Expression and regulation of phospholipase D isoenzymes in human melanoma cells and primary melanocytes

Phospholipase D and cancer metastasis: A focus on exosomes

In mammals, phospholipase D (PLD) enzymes involve 6 isoforms, of which only three have established lipase activity to produce the signaling lipid phosphatidic acid (PA). This phospholipase activity has been postulated to contribute to cancer progression for over three decades now, but the exact mechanisms involved have yet to be uncovered. Indeed, using various models, an altered PLD activity has been proposed altogether to increase cell survival rate, promote angiogenesis, boost rapamycin resistance, and favor metastasis. Although for some part, the molecular pathways by which this increase in PA is pro-oncogenic are partially known, the pleiotropic functions of PA make it quite difficult to distinguish which among these simple signaling pathways is responsible for each of these PLD facets. In this review, we will describe an additional potential contribution of PA generated by PLD1 and PLD2 in the biogenesis, secretion, and uptake of exosomes. Those extracellular vesicles are now viewed as membrane vehicles that carry informative molecules able to modify the fate of receiving cells at distance from the original tumor to favor homing of metastasis. The perspectives for a better understanding of these complex role of PLDs will be discussed.

Upregulated phospholipase D2 expression and activity is related to the metastatic properties of melanoma

The incidence rates of melanoma have increased steadily in recent decades and nearly 25% of the patients diagnosed with early-stage melanoma will eventually develop metastasis, for which there is currently no fully effective treatment. The link between phospholipases and tumors has been studied extensively, particularly in breast and colon cancers. With the aim of finding new biomarkers and therapeutic options for melanoma, the expression of different phospholipases was assessed in 17 distinct cell lines in the present study, demonstrating that phospholipase D2 (PLD2) is upregulated in metastatic melanoma as compared to normal skin melanocytes. These results were corroborated by immunofluorescence and lipase activity assays. Upregulation of PLD2 expression and increased lipase activity were observed in metastatic melanoma relative to normal skin melanocytes. So far, the implication of PLD2 activity in melanoma malignancies has remained elusive. To the best of our knowledge, the present study was the first to demonstrate that the overexpression of PLD2 enhances lipase activity, and its effect to increase the proliferation, migration and invasion capacity of melanoma cells was assessed with XTT and Transwell assays. In addition, silencing of PLD2 in melanoma cells reduced the metastatic potential of these cells. The present study provided evidence that PLD2 is involved in melanoma malignancy and in particular, in its metastatic potential, and established a basis for future studies evaluating PLD2 blockade as a therapeutic strategy to manage this condition.

Restoration of ceramide de novo synthesis by the synthetic retinoid ST1926 as it induces adult T-cell leukemia cell death

Ceramide (Cer) is a bioactive cellular lipid with compartmentalized and tightly regulated levels. Distinct metabolic pathways lead to the generation of Cer species with distinguishable roles in oncogenesis. Deregulation of Cer pathways has emerged as an important mechanism for acquired chemotherapeutic resistance. Adult T-cell leukemia (ATL) cells are defective in Cer synthesis. ATL is an aggressive neoplasm that develops following infection with human T-cell lymphotropic virus-1 (HTLV-1) where the viral oncogene Tax contributes to the pathogenesis of the disease. ATL cells, resistant to all-trans-retinoic acid, are sensitive to pharmacologically achievable concentrations of the synthetic retinoid ST1926. We studied the effects of ST1926 on Cer pathways in ATL cells. ST1926 treatment resulted in early Tax oncoprotein degradation in HTLV-1-treated cells. ST1926 induced cell death and a dose- and time-dependent accumulation of Cer in malignant T cells. The kinetics and degree of Cer production showed an early response upon ST1926 treatment. ST1926 enhanced de novo Cer synthesis via activation of ceramide synthase CerS(s) without inhibiting dihydroceramide desaturase, thereby accumulating Cer rather than the less bioactive dihydroceramide. Using labeling experiments with the unnatural 17-carbon sphinganine and measuring the generated Cer species, we showed that ST1926 preferentially induces the activities of a distinct set of CerS(s). We detected a delay in cell death response and interruption of Cer generation in response to ST1926 in Molt-4 cells overexpressing Bcl-2. These results highlight the potential role of ST1926 in inducing Cer levels, thus lowering the threshold for cell death in ATL cells.

Lack of effective translational regulation of PLD expression and exosome biogenesis in triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that is difficult to treat since cells lack the three receptors (ES, PR, or HER) that the most effective treatments target. We have used a well-established TNBC cell line (MDA-MB-231) from which we found evidence in support for a phospholipase D (PLD)-mediated tumor growth and metastasis: high levels of expression of PLD, as well as the absence of inhibitory miRs (such as miR-203) and 3'-mRNA PARN deadenylase activity in these cells. Such findings are not present in a luminal B cell line, MCF-7, and we propose a new miR•PARN•PLD node that is not uniform across breast cancer molecular subtypes and as such TNBC could be pharmacologically targeted differentially. We review the participation of PLD and phosphatidic acid (PA), its enzymatic product, as new "players" in breast cancer biology, with the aspects of regulation of the tumor microenvironment, macrophage polarization, regulation of PLD transcripts by specific miRs and deadenylases, and PLD-regulated exosome biogenesis. A new signaling miR•PARN•PLD node could serve as new biomarkers for TNBC abnormal signaling and metastatic disease staging, potentially before metastases are able to be visualized using conventional imaging.

The role of lipid signaling in the progression of malignant melanoma

In the past decades, a vast amount of data accumulated on the role of lipid signaling pathways in the progression of malignant melanoma, the most metastatic/aggressive human cancer type. Genomic studies identified that PTEN loss is the leading factor behind the activation of the PI3K-signaling pathway in melanoma, mutations of which are one of the main resistance mechanisms behind target therapy failures. On the other hand, illegitimate expressions of megakaryocytic genes p12-lipoxyganse, cyclooxygenase-2, and phosphodiestherase-2/autotaxin (ATX) are mostly involved in the regulation of motility signaling in melanoma through various G-protein-coupled bioactive lipid receptors. Furthermore, endocannabinoid signaling can also be a novel paracrine survival factor in melanoma. Last but not least, prenylation inhibitors acting even on mutated small GTP-ases, such as NRAS of melanoma may offer novel therapeutic opportunities. As regards melanoma, the most effective therapy nowadays is immunotherapy, with the resistance mechanisms also possibly involving the lipid signaling activities of melanoma cells, which further supports the idea of their being therapeutic targets.

The transcription factors Slug (SNAI2) and Snail (SNAI1) regulate phospholipase D (PLD) promoter in opposite ways towards cancer cell invasion

Slug (SNAI2) and Snail (SNAI1) are master regulatory transcription factors for organogenesis and wound healing, and they are involved in the epithelial to mesenchymal transition (EMT) of cancer cells. We found that the activity of phospholipase D isoform 2 (PLD2) is highly increased in cancers with larger size and poor prognosis (MDA-MB-231 versus MCF-7 cells), so we determined if Snail or Slug were responsible for PLD2 gene transcription regulation. Unexpectedly, we found that PLD2 expression was positively regulated by Slug but negatively regulated by Snail. The differential effects are amplified in breast cancer cells over normal cells and with MDA-MB-231 more robustly than MCF-7. Slug putatively binds to the PLD2 promoter and transactivates it, which is negated when Slug and Snail compete with each other. Meanwhile, PLD2 has a negative effect on Snail expression and a positive effect on Slug, thus closing a feedback loop between the lipase and the transcription factors. Further, PA, the product of PLD2 enzymatic reaction, has profound effects on its own and it further regulates the transcription factors. Thus, we show for the first time that the overexpressed PLD2 in human breast tumors is regulated by Slug and Snail transcription factors. The newly uncovered feedback loops in highly invasive cancer cells have important implications in the process of EMT.

Phosphatidic acid, phospholipase D and tumorigenesis

Phospholipase D (PLD) is a membrane protein with a double role: maintenance of the structural integrity of cellular or intracellular membranes and involvement in cell signaling through the product of the catalytic reaction, PA, and through protein-protein interaction with a variety of partners. Cross-talk during PLD signaling occurs with other cancer regulators (Ras, PDGF, TGF and kinases). Elevation of either PLD1 or PLD2 (the two mammalian isoforms of PLD) is able to transform fibroblasts and contribute to cancer progression. Elevated total PLD activity, as well as overexpression, is present in a wide variety of cancers such as gastric, colorectal, renal, stomach, esophagus, lung and breast. PLD provides survival signals and is involved in migration, adhesion and invasion of cancer cells, and all are increased during PLD upregulation or, conversely, they are decreased during PLD loss of function. Eventhough the end results of PLD action as relates to downstream signaling mechanisms are still currently being elucidated, invasion, a pre-requisite for metastasis, is directly affected by PLD. This review will introduce the classical mammalian PLD's, PLD1 and PLD2, followed by the mechanisms of intracellular regulation and a status of current investigation in the crucial involvement of PLD in cancer, mostly through its role in cell migration, invasion and metastasis, that has grown exponentially in the last few years.

Phospholipase D (PLD) drives cell invasion, tumor growth and metastasis in a human breast cancer xenograph model

Breast cancer is one of the most common malignancies in human females in the world. One protein that has elevated enzymatic lipase activity in breast cancers in vitro is phospholipase D (PLD), which is also involved in cell migration. We demonstrate that the PLD2 isoform, which was analyzed directly in the tumors, is crucial for cell invasion that contributes critically to the growth and development of breast tumors and lung metastases in vivo. We used three complementary strategies in a SCID mouse model and also addressed the underlying molecular mechanism. First, the PLD2 gene was silenced in highly metastatic, aggressive breast cancer cells (MDA-MB-231) with lentivirus-based shRNA, which were xenotransplanted in SCID mice. The resulting mouse primary mammary tumors were reduced in size (65%, p<0.05) and their onset delayed when compared to control tumors. Second, we stably overexpressed PLD2 in low-invasive breast cancer cells (MCF-7) with a biscistronic MIEG retroviral vector and observed that these cells were converted into a highly aggressive phenotype, as primary tumors that formed following xenotransplantation were larger, grew faster and developed lung metastases more readily. Third, we implanted osmotic pumps into SCID xenotransplanted mice that delivered two different small-molecule inhibitors of PLD activity (FIPI and NOPT). These inhibitors led to significant (>70%, p<0.05) inhibition of primary tumor growth, metastatic axillary tumors and lung metastases. In order to define the underlying mechanism, we determined that the machinery of PLD-induced cell invasion is mediated by phosphatidic acid (PA), WASp, Grb2 and Rac2 signaling events that ultimately affect actin polymerization and cell invasion. In summary, this study shows that PLD has a central role in the development, metastasis and level of aggressiveness of breast cancer, raising the possibility that PLD2 could be used as a new therapeutic target.

Serum deprivation confers the MDA-MB-231 breast cancer line with an EGFR/JAK3/PLD2 system that maximizes cancer cell invasion

Our laboratory has reported earlier that in leukocytes, phospholipase D2 (PLD2) is under control of Janus kinase 3 (JAK3), which mediates chemotaxis. Investigating JAK3 in cancer cells led to an important discovery as exponentially growing MDA-MB-231 human breast cancer cells, which are highly proliferative and metastatic, did not substantially use JAK3 to activate PLD2. However, in 2-h or 16-h starved cell cultures, JAK3 switches to a PLD2-enhancing role, consistent with the needs of those cells to enter a "survival state" that relies on an increase in PLD2 activity to withstand serum deprivation. Using a small-molecule tyrosine kinase inhibitor, the flavonoid 4',5,7-trihydroxyflavone (apigenin), as well as RNA silencing, we found that the invasive phenotype of MDA-MB-231 cells is mediated by PLD2 under direct regulation of both JAK3 and the tyrosine kinase, epidermal growth factor receptor (EGFR). Furthermore, serum-deprived cells in culture show an upregulated EGFR/JAK3/PLD2-PA system and are especially sensitive to a combination of JAK3 and PLD2 enzymatic activity inhibitors (30nM apigenin and 300nM 5-fluoro-2-indolyl des-chlorohalopemide (FIPI), respectively). Thus, a multi-layered activation of cell invasion by two kinases (EGFR and JAK3) and a phospholipase (PLD2) provides regulatory flexibility and maximizes the aggressively invasive power of MDA-MB-231 breast cancer cells. This is especially important in the absence of growth factors in serum, coincidental with migration of these cells to new locations.

High-level expression of wild-type p53 in melanoma cells is frequently associated with inactivity in p53 reporter gene assays

BACKGROUND

Inactivation of the p53 pathway that controls cell cycle progression, apoptosis and senescence, has been proposed to occur in virtually all human tumors and p53 is the protein most frequently mutated in human cancer. However, the mutational status of p53 in melanoma is still controversial; to clarify this notion we analysed the largest series of melanoma samples reported to date.

METHODOLOGY/PRINCIPAL FINDINGS

Immunohistochemical analysis of more than 180 melanoma specimens demonstrated that high levels of p53 are expressed in the vast majority of cases. Subsequent sequencing of the p53 exons 5-8, however, revealed only in one case the presence of a mutation. Nevertheless, by means of two different p53 reporter constructs we demonstrate transcriptional inactivity of wild type p53 in 6 out of 10 melanoma cell lines; the 4 other p53 wild type melanoma cell lines exhibit p53 reporter gene activity, which can be blocked by shRNA knock down of p53.

CONCLUSIONS/SIGNIFICANCE

In melanomas expressing high levels of wild type p53 this tumor suppressor is frequently inactivated at transcriptional level.

Caspase cleavage of phospholipase D1 in vitro alters its regulation and reveals a novel property of the "loop" region

Phospholipase D (PLD) has been implicated in mediating vesicular transport, mitosis, differentiation and apoptosis. The product of PLD activity, phosphatidic acid (PA) has mitogenic potential and elevated PLD expression has been detected in many tumor cell lines. Several reports have demonstrated that distinct PLD domains regulate its activity and that truncated forms of PLD retain enzymatic activity. We hypothesized that during apoptosis caspase cleavage of PLDs could result in modification of their activities. To test this idea, we have used in vitro translation of PLD1 and PLD2 which generated active enzymes exhibiting properties mimicking those of the endogenous proteins. Here we demonstrate that PLD1 was rapidly cleaved in vitro by caspases-8, -3 and -7. In contrast, PLD2 cleavage was delayed and its activity was unaffected by incubation with caspase-3. Significantly, following caspase cleavage the response of PLD1 to regulatory stimuli was altered; it was no longer activated by PKC and instead exhibited an increased activity in response to small GTPases. Notably, this enhanced activity was due to cleavage of PLD1 in the "loop" domain, a region previously associated with negative regulatory function. Thus our data have identified a novel regulatory domain in PLD1.