Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids
Phosphatidic acid signaling to mTOR: Signals for the survival of human cancer cells
Introduction
During the past fifteen years many studies have shown that in response to mitogenic signals and activated oncoproteins there is an increase in phospholipase D (PLD) activity. PLD activity is elevated in response to several growth factors including epidermal growth factor (EGF) [1], platelet-derived growth factor [2], fibroblast growth factor [3], [4], insulin [5], insulin-like growth factor 1 [6], and vascular endothelial growth factor [7]. In addition, PLD activity is elevated in response to the oncoproteins v-Src [8], v-Ras [9], [10], v-Fps [11], and v-Raf [12]. The elevated PLD activity in cells transformed by the Ras oncoprotein is required for cell transformation [13]. In addition, elevated expression of either c-Src or the EGF receptor, in combination with elevated expression of PLD, transforms rat fibroblasts [14], [15], [16]. PLD has also been reported to induce anchorage-independent growth and enhance cell cycle progression of mouse fibroblasts [17]. These studies reveal elevated PLD activity is correlated with mitogenic and oncogenic signals.
Elevated PLD expression has also been shown to prevent cell cycle arrest and apoptosis. High intensity Raf signaling induces cell senescence [18], [19] or, if cells are deprived of serum, apoptosis [20]. Elevated expression of PLD suppressed the apoptosis induced by high intensity Raf signals [20]. Similarly, rat fibroblasts overexpressing c-Src undergo apoptosis in response to growth factor deprivation, and elevated PLD expression suppressed this apoptosis [21]. These early studies in rodent fibroblasts indicate that in addition to enhancing cell proliferation, PLD is required for the “survival signals” that suppress default apoptotic programs.
The studies implicating PLD in mitogenic signaling and the suppression of apoptosis suggest that PLD activity might be a factor in human cancer where mitogenic signals are constitutively active and suppression of apoptotic programs is critical. Consistent with this hypothesis, elevated PLD activity and expression has been reported in a variety of human cancer tissues including breast, gastric, kidney, and colon [22], [23], [24], [25]. In addition, elevated PLD activity has been reported in several human cancer cell lines including those derived from breast, lung, bladder, pancreatic, and kidney cancers [26], [27], [28], [29], [30]. Importantly, the elevated PLD activity in these cells was shown to be critical for suppressing apoptosis in these cell lines. Thus, elevated PLD activity in human cancers is likely a critical aspect of tumorigenesis that promotes cell proliferation and suppresses the default apoptotic programs that prevent cancer.
There are two mammalian PLD isoforms – PLD1 and PLD2 – the distinct functions of which are poorly understood [31]. Both catalyze the hydrolysis of phosphatidylcholine to phosphatidic acid (PA) and choline. PA is a central node for lipid signaling and can be generated from lysophosphatidic acid (LPA) and diacylglycerol (DG) as well from phosphatidylcholine (Fig. 1A) — although it is likely that the most relevant source in cancer cells is via the PLD-mediated hydrolysis of phosphatidylcholine [31]. PA is also metabolically converted to the lipid second messengers LPA and DG (Fig. 1A). However, while DG and LPA have important second messenger function, there are several targets of PA have been identified (Fig. 1B) and it is believed that the most significant effects of elevated PLD activity are mediated by targets of PA. Significantly, these include Raf [32] and the mammalian target of rapamycin (mTOR) [33] — both of which are commonly dysregulated in human cancers. mTOR, like PLD, has been implicated in cancer cell survival signals [34], [35], [36]. While the role of PA in regulating mTOR has been controversial [37], recent reports have strongly implicated PLD and its metabolite PA in the regulation of both mTOR complexes — mTORC1 and mTORC2 [38], [39], [40]. This review summarizes the role of PLD in the regulation of mTOR and the potential of combining strategies for targeting PLD and mTOR signals in human cancer.
Section snippets
PA competes with rapamycin for binding to mTOR
The first report directly linking PLD activity with mTOR was the discovery that PA is required for mTOR activity [33]. PA was shown to bind the FKBP12-rapamycin binding (FRB) domain of mTOR in a manner that is competitive with rapamycin-FKBP12 [33]. Consistent with a competition between PA and rapamycin-FKBP12 for mTOR, it was subsequently demonstrated that elevated levels of PLD activity conferred rapamycin resistance in human breast cancer cell lines [42]. The recently reported NMR structure
PA is necessary, but not sufficient to activate mTOR
While recent studies clearly demonstrate a PA requirement for the activation of both mTORC1 and mTORC2, there is also evidence that PA is not sufficient to activate either mTORC1 or mTORC2. Chen and colleagues demonstrated that exogenously provided PA stimulated the activation of the mTORC1 as indicated by increased phosphorylation of the mTORC1 substrates S6 kinase eukaryotic initiation factor 4E binding protein-1 in HEK293 cells, however the effect was dependent on the presence of amino acids
Regulation of PLD1 and PLD2
There are two mammalian PLD isoforms – PLD1 and PLD2 – that can be distinguished by different mechanisms of regulation and sub-cellular distribution [31]. PLD1 has a predominantly peri-nuclear localization and is regulated by members of the Ras family of GTPases including ARF [62], Rho [63], and Ral [64]. Importantly, it was recently reported that the GTPase Rheb activates PLD1 [38]. PLD2 is largely restricted to lipid raft fractions on the plasma membrane and its mode of regulation is not well
Targeting PLD-mTOR survival signals
Targeting mTOR in anti-cancer therapies has attracted much attention in recent years largely due to a link between mTOR and survival signals in human cancer cells [34], [35], [36]. Much has been written about the potential of targeting mTOR because, in principle, the suppression of survival signals in cancer cells should result in apoptotic cell death and tumor regression. While the principle of targeting mTOR-mediated survival signals in human cancer offers an attractive therapeutic option,
Conclusions
Elevated PLD activity has been observed in a large number of human cancers and has been shown to suppress apoptosis [74]. PLD activity is commonly elevated in cancer cell lines in response to the stress of serum withdrawal [27]. It has now become apparent that a key target of PLD survival signals is mTOR. mTOR has been implicated as a key regulator of stress responses by shutting down under conditions of poor nutrition or hypoxia [36], [61]. In order for a cancer cell to survive and
Acknowledgements
Paige Yellen is acknowledged for thoughtful comments on the manuscript. The author of this work was supported by grants from the National Cancer Institute (CA46677) and a SCORE grant from the National Institutes of Health (GM60654). Research Centers in Minority Institutions (RCMI) award RR-03037 from the National Center for Research Resources of the National Institutes of Health, which supports infrastructure and instrumentation in the Biological Sciences Department at Hunter College, is also
References (79)
- et al.
Induction of phosphatidic acid by fibroblast growth factor in cultured baby hamster kidney fibroblasts
FEBS Lett.
(1993) - et al.
Activation and translocation of Rho and ADP ribosylation factor by insulin in rat adipocytes. Apparent involvement of phosphatidylinositol 3-kinase
J. Biol. Chem.
(1997) - et al.
Ras mediates the activation of phospholipase D by v-Src
J. Biol. Chem.
(1995) - et al.
Phosphatidylcholine-specific phospholipase D activity is elevated in v-Fps-transformed cells
Biochem. Biophys. Res. Comm.
(1994) - et al.
Ral and Rho dependent activation of phospholipase D in v-Raf transformed cells
Biochem. Biophys. Res. Comm.
(1999) - et al.
Transformation of cells overexpressing a tyrosine kinase by phospholipase D1 and D2
Biochem. Biophys. Res. Comm.
(2001) - et al.
Phospholipase D prevents apoptosis in v-Src-transformed rat fibroblasts and MDA-MB-231 breast cancer cells
Biochem. Biophys. Res. Comm.
(2003) - et al.
Overexpression of phospholipase D1 in human breast cancer tissues
Cancer Lett.
(2000) - et al.
Increased activity and intranuclear expression of phospholipase D2 in human renal cancer
Biochem. Biophys. Res. Commun.
(2000) - et al.
Phospholipase D couples survival and migration signals in response to stress in human breast cancer cells
J. Biol. Chem.
(2006)
Phospholipase D provides a survival signal in human cancer cells with activated H-Ras or K-Ras
Cancer Lett.
Phospholipase D and its product, phosphatidic acid, mediate agonist-dependent raf-1 translocation to the plasma membrane and the activation of the mitogen-activated protein kinase pathway
J. Biol. Chem.
Will mTOR inhibitors make it as cancer drugs?
Cancer Cell
Defining the role of mTOR in cancer
Cancer Cell
Differential dependence of HIF1α and HIF2α on mTORC1 and mTORC2
J. Biol. Chem.
A novel pathway regulating the mammalian target of rapamycin (mTOR) signaling
Biochem. Pharmacol.
Two TOR complexes, only one of which is rapamycin sensitive, have distinct roles in cell growth control
Mol. Cell.
Prolonged rapamycin treatment inhibits mTORC2 assembly and Akt/PKB
Mol. Cell
Rapamycin derivatives reduce mTORC2 signaling and inhibit AKT activation in AML
Blood
Inhibition of HIF is necessary for tumor suppression by the von Hippel-Lindau protein
Cancer Cell
AKT/PKB signaling: navigating downstream
Cell
mTOR, translational control and human disease
Semin. Cell Dev. Biol.
Re-evaluating the roles of proposed modulators of mammalian target of rapamycin complex 1 (mTORC1) signaling
J. Biol. Chem.
SIN1/MIP1 maintains rictor-mTOR complex integrity and regulates Akt phosphorylation and substrate specificity
Cell
Ablation in mice of the mTORC components raptor, rictor, or mLST8 reveals that mTORC2 is required for signaling to Akt-FOXO and PKCα, but not S6 K1. Dev
Cell.
TOR, a central controller of cell growth
Cell
Evidence for Rho-mediated agonist stimulation of phospholipase D in rat1 fibroblasts. Effects of Clostridium botulinum C3 exoenzyme
J. Biol. Chem.
Phosphatidic acid is a specific activator of phosphatidylinositol-4-phosphate kinase
J. Biol. Chem.
PLD1 regulates mTOR signaling and mediates Cdc42 activation of S6 K1
Curr. Biol.
PLD2 forms a functional complex with mTOR/raptor to transduce mitogenic signals
Cell Signal
mTOR-dependent suppression of protein phosphatase 2A is critical for phospholipase D survival signals in human breast cancer cells
J. Biol. Chem.
Honokiol, a small molecular weight natural product, inhibits angiogenesis in vitro and tumor growth in vivo
J. Biol. Chem.
Optimization of halopemide for phospholipase D2 inhibition
Bioorg. Med. Chem. Lett.
EGF induces the production of biologically distinguishable diglyceride species from phosphatidylinositol and phosphatidylcholine: evidence for the independent activation of type C and type D phospholipases
Cell Growth Differ.
Multiple sources of sn-1,2-diacylglycerol in platelet-derived-growth factor-stimulated swiss 3T3 fibroblasts
Biochem. J.
Basic fibroblast growth factor stimulates cytosolic phospholipase A2, phospholipase C-γ1 and phospholipase D through distinguishable signaling mechanisms
Mol. Cell. Biochem.
Involvement of phospholipase D in insulin-like growth factor-I-induced activation of extracellular signal-regulated kinase, but not phosphatidylinositol 3-kinase or Akt, in Chinese hamster ovary cells
Biochem. J.
Vascular endothelial growth factor stimulates protein kinase C-dependent phospholipase D activity in endothelial cells
Lab. Invest.
v-Src increases diacylglycerol levels via a type D phospholipase-mediated hydrolysis of phosphatidylcholine
Mol. Cell. Biol.
Cited by (174)
Exploring phospholipase D signaling in the Warburg effect and cancer
2023, Phospholipases in Physiology and Pathology: Volumes 1-7Biochemical mechanisms in the regulation of phospholipases
2023, Phospholipases in Physiology and Pathology: Volumes 1-7The emerging role of phospholipase D in cancer progression and therapeutics
2023, Phospholipases in Physiology and Pathology: Volumes 1-7Underpinning the role of phospholipase D as a therapeutic target in cancer
2023, Phospholipases in Physiology and Pathology: Volumes 1-7Prostate cancer-derived exosomes promote osteoblast differentiation and activity through phospholipase D2
2020, Biochimica et Biophysica Acta - Molecular Basis of DiseaseCitation Excerpt :PLD catalyzes the hydrolysis of membrane phospholipids, mainly phosphatidylcholine, which yields to the production of phosphatidic acid (PA) and choline. PA is a potent regulator of mTOR [25], actin cytoskeleton organization [26,27], and it also interacts with a vast array of proteins [28,29]. Above all, PA is negatively charged and has a “cone” like shape which is thought to be helping with vesicles formation and stability [30], facilitating exosome biogenesis.