Protean agonism of the lysophosphatidic acid receptor-1 with Ki16425 reduces nerve growth factor-induced neurite outgrowth in pheochromocytoma 12 cells

Alteration of chromosome structure impacts gene expressions implicated in pancreatic ductal adenocarcinoma cells

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is a lethal malignancy with a five-year survival rate of approximately 10%. Genetic mutations are pivotal drivers in PDAC pathogenesis, but recent investigations also revealed the involvement of non-genetic alterations in the disease development. In this study, we undertook a multi-omics approach, encompassing ATAC-seq, RNA-seq, ChIP-seq, and Hi-C methodologies, to dissect gene expression alterations arising from changes in chromosome accessibility and chromatin three-dimensional interactions in PDAC.

RESULTS

Our findings indicate that chromosomal structural alterations can lead to abnormal expressions on key genes during PDAC development. Notably, overexpression of oncogenes FGFR2, FOXA2, CYP2R1, and CPOX can be attributed to the augmentation of promoter accessibility, coupled with long-range interactions with distal elements. Additionally, our findings indicate that chromosomal structural alterations caused by genomic instability can lead to abnormal expressions in PDACs. As an example, by analyzing chromosomal changes, we identified a putative oncogenic gene, LPAR1, which shows upregulated expression in both PDAC cell lines and clinical samples. The overexpression is correlated with alterations in LPAR1-associated 3D genome structure and chromatin state. We further demonstrated that high LPAR1 activity is required for enhanced PDAC cell migration in vitro.

CONCLUSIONS

Collectively, our findings reveal that the chromosomal conformational alterations, in addition to the well-known genetic mutations, are critical for PDAC tumorigenesis.

Neurotensin receptor type 2 protects B-cell chronic lymphocytic leukemia cells from apoptosis

B-cell chronic lymphocytic leukemia (B-CLL) cells are resistant to apoptosis, and consequently accumulate to the detriment of normal B cells and patient immunity. Because current therapies fail to eradicate these apoptosis-resistant cells, it is essential to identify alternative survival pathways as novel targets for anticancer therapies. Overexpression of cell-surface G protein-coupled receptors drives cell transformation, and thus plays a critical role in malignancies. In this study, we identified neurotensin receptor 2 (NTSR2) as an essential driver of apoptosis resistance in B-CLL. NTSR2 was highly expressed in B-CLL cells, whereas expression of its natural ligand, neurotensin (NTS), was minimal in both B-CLL cells and patient plasma. Surprisingly, NTSR2 remained in a constitutively active phosphorylated state, caused not by a mutation-induced gain-of-function but rather by an interaction with the oncogenic tyrosine kinase receptor TrkB. Functional and biochemical characterization revealed that the NTSR2-TrkB interaction acts as a conditional oncogenic driver requiring the TrkB ligand brain-derived neurotrophic factor (BDNF), which unlike NTS is highly expressed in B-CLL cells. Together, NTSR2, TrkB and BDNF induce autocrine and/or paracrine survival pathways that are independent of mutation status and indolent or progressive disease course. The NTSR2-TrkB interaction activates survival signaling pathways, including the Src and AKT kinase pathways, as well as expression of the anti-apoptotic proteins Bcl-2 and Bcl-xL. When NTSR2 was downregulated, TrkB failed to protect B-CLL cells from a drastic decrease in viability via typical apoptotic cell death, reflected by DNA fragmentation and Annexin V presentation. Together, our findings demonstrate that the NTSR2-TrkB interaction plays a crucial role in B-CLL cell survival, suggesting that inhibition of NTSR2 represents a promising targeted strategy for treating B-CLL malignancy.

Positive and Negative Cross-Talk between Lysophosphatidic Acid Receptor 1, Free Fatty Acid Receptor 4, and Epidermal Growth Factor Receptor in Human Prostate Cancer Cells

Lysophosphatidic acid (LPA) is a lipid mediator that mediates cellular effects via G protein-coupled receptors (GPCRs). Epidermal growth factor (EGF) is a peptide that acts via a receptor tyrosine kinase. LPA and EGF both induce proliferation of prostate cancer cells and can transactivate each other's receptors. The LPA receptor LPA1 is particularly important for LPA response in human prostate cancer cells. Previous work in our laboratory has demonstrated that free fatty acid 4 (FFA4), a GPCR activated by ω-3 fatty acids, inhibits responses to both LPA and EGF in these cells. One potential mechanism for the inhibition involves negative interactions between FFA4 and LPA1, thereby suppressing responses to EGF that require LPA1 In the current study, we examined the role of LPA1 in mediating EGF and FFA4 agonist responses in two human prostate cancer cell lines, DU145 and PC-3. The results show that an LPA1-selective antagonist inhibits proliferation and migration to both LPA and EGF. Knockdown of LPA1 expression, using silencing RNA, blocks responses to LPA and significantly inhibits responses to EGF. The partial response to EGF that is observed after LPA1 knockdown is not inhibited by FFA4 agonists. Finally, the role of arrestin-3, a GPCR-binding protein that mediates many actions of activated GPCRs, was tested. Knockdown of arrestin-3 completely inhibits responses to both LPA and EGF in prostate cancer cells. Taken together, these results suggest that LPA1 plays a critical role in EGF responses and that FFA4 agonists inhibit proliferation by suppressing positive cross-talk between LPA1 and the EGF receptor.

LPA1extracellular loop residues 115 and 191 are not required for receptor activation but prevent Ki16425 super-antagonism

Extracellular loop residues (R115 and D191) are not required for receptor activation but repress Ki16425-type super-antagonism but not LPA-analogue antagonists using a combination of site-directed mutagenesis and intracellular calcium assay procedures.

TRPM2 mediates the lysophosphatidic acid-induced neurite retraction in the developing brain

Intracellular Ca(2+) signal is a key regulator of axonal growth during brain development. As transient receptor potential (TRP) channels are permeable to Ca(2+) and mediate numerous brain functions, it is conceivable that many TRP channels would regulate neuronal differentiation. We therefore screened TRP channels that are involved in the regulation of neurite growth. Among the TRP channels, the Trpm2 level was inversely associated with neurite growth. TRPM2 was highly expressed in embryonic brain. Pharmacological perturbation or knockdown of TRPM2 markedly increased the axonal growth, whereas its overexpression inhibited the axonal growth. Addition of ADP ribose, an endogenous activator of TRPM2, to PC12 cells significantly repressed the axonal growth. TRPM2 was actively involved in the neuronal retraction induced by cerebrospinal fluid-rich lysophosphatidic acid (LPA). More importantly, neurons isolated from the brain of Trpm2-deficient mice have significantly longer neurites with a greater number of spines than those obtained from the brain of wild-type mice. Therefore, we conclude that TRPM2 mediates the LPA-induced suppression of axonal growth, which provides a long-sought mechanism underlying the effect of LPA on neuronal development.

Receptor tyrosine kinase-G-protein-coupled receptor signalling platforms: out of the shadow?

Receptor tyrosine kinases (RTKs) and G-protein-coupled receptors (GPCRs) can form platforms in which protein signalling components specific for each receptor are shared (owing to close proximity) to produce an integrated response upon engagement of ligands. RTK-GPCR signalling platforms respond to growth factors and GPCR agonists to increase gain over and above that which is normally produced by separate receptors. They can also function to change the spatial context of signalling in response to growth factor activation. The function of RTK-GPCR signalling platforms can be modulated with conformational-specific inhibitors that stabilise defined GPCR states to abrogate both GPCR agonist- and growth factor-stimulated cell responses. In this paper, we provide an opinion of the biology and unusual pharmacology of RTK-GPCR signalling platforms and make comparisons with a more traditional model of crosstalk between RTKs and GPCRs.

Deletion of lysophosphatidic acid receptor LPA1 reduces neurogenesis in the mouse dentate gyrus

Neurogenesis persists in certain regions of the adult brain including the subgranular zone of the hippocampal dentate gyrus wherein its regulation is essential, particularly in relation to learning, stress and modulation of mood. Lysophosphatidic acid (LPA) is an extracellular signaling phospholipid with important neural regulatory properties mediated by specific G protein-coupled receptors, LPA(1-5). LPA(1) is highly expressed in the developing neurogenic ventricular zone wherein it is required for normal embryonic neurogenesis, and, by extension may play a role in adult neurogenesis as well. By means of the analyses of a variant of the original LPA(1)-null mutant mouse, termed the Malaga variant or "maLPA(1)-null," which has recently been reported to have defective neurogenesis within the embryonic cerebral cortex, we report here a role for LPA(1) in adult hippocampal neurogenesis. Proliferation, differentiation and survival of newly formed neurons are defective in the absence of LPA(1) under normal conditions and following exposure to enriched environment and voluntary exercise. Furthermore, analysis of trophic factors in maLPA(1)-null mice demonstrated alterations in brain-derived neurotrophic factor and insulin growth factor 1 levels after enrichment and exercise. Morphological analyses of doublecortin positive cells revealed the anomalous prevalence of bipolar cells in the subgranular zone, supporting the operation of LPA(1) signaling pathways in normal proliferation, maturation and differentiation of neuronal precursors.

Lysophosphatidic acid-induced transactivation of epidermal growth factor receptor regulates cyclo-oxygenase-2 expression and prostaglandin E(2) release via C/EBPbeta in human bronchial epithelial cells

We have demonstrated that LPA (lysophosphatidic acid)-induced IL (interleukin)-8 secretion was partly mediated via transactivation of EGFR [EGF (epidermal growth factor) receptor] in HBEpCs (human bronchial epithelial primary cells). The present study provides evidence that LPA-induced transactivation of EGFR regulates COX (cyclo-oxygenase)-2 expression and PGE(2) [PG (prostaglandin) E(2)] release through the transcriptional factor, C/EBPbeta (CCAAT/enhancer-binding protein beta), in HBEpCs. Treatment with LPA (1 microM) stimulated COX-2 mRNA and protein expression and PGE(2) release via G(alphai)-coupled LPARs (LPA receptors). Pretreatment with inhibitors of NF-kappaB (nuclear factor-kappaB), JNK (Jun N-terminal kinase), or down-regulation of c-Jun or C/EBPbeta with specific siRNA (small interference RNA) attenuated LPA-induced COX-2 expression. Downregulation of EGFR by siRNA or pretreatment with the EGFR tyrosine kinase inhibitor, AG1478, partly attenuated LPA-induced COX-2 expression and phosphorylation of C/EBPbeta; however, neither of these factors had an effect on the NF-kappaB and JNK pathways. Furthermore, LPA-induced EGFR transactivation, phosphorylation of C/EBPbeta and COX-2 expression were attenuated by overexpression of a catalytically inactive mutant of PLD2 [PLD (phospholipase D) 2], PLD2-K758R, or by addition of myristoylated PKCzeta [PKC (protein kinase C) zeta] peptide pseudosubstrate. Overexpression of the PLD2-K758R mutant also attenuated LPA-induced phosphorylation and activation of PKCzeta. These results demonstrate that LPA induces COX-2 expression and PGE(2) production through EGFR transactivation-independent activation of transcriptional factors NF-kappaB and c-Jun, and EGFR transactivation-dependent activation of C/EBPbeta in HBEpCs. Since COX-2 and PGE(2) have been shown to be anti-inflammatory in airway inflammation, the present data suggest a modulating and protective role of LPA in regulating innate immunity and remodelling of the airways.

Betagamma subunits of G(i/o) suppress EGF-induced ERK5 phosphorylation, whereas ERK1/2 phosphorylation is enhanced

Extracellular signal-regulated kinases (ERKs) play important physiological roles in proliferation, differentiation and gene expression. ERK5 is twice the size of ERK1/2, the amino-terminal half contains the kinase domain that shares the homology with ERK1/2 and TEY activation motif, whereas the carboxy-terminal half is unique. In this study, we examined the cross-talk mechanism between G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases, focusing on ERK1/2 and 5. The pretreatment of rat pheochromocytoma cells (PC12) with pertussis toxin (PTX) specifically enhanced epidermal growth factor (EGF)-induced ERK5 phosphorylation. In addition, lysophosphatidic acid (LPA) attenuated the EGF-induced ERK5 phosphorylation in LPA(1) receptor- and G(i/o)-dependent manners. On the other hand, LPA alone activated ERK1/2 via Gbetagamma subunits and Ras and potentiated EGF-induced ERK1/2 phosphorylation at late time points. These results suggest G(i/o) negatively regulates ERK5, while it positively regulates ERK1/2. LPA did not affect cAMP levels after EGF treatment, and the reagents promoting cAMP production such as forskolin and cholera toxin also attenuated the EGF-induced ERK5 phosphorylation, indicating that the inhibitory effect of LPA on ERK5 inhibition via G(i/o) is not due to inhibition of adenylyl cyclase by Galpha(i/o). However, the inhibitory effect of LPA on ERK5 was abolished in PC12 cells stably overexpressing C-terminus of GPCR kinase2 (GRK2), and overexpression of Gbeta(1) and gamma(2) subunits also suppressed ERK5 phosphorylation by EGF. In response to LPA, Gbetagamma subunits interacted with EGF receptor in a time-dependent manner. These results strongly suggest that LPA negatively regulates the EGF-induced ERK5 phosphorylation through Gbetagamma subunits.

GPCR-jacking: from a new route in RTK signalling to a new concept in GPCR activation

A large body of evidence indicates that agonists of some G protein-coupled receptors (GPCRs) can activate growth factor receptor tyrosine kinases (RTKs) in the absence of added growth factor. This phenomenon, called transactivation, is an important pathway that contributes to growth-promoting activity of many GPCR ligands. Reciprocally, recent advances indicate that RTKs utilize GPCR signalling molecules to transduce signals and that RTK ligands themselves can transactivate GPCRs. This novel transactivation process, which places GPCR signalling downstream of RTKs, either requires the production of a GPCR ligand of the transactivated GPCR or occurs in a ligand independent manner within an integrated signalling network. Here, we provide an overview of the molecular mechanisms involved in this novel cross-communication between GPCRs and RTKs and discuss its relevance in the specification of growth factor signalling and functions.

Constitutive activity of the histamine H3 receptor

Constitutive activity has been mainly recorded for numerous overexpressed and/or mutated receptors. The histamine H(3) receptor (H(3)R) is a target of choice to study the physiological relevance of this process. In rodent brain, postsynaptic H(3)Rs show high constitutive activity, and presynaptic H(3) autoreceptors that show constitutive activity have a predominant role in inhibiting the activity of histamine neurons. H(3)R inverse agonists abrogate this constitutive brake and enhance histamine release in vivo. Some of these inverse agonists have entered clinical trials for the treatment of cognitive and food intake disorders. Studies performed in vitro and in vivo with proxyfan show that this H(3)R ligand is a 'protean agonist' - that is, a ligand with a spectrum of activity ranging from full agonism to full inverse agonism depending on the level of H(3)R constitutive activity. Consistent with its physiological and therapeutic relevance, the constitutive activity of H(3)R thus has a major function in the brain and regulates the activity of H(3)R-targeted drugs.

Integrin signalling regulates the nuclear localization and function of the lysophosphatidic acid receptor-1 (LPA1) in mammalian cells

We show that LPA1 (lysophosphatidic acid receptor-1) is constitutively localized in the nucleus of mammalian cells. LPA1 also traffics from cell membranes to the nucleus in response to LPA (lysophosphatidic acid). Several lines of evidence suggest an important role for cell-matrix interaction in regulating the constitutive nuclear localization of LPA1. First, the RGDS peptide, which blocks cell matrix-induced integrin clustering and cytoskeletal rearrangement, reduced the number of cells containing LPA1 in the nucleus. Secondly, a higher proportion of cells contained nuclear LPA1 when adhesion on fibronectin-coated glass was compared with adherence to polylysine-coated glass. Thirdly, pre-treatment of cells with the Rho kinase inhibitor (Y27632) or the myosin light chain kinase inhibitor (ML9) reduced the number of cells containing nuclear LPA1. The addition of LPA and/or Ki16425 (which binds to LPA1) to isolated nuclei containing LPA1 induced the phosphorylation of several proteins with molecular masses of 34, 32, 14 and 11 kDa. These findings demonstrate that trafficking of LPA1 to the nucleus is influenced by cell-matrix interactions and that nuclear LPA1 may be involved in regulating intranuclear protein phosphorylation and signalling.