Mitogenic Signaling by the gep Oncogene Involves the Upregulation of S-Phase Kinase-Associated Protein 2

Lysophosphatidic Acid Mediates Imiquimod-Induced Psoriasis-like Symptoms by Promoting Keratinocyte Proliferation through LPAR1/ROCK2/PI3K/AKT Signaling Pathway

Psoriasis is a chronic inflammatory skin disease. Recently, lysophosphatidic acid (LPA)/LPAR5 signaling has been reported to be involved in both NLRP3 inflammasome activation in macrophages and keratinocyte activation to produce inflammatory cytokines, contributing to psoriasis pathogenesis. However, the effect and molecular mechanisms of LPA/LPAR signaling in keratinocyte proliferation in psoriasis remain unclear. In this study, we investigated the effects of LPAR1/3 inhibition on imiquimod (IMQ)-induced psoriasis-like mice. Treatment with the LPAR1/3 antagonist, ki16425, alleviated skin symptoms in IMQ-induced psoriasis-like mouse models and decreased keratinocyte proliferation in the lesion. It also decreased LPA-induced cell proliferation and cell cycle progression via increased cyclin A2, cyclin D1, cyclin-dependent kinase (CDK)2, and CDK4 expression and decreased p27^Kip1 expression in HaCaT cells. LPAR1 knockdown in HaCaT cells reduced LPA-induced proliferation, suppressed cyclin A2 and CDK2 expression, and restored p27^Kip1 expression. LPA increased Rho-associated protein kinase 2 (ROCK2) expression and PI3K/AKT activation; moreover, the pharmacological inhibition of ROCK2 and PI3K/AKT signaling suppressed LPA-induced cell cycle progression. In conclusion, we demonstrated that LPAR1/3 antagonist alleviates IMQ-induced psoriasis-like symptoms in mice, and in particular, LPAR1 signaling is involved in cell cycle progression via ROCK2/PI3K/AKT pathways in keratinocytes.

Identification of GNA12-driven gene signatures and key signaling networks in ovarian cancer

With the focus on defining the oncogenic network stimulated by lysophosphatidic acid (LPA) in ovarian cancer, the present study sought to interrogate the oncotranscriptome regulated by the LPA-mediated signaling pathway. LPA, LPA-receptor (LPAR) and LPAR-activated G protein 12 α-subunit, encoded by G protein subunit α 12 (GNA12), all serve an important role in ovarian cancer progression. While the general signaling mechanism regulated by LPA/LPAR/GNA12 has previously been characterized, the global transcriptomic network regulated by GNA12 in ovarian cancer pathophysiology remains largely unknown. To define the LPA/LPAR/GNA12-orchestrated oncogenic networks in ovarian cancer, transcriptomic and bioinformatical analyses were conducted using SKOV3 cells, in which the expression of GNA12 was silenced. Array analysis was performed in Agilent SurePrint G3 Human Comparative Genomic Hybridization 8×60 microarray platform. The array results were validated using Kuramochi cells. Gene and functional enrichment analyses were performed using Database for Annotation, Visualization and Integrated Discovery, Search Tool for Retrieval of Interacting Genes and Cytoscape algorithms. The results indicated a paradigm in which GNA12 drove ovarian cancer progression by upregulating a pro-tumorigenic network with AKT1, VEGFA, TGFB1, BCL2L1, STAT3, insulin-like growth factor 1 and growth hormone releasing hormone as critical hub and/or bottleneck nodes. Moreover, GNA12 downregulated a growth-suppressive network involving proteasome 20S subunit (PSM) β6, PSM α6, PSM ATPase 5, ubiquitin conjugating enzyme E2 E1, PSM non-ATPase 10, NDUFA4 mitochondrial complex-associated, NADH:ubiquinone oxidoreductase subunit B8 and anaphase promoting complex subunit 1 as hub or bottleneck nodes. In addition to providing novel insights into the LPA/LPAR/GNA12-regulated oncogenic networks in ovarian cancer, the present study identified several potential nodes in this network that could be assessed for targeted therapy.

Determinant role for the gep oncogenes, Gα12/13, in ovarian cancer cell proliferation and xenograft tumor growth

Recent studies have shown that the gip2 and gep oncogenes defined by the α-subunits of Gi2 and G12 family of G proteins, namely Gαi2 and Gα12/13, stimulate oncogenic signaling pathways in cancer cells including those derived from ovarian cancer. However, the critical α-subunit involved in ovarian cancer growth and progression in vivo remains to be identified. Using SKOV3 cells in which the expressions of individual Gα-subunits were silenced, we demonstrate that the silencing of Gα12 and Gα13 drastically attenuated serum- or lysophosphatidic acid-stimulated proliferation. In contrast, the invasive migration of these cells were reduced only by the silencing of Gαi2 or Gα13. Analyses of the xenograft tumors derived from these Gα-silenced cells indicated that only the silencing of Gα13 drastically reduced xenograft tumor growth and prolonged the survival of the mice. Similar, but albeit reduced, effect was seen with the silencing of Gα12. On the contrary, the silencing of Gαi2 or Gαq failed to exert such effect. Thus, our studies establish for the first time that Gα12/13, the putative gep oncogenes, are the determinant α-subunits involved in ovarian cancer growth in vivo and their increased oncogenicity can be correlated with its ability to stimulate both proliferation and invasive migration.

The gep proto-oncogene Gα12 mediates LPA-stimulated activation of CREB in ovarian cancer cells

Lysophosphatidic acid (LPA) plays a critical role in the pathophysiology of ovarian cancers. Previous studies have shown that LPA stimulates the proliferation of ovarian cancer cells via Gα12. The present study utilizing Protein/DNA array analyses of LPA-stimulated HeyA8 cells in which the expression of Gα12 was silenced, demonstrates for the first time that Gα12-dependent mitogenic signaling by LPA involves the atypical activation cAMP-response element binding protein (CREB). Results indicate that the robust activation of CREB by LPA is an early event that can be monitored by the phosphorylation of SER133 of CREB as early as 3min. The findings that the expression of the constitutively activated mutant of Gα12 stimulates CREB even in the absence of LPA in multiple ovarian cancer cell lines confirm the direct role of Gα12 in the activation of CREB. This is further substantiated by the observation that the silencing of Gα12 drastically attenuates LPA-stimulated phosphorylation of CREB. Our results also establish that LPA-Gα12-dependent activation of CREB is through a cAMP-independent, but Ras-ERK-dependent mechanism. More significantly, our findings indicate that the expression of the dominant negative S133A mutant of CREB leads to a reduction in LPA-stimulated proliferation of HeyA8 ovarian cancer cells. Thus, results presented here demonstrate for the first time that CREB is a critical signaling node in LPA-LPAR and Gα12/gep proto-oncogene stimulated oncogenic signaling in ovarian cancer cells.

The gep proto-oncogene Gα13 mediates lysophosphatidic acid-mediated migration of pancreatic cancer cells

OBJECTIVES

Tumor microenvironment, defined by a variety of growth factors including lysophosphatidic acid (LPA), whose levels are increased in pancreatic cancer patients, plays a major role in the genesis and progression of pancreatic cancer. Because the gep proto-oncogenes, Gα12 and Gα13, are implicated in LPA-stimulated oncogenic signaling, this study is focused on evaluating the role of these proto-oncogenes in LPA-stimulated invasive migration of pancreatic cancer cells.

METHODS

Effect of LPA on the migration and proliferation of pancreatic cancer cells was assessed using BxPC3, Dan-G, MDAPanc-28, Panc-1, and PaCa-2 cell lines. The role of Gα13 in the migration of pancreatic cancer cells was interrogated by disrupting lysophosphatidic acid receptor-Gα13 interaction using CT13, a dominant negative mutant of Gα13, and by silencing the expression of Gα13.

RESULTS

Results indicate that LPA stimulates the migration of pancreatic cancer cells and such LPA-stimulated migratory response is mediated by Gα13. Furthermore, the results establish that the silencing of Gα13, but not Gα12, abrogates LPA-stimulated invasive migration of pancreatic cancer cells.

CONCLUSIONS

These results report for the first time a critical role for Gα13 in LPA-stimulated invasive migration of pancreatic cancer cells. These findings identify LPA-lysophosphatidic acid receptor-Gα13 signaling node as a novel therapeutic target for pancreatic cancer treatment and control.

Lysophosphatidic Acid Stimulates the Proliferation of Ovarian Cancer Cells via the gep Proto-Oncogene Gα(12)

Lysophosphatidic acid (LPA), an agonist that activates specific G protein-coupled receptors, is present at an elevated concentration in the serum and ascitic fluid of ovarian cancer patients. Although the increased levels of LPA have been linked to the genesis and progression of different cancers including ovarian carcinomas, the specific signaling conduit utilized by LPA in promoting different aspects of oncogenic growth has not been identified. Here, we show that LPA stimulates both migration and proliferation of ovarian cancer cells. Using multiple approaches, we demonstrate that the stimulation of ovarian cancer cells with LPA results in a robust and statistically significant proliferative response. Our results also indicate that Gα(12), the gep proto-oncogene, which can be stimulated by LPA via specific LPA receptors, is overtly activated in a large array of ovarian cancer cells. We further establish that LPA stimulates the rapid activation of Gα(12) in SKOV-3 cells and the expression of CT12, an inhibitory minigene of Gα(12) that disrupts LPAR-Gα(12) interaction and potently inhibits such activation. Using this inhibitory molecule as well as the shRNA approach, we show that the inhibition of Gα(12) or silencing of its expression drastically and significantly attenuates LPA-mediated proliferation of ovarian cancer cell lines such as SKOV3, Hey, and OVCAR-3. Together with our findings that the silencing of Gα(12) does not have any significant effect on LPA-mediated migratory response of SKOV3 cells, our results point to a critical role for LPA-LPAR-Gα(12) signaling in ovarian cancer cell proliferation and not in migration. Thus, results presented here for the first time demonstrate that the gep proto-oncogene forms a specific node in LPA-LPAR-mediated mitogenic signaling in ovarian cancer cells.

Neoplastic transformation induced by the gep oncogenes involves the scaffold protein JNK-interacting leucine zipper protein

The activated mutants of the α-subunits of G proteins G(12) and G(13) have been designated as the gep oncogenes owing to their ability to stimulate diverse oncogenic signaling pathways that lead to neoplastic transformation of fibroblast cell lines and tumorigenesis in nude mice models. Studies from our laboratory as well as others have shown that the growth-promoting activities of Gα(12) and Gα(13) involve potent activation of c-Jun N-terminal kinases (JNKs). Our previous studies have indicated that the JNK-interacting leucine zipper protein (JLP), a scaffold protein involved in the structural and functional organization of the JNK/p38 mitogen-activated protein kinase module, tethers Gα(12) and Gα(13) to the JNK signaling module. In the present study, in addition to demonstrating the physical association between JLP and Gα(12), we show that this interaction is enhanced by the receptor- or mutation-mediated activation of Gα(12). We also establish that JLP interacts with Gα(12) through the C-terminal domain that has been previously identified to be involved in binding to Gα(13). Furthermore, using this C-terminal domain as a competitively inhibitor of JLP that can disrupt Gα(12)-JLP interaction, we demonstrate that JLP is required for the stimulation of JNK by Gα(12). Our results also indicate that such JLP interaction is required for Gα(12) as well as Gα(13)-mediated neoplastic transformation of JLP. These studies demonstrate for the first time a functional role for JLP in the gep oncogene-regulated neoplastic signaling pathway.