisplatin in genetically engineered mouse model (GEMM) of PDAC. Overall, our study not only establishes Ent as a novel therapeutic agent for PDAC but also reveals the mechanisms underlying its benefits.

Citation Format: Gaoyang Liang, Nasun Hah, Yu Shi, Morgan L. Truitt, Corina E. Antal, Annette R. Atkins, Cory Fraser, Ester Banayo, Senada Bashi, Yang Dai, Mara Sherman, Christopher Liddle, Ruth T. Yu, Tony Hunter, Haiyong Han, Daniel D. Von Hoff, Michael Downes, Ronald M. Evans. Bicompartmental regulation of disease-related gene networks by histone deacetylase inhibition curbs pancreatic cancer progression [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr A29.

Targeting LIF-mediated paracrine interaction for pancreatic cancer therapy and monitoring

Pancreatic ductal adenocarcinoma (PDAC) has a dismal prognosis largely owing to inefficient diagnosis and tenacious drug resistance. Activation of pancreatic stellate cells (PSCs) and consequent development of dense stroma are prominent features accounting for this aggressive biology^1,2. The reciprocal interplay between PSCs and pancreatic cancer cells (PCCs) not only enhances tumour progression and metastasis but also sustains their own activation, facilitating a vicious cycle to exacerbate tumorigenesis and drug resistance^3-7. Furthermore, PSC activation occurs very early during PDAC tumorigenesis^8-10, and activated PSCs comprise a substantial fraction of the tumour mass, providing a rich source of readily detectable factors. Therefore, we hypothesized that the communication between PSCs and PCCs could be an exploitable target to develop effective strategies for PDAC therapy and diagnosis. Here, starting with a systematic proteomic investigation of secreted disease mediators and underlying molecular mechanisms, we reveal that leukaemia inhibitory factor (LIF) is a key paracrine factor from activated PSCs acting on cancer cells. Both pharmacologic LIF blockade and genetic Lifr deletion markedly slow tumour progression and augment the efficacy of chemotherapy to prolong survival of PDAC mouse models, mainly by modulating cancer cell differentiation and epithelial-mesenchymal transition status. Moreover, in both mouse models and human PDAC, aberrant production of LIF in the pancreas is restricted to pathological conditions and correlates with PDAC pathogenesis, and changes in the levels of circulating LIF correlate well with tumour response to therapy. Collectively, these findings reveal a function of LIF in PDAC tumorigenesis, and suggest its translational potential as an attractive therapeutic target and circulating marker. Our studies underscore how a better understanding of cell-cell communication within the tumour microenvironment can suggest novel strategies for cancer therapy.

Gain-of-function mutations in protein kinase Cα (PKCα) may promote synaptic defects in Alzheimer's disease

Alzheimer's disease (AD) is a progressive dementia disorder characterized by synaptic degeneration and amyloid-β (Aβ) accumulation in the brain. Through whole-genome sequencing of 1345 individuals from 410 families with late-onset AD (LOAD), we identified three highly penetrant variants in PRKCA, the gene that encodes protein kinase Cα (PKCα), in five of the families. All three variants linked with LOAD displayed increased catalytic activity relative to wild-type PKCα as assessed in live-cell imaging experiments using a genetically encoded PKC activity reporter. Deleting PRKCA in mice or adding PKC antagonists to mouse hippocampal slices infected with a virus expressing the Aβ precursor CT100 revealed that PKCα was required for the reduced synaptic activity caused by Aβ. In PRKCA(-/-) neurons expressing CT100, introduction of PKCα, but not PKCα lacking a PDZ interaction moiety, rescued synaptic depression, suggesting that a scaffolding interaction bringing PKCα to the synapse is required for its mediation of the effects of Aβ. Thus, enhanced PKCα activity may contribute to AD, possibly by mediating the actions of Aβ on synapses. In contrast, reduced PKCα activity is implicated in cancer. Hence, these findings reinforce the importance of maintaining a careful balance in the activity of this enzyme.

Intramolecular C2 Domain-Mediated Autoinhibition of Protein Kinase C βII

The signaling output of protein kinase C (PKC) is exquisitely controlled, with its disruption resulting in pathophysiologies. Identifying the structural basis for autoinhibition is central to developing effective therapies for cancer, where PKC activity needs to be enhanced, or neurodegenerative diseases, where PKC activity should be inhibited. Here, we reinterpret a previously reported crystal structure of PKCβII and use docking and functional analysis to propose an alternative structure that is consistent with previous literature on PKC regulation. Mutagenesis of predicted contact residues establishes that the Ca(2+)-sensing C2 domain interacts intramolecularly with the kinase domain and the carboxyl-terminal tail, locking PKC in an inactive conformation. Ca(2+)-dependent bridging of the C2 domain to membranes provides the first step in activating PKC via conformational selection. Although the placement of the C1 domains remains to be determined, elucidation of the structural basis for autoinhibition of PKCβII unveils a unique direction for therapeutically targeting PKC.

Abstract 125: Protein kinase C loss-of-function mutations in cancer reveal role as tumor suppressor

Protein kinase C (PKC) remains an elusive chemotherapeutic target despite decades of research. To determine whether PKC isozymes function as oncogenes or tumor suppressors, we analyzed 8% of PKC mutations identified in human cancers. Surprisingly, the majority were loss-of-function and none were activating. Loss-of-function mutations were found in all PKC subgroups and acted by impeding 2nd messenger binding or preventing processing phosphorylations. Bioinformatic analysis revealed that PKC mutations might cooperate with co-occurring mutations in cancer drivers. Correction of a patient-identified, loss-of-function PKCβ mutation by CRISPR-mediated genome editing, in a colon cancer cell line, suppressed anchorage-independent growth and reduced tumor growth in xenograft models. Hemizygous deletion provided an anchorage-independent growth advantage, revealing PKC is haploinsufficient for tumor suppression. Several mutations were dominant-negative, suppressing global PKC signaling output. These data establish that PKC isozymes generally function as tumor suppressors, indicating that therapeutic strategies should focus on restoring PKC activity, not inhibiting it.

Citation Format: Corina E. Antal, Andrew M. Hudson, Emily Kang, Ciro Zanca, Christopher Wirth, Natalie L. Stephenson, Eleanor W. Trotter, Lisa L. Gallegos, Crispin Miller, Frank Furnari, Tony Hunter, John Brognard, Alexandra C. Newton. Protein kinase C loss-of-function mutations in cancer reveal role as tumor suppressor. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 125. doi:10.1158/1538-7445.AM2015-125

Cancer-associated protein kinase C mutations reveal kinase's role as tumor suppressor

Protein kinase C (PKC) isozymes have remained elusive cancer targets despite the unambiguous tumor promoting function of their potent ligands, phorbol esters, and the prevalence of their mutations. We analyzed 8% of PKC mutations identified in human cancers and found that, surprisingly, most were loss of function and none were activating. Loss-of-function mutations occurred in all PKC subgroups and impeded second-messenger binding, phosphorylation, or catalysis. Correction of a loss-of-function PKCβ mutation by CRISPR-mediated genome editing in a patient-derived colon cancer cell line suppressed anchorage-independent growth and reduced tumor growth in a xenograft model. Hemizygous deletion promoted anchorage-independent growth, revealing that PKCβ is haploinsufficient for tumor suppression. Several mutations were dominant negative, suppressing global PKC signaling output, and bioinformatic analysis suggested that PKC mutations cooperate with co-occurring mutations in cancer drivers. These data establish that PKC isozymes generally function as tumor suppressors, indicating that therapies should focus on restoring, not inhibiting, PKC activity.

Abstract 5270: Prevalence of inactivating protein kinase C mutations in human cancers

Protein kinase C (PKC) levels are aberrant in diverse cancers and over 400 mutations in PKC have been identified in human cancers. Here we characterized PKC mutations identified in human cancers to investigate whether PKCs function as oncogenes or tumor suppressors or whether their function is isozyme and/or context dependent. Live-cell imaging was used to determine how these mutations affect PKC translocation, subcellular localization, and activity. Cell-based assays were used to determine how PKC mutations alter proliferation, apoptosis, migration, and anchorage-independent growth. Of 37 mutations we examined thus far, 24 of them reduced or completely abolished PKC activity, while only 1 increased it. These mutations reside within the diacylglycerol-binding C1 domain, the Ca2+-binding C2 domain, or the kinase domain of conventional, novel, or atypical PKCs. The inactivating mutations ablate activity by either impeding Ca2+ or diacylglycerol binding, or by yielding a misfolded kinase. Our results suggest that PKC can act as a tumor suppressor in certain cancers. Understanding whether mutations are activating or inactivating in specific cancers would allow us to determine whether therapeutic strategies should target ways to inactivate or activate PKC in the respective cancers.

Citation Format: Corina E. Antal, Emily Kang, Natalie L. Stephenson, Eleanor W. Trotter, Tony Hunter, John Brognard, Alexandra Newton. Prevalence of inactivating protein kinase C mutations in human cancers. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5270. doi:10.1158/1538-7445.AM2014-5270

Intramolecular conformational changes optimize protein kinase C signaling

Optimal tuning of enzyme signaling is critical for cellular homeostasis. We use fluorescence resonance energy transfer reporters in live cells to follow conformational transitions that tune the affinity of a multidomain signal transducer, protein kinase C (PKC), for optimal response to second messengers. This enzyme comprises two diacylglycerol sensors, the C1A and C1B domains, that have a sufficiently high intrinsic affinity for ligand so that the enzyme would be in a ligand-engaged, active state if not for mechanisms that mask its domains. We show that both diacylglycerol sensors are exposed in newly synthesized PKC and that conformational transitions following priming phosphorylations mask the domains so that the lower affinity sensor, the C1B domain, is the primary diacylglycerol binder. The conformational rearrangements of PKC serve as a paradigm for how multimodule transducers optimize their dynamic range of signaling.

Spatiotemporal dynamics of phosphorylation in lipid second messenger signaling

The plasma membrane serves as a dynamic interface that relays information received at the cell surface into the cell. Lipid second messengers coordinate signaling on this platform by recruiting and activating kinases and phosphatases. Specifically, diacylglycerol and phosphatidylinositol 3,4,5-trisphosphate activate protein kinase C and Akt, respectively, which then phosphorylate target proteins to transduce downstream signaling. This review addresses how the spatiotemporal dynamics of protein kinase C and Akt signaling can be monitored using genetically encoded reporters and provides information on how the coordination of signaling at protein scaffolds or membrane microdomains affords fidelity and specificity in phosphorylation events.

Electrostatic and hydrophobic interactions differentially tune membrane binding kinetics of the C2 domain of protein kinase Cα

The cellular activation of conventional protein kinase C (PKC) isozymes is initiated by the binding of their C2 domains to membranes in response to elevations in intracellular Ca(2+). Following this C2 domain-mediated membrane recruitment, the C1 domain binds its membrane-embedded ligand diacylglycerol, resulting in activation of PKC. Here we explore the molecular mechanisms by which the C2 domain controls the initial step in the activation of PKC. Using stopped-flow fluorescence spectroscopy to measure association and dissociation rate constants, we show that hydrophobic interactions are the major driving force in the binding of the C2 domain to anionic membranes, whereas electrostatic interactions dominate in membrane retention. Specifically, mutation of select hydrophobic or select basic residues in the Ca(2+)-binding loops reduces membrane affinity by distinct mechanisms; mutation of hydrophobic residues primarily alters association rate constants, whereas mutation of charged residues affects dissociation rate constants. Live cell imaging reveals that introduction of these mutations into full-length PKCα not only reduces the Ca(2+)-dependent translocation to plasma membrane but, by impairing the plasma membrane-sensing role of the C2 domain, causes phorbol ester-triggered redistribution of PKCα to other membranes, such as the Golgi. These data underscore the key role of the C2 domain in driving conventional PKC isozymes to the plasma membrane and reveal that not only the amplitude but also the subcellular location of conventional PKC signaling can be tuned by altering the affinity of this module for membranes.

Euphohelioscopin A is a PKC activator capable of inducing macrophage differentiation

To identify small molecules that selectively control hematopoietic stem cell differentiation, we performed an unbiased screen using primary human CD34(+) cells. We identified a plant-derived natural product, euphohelioscopin A, capable of selectively differentiating CD34(+) cells down the granulocyte/monocytic lineage. Euphohelioscopin A also inhibits proliferation and induces differentiation of the myeloid leukemia cell lines THP-1 and HL-60. Mechanistic studies revealed that euphohelioscopin A is an activator of protein kinase C (PKC), and that the promonocytic effects of this natural product are mediated by PKC activation. In addition to shedding insights into normal hematopoiesis, this work may ultimately facilitate the application of stem cell therapies to a host of myeloid dysfunctions.

The effect of short-term hypoxic exposure on metabolic gene expression

The long-term effect of hypoxia is to decrease both the production and use of ATP and thus decrease the reliance on mitochondrial oxidative energy production. Yet, recent studies include more immediate affects of hypoxia on gene expression and these data suggest the maintenance of mitochondrial function. To better understand the short-term physiological response to hypoxia, we quantified metabolic mRNA expression in the heart ventricles and livers of the teleost fish Fundulus grandis exposed to partial oxygen pressure of 2.8 kPa (-13.5% air saturation).Twenty-eight individuals from a single population were exposed to hypoxia for 0, 4, 8, 12, 24, 48, and 96  hr. Liver and cardiac tissues were sampled from the same individuals at 0-48  hr. At 96  hr, only cardiac tissue was assayed. Gene expression was significantly different (ANOVA, P < 0.05) for 17 of 226 metabolic genes (7.5%) in cardiac tissue and for 20 of 256 (7.8%) metabolic genes in hepatic tissue. For the two tissues examined in this study, the maximum response occurred at different times. For cardiac tissue, using Dunnett's post hoc test, most of these significant differences occurred at 96  hr of exposure. For liver, all but one significant difference occurred at 4  hr. Surprisingly, too many (relative to random expectations) of the genes with significant increase in mRNA are involved in the oxidative phosphorylation pathway: 44% of the significant genes at 96  hr in the heart and 33% of the significant genes at 4  hr in the liver are involved in the oxidative phosphorylation pathway. These data indicate that there are tissue-specific differences in the timing of the response to hypoxia, yet both cardiac and hepatic tissues have increases in mRNA that code for enzyme in the oxidative phosphorylation pathway. If these changes in mRNA produce a similar change in protein, then these data suggest that the initial response to hypoxia involves an increase in the oxidative pathway potentially as a mechanism to maintain ATP production.

Active site inhibitors protect protein kinase C from dephosphorylation and stabilize its mature form

Conformational changes acutely control protein kinase C (PKC). We have previously shown that the autoinhibitory pseudosubstrate must be removed from the active site in order for 1) PKC to be phosphorylated by its upstream kinase phosphoinositide-dependent kinase 1 (PDK-1), 2) the mature enzyme to bind and phosphorylate substrates, and 3) the mature enzyme to be dephosphorylated by phosphatases. Here we show an additional level of conformational control; binding of active site inhibitors locks PKC in a conformation in which the priming phosphorylation sites are resistant to dephosphorylation. Using homogeneously pure PKC, we show that the active site inhibitor Gö 6983 prevents the dephosphorylation by pure protein phosphatase 1 (PP1) or the hydrophobic motif phosphatase, pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP). Consistent with results using pure proteins, treatment of cells with the competitive inhibitors Gö 6983 or bisindolylmaleimide I, but not the uncompetitive inhibitor bisindolylmaleimide IV, prevents the dephosphorylation and down-regulation of PKC induced by phorbol esters. Pulse-chase analyses reveal that active site inhibitors do not affect the net rate of priming phosphorylations of PKC; rather, they inhibit the dephosphorylation triggered by phorbol esters. These data provide a molecular explanation for the recent studies showing that active site inhibitors stabilize the phosphorylation state of protein kinases B/Akt and C.