The role of protein kinase-C theta in control of epithelial to mesenchymal transition and cancer stem cell formation

Chronic Stress: Impacts on Tumor Microenvironment and Implications for Anti-Cancer Treatments

Chronic stress is common among cancer patients due to the psychological, operative, or pharmaceutical stressors at the time of diagnosis or during the treatment of cancers. The continuous activations of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS), as results of chronic stress, have been demonstrated to take part in several cancer-promoting processes, such as tumorigenesis, progression, metastasis, and multi-drug resistance, by altering the tumor microenvironment (TME). Stressed TME is generally characterized by the increased proportion of cancer-promoting cells and cytokines, the reduction and malfunction of immune-supportive cells and cytokines, augmented angiogenesis, enhanced epithelial-mesenchymal transition, and damaged extracellular matrix. For the negative effects that these alterations can cause in terms of the efficacies of anti-cancer treatments and prognosis of patients, supplementary pharmacological or psychotherapeutic strategies targeting HPA, SNS, or psychological stress may be effective in improving the prognosis of cancer patients. Here, we review the characteristics and mechanisms of TME alterations under chronic stress, their influences on anti-cancer therapies, and accessory interventions and therapies for stressed cancer patients.

The Emerging Function of PKCtheta in Cancer

Protein Kinase C theta (PKCθ) is a serine/threonine kinase that belongs to the novel PKC subfamily. In normal tissue, its expression is restricted to skeletal muscle cells, platelets and T lymphocytes in which PKCθ controls several essential cellular processes such as survival, proliferation and differentiation. Particularly, PKCθ has been extensively studied for its role in the immune system where its translocation to the immunological synapse plays a critical role in T cell activation. Beyond its physiological role in immune responses, increasing evidence implicates PKCθ in the pathology of various diseases, especially autoimmune disorders and cancers. In this review, we discuss the implication of PKCθ in various types of cancers and the PKCθ-mediated signaling events controlling cancer initiation and progression. In these types of cancers, the high PKCθ expression leads to aberrant cell proliferation, migration and invasion resulting in malignant phenotype. The recent development and application of PKCθ inhibitors in the context of autoimmune diseases could benefit the emergence of treatment for cancers in which PKCθ has been implicated.

Potentials of PKC in Cancer Progression and Anticancer Drug Development

PKC is a family of serine-threonine kinases which play crucial roles in the regulation of important signal transduction pathways in mammalian cell-biology. These enzymes are themselves regulated by various molecules that can serve as ligands to the regulatory domains and translocate PKC to membrane for activity. The role of PKC in the modulation of both proliferative and apoptotic signaling in cancer has become a subject of immense interest after it was discovered that PKC regulates a myriad of enzymes and transcription factors involved in carcinogenic signaling. Therefore, PKC has served as an attractive target for the development of newer generation of anti-cancer drugs. The following review discusses the potential of PKC to be regarded as a target for anti-cancer therapy. We also review all the molecules that have been discovered so far to be regulators/activators/inhibitors of PKC and also how far these molecules can be considered as potential candidates for anti-cancer drug development based on PKC.

Mammalian target of rapamycin inhibitors, temsirolimus and torin 1, attenuate stemness-associated properties and expression of mesenchymal markers promoted by phorbol-myristate-acetate and oncostatin-M in glioblastoma cells

The phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin signaling pathway is crucial for tumor survival, proliferation, and progression, making it an attractive target for therapeutic intervention. In glioblastoma, activated mammalian target of rapamycin promotes invasive phenotype and correlates with poor patient survival. A wide range of mammalian target of rapamycin inhibitors are currently being evaluated for cytotoxicity and anti-proliferative activity in various tumor types but are not explored sufficiently for controlling tumor invasion and recurrence. We recently reported that mammalian target of rapamycin inhibitors-rapamycin, temsirolimus, torin 1, and PP242-suppressed invasion and migration promoted by tumor necrosis factor-alpha and phorbol-myristate-acetate in glioblastoma cells. As aggressive invasion and migration of tumors are associated with mesenchymal and stem-like cell properties, this study aimed to examine the effect of mammalian target of rapamycin inhibitors on these features in glioblastoma cells. We demonstrate that temsirolimus and torin 1 effectively reduced the constitutive as well as phorbol-myristate-acetate/oncostatin-M-induced expression of mesenchymal markers (fibronectin, vimentin, and YKL40) and neural stem cell markers (Sox2, Oct4, nestin, and mushashi1). The inhibitors significantly abrogated the neurosphere-forming capacity induced by phorbol-myristate-acetate and oncostatin-M. Furthermore, we demonstrate that the drugs dephosphorylated signal transducer and activator transcription factor 3, a major regulator of mesenchymal and neural stem cell markers implicating the role of signal transducer and activator transcription factor 3 in the inhibitory action of these drugs. The findings demonstrate the potential of mammalian target of rapamycin inhibitors as "stemness-inhibiting drugs" and a promising therapeutic approach to target glioma stem cells.

PRKCQ promotes oncogenic growth and anoikis resistance of a subset of triple-negative breast cancer cells

Background

The protein kinase C (PKC) family comprises distinct classes of proteins, many of which are implicated in diverse cellular functions. Protein tyrosine kinase C theta isoform (PRKCQ)/PKCθ, a member of the novel PKC family, may have a distinct isoform-specific role in breast cancer. PKCθ is preferentially expressed in triple-negative breast cancer (TNBC) compared to other breast tumor subtypes. We hypothesized that PRKCQ/PKCθ critically regulates growth and survival of a subset of TNBC cells.

Methods

To elucidate the role of PRKCQ/PKCθ in regulating growth and anoikis resistance, we used both gain and loss of function to modulate expression of PRKCQ. We enhanced expression of PKCθ (kinase-active or inactive) in non-transformed breast epithelial cells (MCF-10A) and assessed effects on epidermal growth factor (EGF)-independent growth, anoikis, and migration. We downregulated expression of PKCθ in TNBC cells, and determined effects on in vitro and in vivo growth and survival. TNBC cells were also treated with a small molecule inhibitor to assess requirement for PKCθ kinase activity in the growth of TNBC cells.

Results

PRKCQ/PKCθ can promote oncogenic phenotypes when expressed in non-transformed MCF-10A mammary epithelial cells; PRKCQ/PKCθ enhances anchorage-independent survival, growth-factor-independent proliferation, and migration. PKCθ expression promotes retinoblastoma (Rb) phosphorylation and cell-cycle progression under growth factor-deprived conditions that typically induce cell-cycle arrest of MCF-10A breast epithelial cells. Proliferation and Rb phosphorylation are dependent on PKCθ-stimulated extracellular signal-related kinase (Erk)/mitogen-activated protein kinase (MAPK) activity. Enhanced Erk/MAPK activity is dependent on the kinase activity of PKCθ, as overexpression of kinase-inactive PKCθ does not stimulate Erk/MAPK or Rb phosphorylation or promote growth-factor-independent proliferation. Downregulation of PRKCQ/PKCθ in TNBC cells enhances anoikis, inhibits growth in 3-D Matrigel^TM cultures, and impairs triple-negative tumor xenograft growth. AEB071, an inhibitor of PKCθ kinase activity, also inhibits growth and invasive branching of TNBC cells in 3-D cultures, further supporting a role for PKCθ kinase activity in triple-negative cancer cell growth.

Conclusions

Enhanced PRKCQ/PKCθ expression can promote growth-factor-independent growth, anoikis resistance, and migration. PRKCQ critically regulates growth and survival of a subset of TNBC. Inhibition of PKCθ kinase activity may be an attractive therapeutic approach for TNBC, a subtype in need of improved targeted therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-016-0749-6) contains supplementary material, which is available to authorized users.

Identification of chromatin accessibility domains in human breast cancer stem cells

Epithelial-to-mesenchymal transition (EMT) is physiological in embryogenesis and wound healing but also associated with the formation of cancer stem cells (CSCs). Many EMT signaling pathways are implicated in CSC formation, but the precise underlying mechanisms of CSC formation remain elusive. We have previously demonstrated that PKC is critical for EMT induction and CSC formation in inducible breast EMT/CSC models. Here, we used formaldehyde-assisted isolation of regulatory elements-sequencing (FAIRE-seq) to investigate DNA accessibility changes after PKC activation and determine how they influence EMT and CSC formation. During EMT, DNA accessibility principally increased in regions distant from transcription start sites, low in CpG content, and enriched with chromatin enhancer marks. ChIP-sequencing revealed that a subset of these regions changed from poised to active enhancers upon stimulation, with some even more acteylated in CSCs. While regions with increased accessibility were enriched for FOX, AP-1, TEAD, and TFAP2 motifs, those containing FOX and AP-1 motif were associated with increased expression of CSC-associated genes, while those with TFAP2 were associated with genes with increased expression in non-CSCs. Silencing of 2 members of the FOX family, FOXN2 and FOXQ1, repressed CSCs and the mesenchymal phenotype and inhibited the CSC gene signature. These novel, PKC-induced DNA accessibility regions help explain how the epigenomic plasticity of cells undergoing EMT leads to CSC gene activation.

Differential Roles for DUSP Family Members in Epithelial-to-Mesenchymal Transition and Cancer Stem Cell Regulation in Breast Cancer

Dual-specificity phosphatases (DUSPs) dephosphorylate threonine/serine and tyrosine residues on their substrates. Here we show that DUSP1, DUSP4, and DUSP6 are involved in epithelial-to-mesenchymal transition (EMT) and breast cancer stem cell (CSC) regulation. DUSP1, DUSP4, and DUSP6 are induced during EMT in a PKC pathway signal-mediated EMT model. We show for the first time that the key chromatin-associated kinase PKC-θ directly regulates a subset of DUSP family members. DUSP1, DUSP4, and DUSP6 globally but differentially co-exist with enhancer and permissive active histone post-translational modifications, suggesting that they play distinct roles in gene regulation in EMT/CSCs. We show that nuclear DUSP4 associates with the key acetyltransferase p300 in the context of the chromatin template and dynamically regulates the interplay between two key phosphorylation marks: the 1834 (active) and 89 (inhibitory) residues central to p300’s acetyltransferase activity. Furthermore, knockdown with small-interfering RNAs (siRNAs) shows that DUSP4 is required for maintaining H3K27ac, a mark mediated by p300. DUSP1, DUSP4, and DUSP6 knockdown with siRNAs shows that they participate in the formation of CD44^hi/CD24^lo/EpCAM⁺ breast CSCs: DUSP1 knockdown reduces CSC formation, while DUSP4 and DUSP6 knockdown enhance CSC formation. Moreover, DUSP6 is overexpressed in patient-derived HER2⁺ breast carcinomas compared to benign mammary tissue. Taken together, these findings illustrate novel pleiotropic roles for DUSP family members in EMT and CSC regulation in breast cancer.