Protein kinase C-betaII represses hepatocyte growth factor-induced invasion by preventing the association of adapter protein Gab1 and phosphatidylinositol 3-kinase in melanoma cells

Targeting GPCRs and Their Signaling as a Therapeutic Option in Melanoma

Simple Summary

Sixteen G-protein-coupled receptors (GPCRs) have been involved in melanogenesis or melanomagenesis. Here, we review these GPCRs, their associated signaling, and therapies.

Abstract

G-protein-coupled receptors (GPCRs) serve prominent roles in melanocyte lineage physiology, with an impact at all stages of development, as well as on mature melanocyte functions. GPCR ligands are present in the skin and regulate melanocyte homeostasis, including pigmentation. The role of GPCRs in the regulation of pigmentation and, consequently, protection against external aggression, such as ultraviolet radiation, has long been established. However, evidence of new functions of GPCRs directly in melanomagenesis has been highlighted in recent years. GPCRs are coupled, through their intracellular domains, to heterotrimeric G-proteins, which induce cellular signaling through various pathways. Such signaling modulates numerous essential cellular processes that occur during melanomagenesis, including proliferation and migration. GPCR-associated signaling in melanoma can be activated by the binding of paracrine factors to their receptors or directly by activating mutations. In this review, we present melanoma-associated alterations of GPCRs and their downstream signaling and discuss the various preclinical models used to evaluate new therapeutic approaches against GPCR activity in melanoma. Recent striking advances in our understanding of the structure, function, and regulation of GPCRs will undoubtedly broaden melanoma treatment options in the future.

Topography of Protein Kinase C βII in Benign and Malignant Melanocytic Lesions

BACKGROUND

Protein kinase C βII promotes melanogenesis and affects proliferation of melanocytic cells but is frequently absent or decreased in melanoma cells in vitro.

OBJECTIVE

To investigate PKC-βII expression and spatial distribution within a lesion in various benign and malignant melanocytic proliferations.

METHODS

Expression of PKC-βII was semiquantitatively assessed in the various existing compartments (intraepidermal [not nested], junctional [nested], and dermal) of benign (n = 43) and malignant (n = 28) melanocytic lesions by immunohistochemistry.

RESULTS

Melanocytes in the basal layer of normal skin or in lentigo simplex stained strongly for PKC-βII. Common nevi lacked completely PKC-βII. All other lesions expressed variably PKC-βII, with cutaneous melanoma metastases displaying the lowest rate of positivity (14%). In the topographical analysis within a lesion, PKC-βII expression was largely retained in the intraepidermal and junctional part of all other lesions (dysplastic nevus, lentigo maligna, and melanoma). Reduced expression of PKC-βII was found in the dermal component of benign and malignant lesions ( P = .041 vs intraepidermal). PKC-βII expression in the various compartments did not differ significantly between benign and malignant lesions.

CONCLUSIONS

The current study revealed a significant correlation between PKC-βII expression and spatial localization of melanocytes, with the lowest expression found in the dermal compartment and the highest in the epidermal compartment.

Expression of Gab1 Is Associated with Poor Prognosis of Patients with Epithelial Ovarian Cancer

Growth factor receptor-bound protein-2 (Grb2) can act as the scaffold protein recruiting other molecules to the stimulated receptors. Grb2-associated binding protein 1 (Gab1) is involved in cell proliferation, and its expression may enhance the carcinogenesis and cancer progression. However, the function of Gab1 remains to be investigated. Epithelial ovarian cancer (EOC) is the most lethal malignancy in the female reproductive system with increasing incidence and unsatisfied overall survival (OS). We investigated the expression of Gab1 in EOC tissues and the correlations between Gab1 expression and the clinicopathological characteristics of patients with EOC using Spearman rank test. The staining results were evaluated based on both the percentage of Gab1-positive tumor cells and the staining intensity for Gab1 expression. Kaplan-Meier survival analysis and Cox proportional hazards analysis were used to compare the postoperative OS between EOC patients with high Gab1 expression and those with low Gab1 expression. The high expression of Gab1 was positively correlated with advanced FIGO stage and lymph node metastasis of EOC. Univariate analysis showed that advanced FIGO stage, pathological grade, lymph node metastasis or Gab1 expression were associated with poor OS. Moreover, multivariate analysis revealed that Gab1 expression could be an independent prognostic factor for the poor OS of EOC patients (P = 0.042). We propose that Gab1 expression is correlated with poor prognosis of EOC patients and may act as an independent prognostic indicator.

MicroRNA-200a Suppresses Cell Invasion and Migration by Directly Targeting GAB1 in Hepatocellular Carcinoma

MicroRNA-200a (miR-200a) is frequently downregulated in most cancer types and plays an important role in carcinogenesis and cancer progression. In this study, we determined that miR-200a was downregulated in hepatocellular carcinoma (HCC) tissues and cell lines, consistent with the results of our previous study. Because a previous study suggested that downregulation of miR-200a is correlated with HCC metastasis, we aimed to elucidate the mechanism underlying the role of miR-200a in metastasis in HCC. Here we observed that overexpression of miR-200a resulted in suppression of HCC metastatic ability, including HCC cell migration, invasion, and metastasis, in vitro and in vivo. Furthermore, bioinformatics and luciferase reporter assays indicated that GAB1 is a direct target of miR-200a. Inhibition of GAB1 resulted in substantially decreased cell invasion and migration similar to that observed with overexpression of miR-200a in HCC cell lines, whereas restoration of GAB1 partially rescued the inhibitory effects of miR-200a. Taken together, these data provide novel information for comprehending the tumor-suppressive role of miR-200a in HCC pathogenesis through inhibition of GAB1 translation.

MicroRNA-409-3p suppresses colorectal cancer invasion and metastasis partly by targeting GAB1 expression

Colorectal cancer (CRC) is one of the most common cancers worldwide and its metastasis accounts for the majority of deaths. However, the molecular mechanisms underlying CRC progression are not well characterized. In this study, we identified miR-409-3p as a tumor suppressor of CRC. MiR-409-3p expression was significantly downregulated in CRC tissue compared to adjacent non-tumor tissue, and reduced miR-409-3p expression was correlated with CRC metastasis. In vitro and in vivo studies revealed that miR-409-3p negatively regulated CRC metastatic capacities, including suppressing cancer cell migration, invasion and metastasis. To explore the mechanism of action of miR-409-3p, we adopted a pathway and pathophysiological event-based target screening and validation approach, and found nine known metastasis-related genes as potential targets. The 3'-UTR binding assays between the candidates and miR-409-3p suggested that only GAB1, NR4A2 and LMO4 were directly regulated by the miRNA. However, endogenous expression analysis revealed that only GAB1 was modulated by miR-409-3p in CRC cells at both the mRNA and protein levels. Furthermore, we provided evidence to conclude that GAB1 was partially responsible for miR-409-3p-mediated metastasis. Taken together, our data demonstrate that miR-409-3p is a metastatic suppressor, and post-transcriptional inhibition of the oncoprotein GAB1 is one of the mechanisms of action of this miRNA. Our finding suggests miR-409-3p might be a novel target for CRC metastasis treatment.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

Down-regulation of Gab1 inhibits cell proliferation and migration in hilar cholangiocarcinoma

Hilar cholangiocarcinoma is a highly aggressive malignancy originating from the hilar biliary duct epithelium. Due to few effective comprehensive treatments, the prognosis of hilar cholangiocarcinoma is poor. In this study, immunohistochemistry was first used to detect and analyze the expression of Gab1, VEGFR-2, and MMP-9 in hilar cholangiocarcinoma solid tumors and the relationships to the clinical pathological features. Furthermore, Gab1 and VEGFR-2 siRNA were used to interfere the hilar cholangiocarcinoma cell line ICBD-1 and then detect the PI3K/Akt signaling pathway, MMP-9 levels and malignant biological behaviors of tumor cells. The data showed that 1. Gab1, VEGFR-2, and MMP-9 were highly expressed and positively correlated with each other in hilar cholangiocarcinoma tissues, which were related to lymph node metastasis and differentiation. 2. After Gab1 or VEGFR-2 siRNA interference, PI3K/Akt pathway activity and MMP-9 levels were decreased in ICBD-1 cells. At the same time, cell proliferation decreased, cell cycle arrested in G1 phase, apoptosis increased and invasion decreased. These results suggest that the expression of Gab1, VEGFR-2, and MMP-9 are significantly related to the malignant biological behavior of hilar cholangiocarcinoma. Gab1 regulates growth, apoptosis and invasion through the VEGFR-2/Gab1/PI3K/Akt signaling pathway in hilar cholangiocarcinoma cells and influences the invasion of tumor cells via MMP-9.

Specifying protein kinase C functions in melanoma

The protein kinase C (PKC) family of serine/threonine protein kinases is a heterogeneous group of enzymes receiving and integrating signals involved in both normal melanocyte biology and melanoma pathology. Alterations in PKC enzyme expression and activation contribute to the malignant phenotype of melanoma in both oncogenic and tumor suppressive roles. Delineating the diverse and often context-dependent functions of PKC enzymes in melanocyte/melanoma biology is key to capitalize on these kinases as drug targets. This review summarizes several of the diverse functions of PKC in melanocyte and melanoma biology with a focus on PKC enzyme regulation and function.

Role and regulation of STAT3 phosphorylation at Ser727 in melanocytes and melanoma cells

The transcription factor signal transducer and activator of transcription 3 (STAT3) has two important phosphorylation sites, Tyr705 and Ser727, for its activation. Ser727 phosphorylation has been considered to be a secondary event after Tyr705 phosphorylation. In this study, the role and regulation of Ser727 phosphorylation in STAT3 in melanocytic cells were examined. STAT3 was phosphorylated on Ser727 in the absence of Tyr705 phosphorylation in melanocytes. 12-O-tetradecanoylphorbol-13-acetate-induced increase in cell survival activity and nuclear translocation of STAT3 was associated with Ser727 phosphorylation. Ser727 was constitutively phosphorylated in all melanoma cell lines examined irrespective of Tyr705 phosphorylation. The possible involvement of Ser727 phosphorylation in STAT3 in cell survival activity and nuclear translocation of STAT3 in melanocytes was demonstrated also in melanoma cells. The constitutive Ser727 phosphorylation in melanoma cells was partially mediated by the B-Raf-MEK-ERK1/2 pathway. Immunohistochemical studies on specimens of primary lesions of acral lentiginous melanoma revealed that Ser727 phosphorylation precedes Tyr705 phosphorylation in the early stages of melanoma progression. Our results indicate that Ser727 phosphorylation on STAT3 is not necessarily a secondary event after Tyr705 phosphorylation and suggest that it has a role in the regulation of cell survival activity and nuclear translocation of STAT3 in melanocytic cells.

Hepatocyte growth factor-induced c-Src-phosphatidylinositol 3-kinase-AKT-mammalian target of rapamycin pathway inhibits dendritic cell activation by blocking IκB kinase activity

Hepatocyte growth factor modulates activation and antigen-presenting cell function of dendritic cells. However, the molecular basis for immunoregulation of dendritic cells by hepatocyte growth factor is undefined. In the current study, we demonstrate that hepatocyte growth factor exhibits inhibitory effect on dendritic cell activation by blocking IκB kinase activity and subsequent nuclear factor-κB activation. Inhibition of IκB kinase is mediated by hepatocyte growth factor-induced activation of c-Src. Proximal signaling events induced in dendritic cells by hepatocyte growth factor include a physical association of c-Src with the hepatocyte growth factor receptor c-MET and concomitant activation of c-Src. Activation of c-Src in turn establishes a complex consisting of phosphatidylinositol 3-kinase and c-MET, and promotes downstream activation of the phosphatidylinositol 3-kinase/AKT pathway and mammalian target of rapamycin. Blocking activation of c-Src, phosphatidylinositol 3-kinase and mammalian target of rapamycin prevents hepatocyte growth factor-induced inhibition of IκB kinase, nuclear factor-κB and dendritic cell activation. Notably, hepatocyte growth factor-stimulated c-Src activation results in induction of phosphatidylinositol 3-kinase complexes p85α/p110α and p85α/p110δ, which is required for activation of mammalian target of rapamycin, and consequent inhibition of IκB kinase and nuclear factor-κB activation. Our findings, for the first time, have identified the c-Src-phosphatidylinositol 3-kinase-AKT-mammalian target of rapamycin pathway that plays a pivotal role in mediating the inhibitory effects of hepatocyte growth factor on dendritic cell activation by blocking nuclear factor-κB signaling.

Functional alterations in protein kinase C beta II expression in melanoma

Protein kinase C (PKC) is a heterogeneous family of serine/threonine protein kinases that have different biological effects in normal and neoplastic melanocytes (MCs). To explore the mechanism behind their differential response to PKC activation, we analyzed the expression profile of all nine PKC isoforms in normal human MCs, HPV16 E6/E7 immortalized MCs, and a panel of melanoma cell lines. We found reduced PKCbeta and increased PKCzeta and PKCiota expression at both the protein and mRNA levels in immortalized MCs and melanoma lines. We focused on PKCbeta as it has been functionally linked to melanin production and oxidative stress response. Re-expression of PKCbeta in melanoma cells inhibited colony formation in soft agar, indicating that PKCbeta loss in melanoma is important for melanoma growth. PKCbetaII, but not PKCbetaI, was localized to the mitochondria, and inhibition of PKCbeta significantly reduced UV-induced reactive oxygen species (ROS) in MCs with high PKCbeta expression. Thus alterations in PKCbeta expression in melanoma contribute to their neoplastic phenotype, possibly by reducing oxidative stress, and may constitute a selective therapeutic target.