Reflections on the days of phospholipase C

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

Phospholipase Signaling in Breast Cancer

Breast cancer progression results from subversion of multiple intra- or intercellular signaling pathways in normal mammary tissues and their microenvironment, which have an impact on cell differentiation, proliferation, migration, and angiogenesis. Phospholipases (PLC, PLD and PLA) are essential mediators of intra- and intercellular signaling. They hydrolyze phospholipids, which are major components of cell membrane that can generate many bioactive lipid mediators, such as diacylglycerol, phosphatidic acid, lysophosphatidic acid, and arachidonic acid. Enzymatic processing of phospholipids by phospholipases converts these molecules into lipid mediators that regulate multiple cellular processes, which in turn can promote breast cancer progression. Thus, dysregulation of phospholipases contributes to a number of human diseases, including cancer. This review describes how phospholipases regulate multiple cancer-associated cellular processes, and the interplay among different phospholipases in breast cancer. A thorough understanding of the breast cancer-associated signaling networks of phospholipases is necessary to determine whether these enzymes are potential targets for innovative therapeutic strategies.

PLCD3, a flotillin2-interacting protein, is involved in proliferation, migration and invasion of nasopharyngeal carcinoma cells

Phospholipase C (PLC) is a pivotal enzyme in the phosphoinositide pathway that promotes the second messengers, diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), to participate in eukaryotic signal transduction. Several PLC isozymes are associated with cancer, such as PLC-β1, PLC-δ1, PLC-ε and PLC-γ1. However, the role of PLC-δ3 (PLCD3) in nasopharyngeal carcinoma (NPC) has not been investigated to date. In our previous study, we demonstrated that flotillin2 (Flot2) plays a pro-neoplastic role in NPC and is involved in tumour progression and metastasis. In the present study, we screened the interacting proteins of Flot2 using the yeast two-hybrid (Y2H) method and verified the interaction between PLCD3 and Flot2 by co-immunoprecipitation. We also investigated the biological functions of PLCD3 in NPC. Inhibition of PLCD3 expression impaired the malignant potential of 5–8F, a highly metastatic NPC cell line, by restraining its growth, proliferation, mobility and migration. The present study demonstrated that PLCD3 may be an oncogenic protein in NPC and that it plays an important role in the progression of NPC partially by interacting with Flot2.

BMP-2 Induced Expression of PLCβ1 That is a Positive Regulator of Osteoblast Differentiation

Bone morphogenetic protein 2 (BMP-2) is a critical growth factor that directs osteoblast differentiation and bone formation. Phosphoinositide-phospholipase Cβ 1 (PLCβ1) plays a crucial role in the initiation of the genetic program responsible for muscle differentiation. Differentiation of C2C12 mouse myoblasts in response to insulin stimulation is characterized by a marked increase in nuclear PLCβ1. Here, the function of PLCβ1 in the osteogenic differentiation was investigated. Briefly, in C2C12 cells treated with BMP-2 we assist to a remarkable increase in PLCβ1 protein and mRNA expression. The data regarding the influence on differentiation demonstrated that PLCβ1 promotes osteogenic differentiation by up-regulating alkaline phosphatase (ALP). Moreover, PLCβ1 is present in the nuclear compartment of these cells and overexpression of a cytosolic-PLCβ1mutant (cyt-PLCβ1), which lacks a nuclear localization sequence, prevented the differentiation of C2C12 cells into osteocytes. Recent evidence indicates that miRNAs act as important post transcriptional regulators in a large number of processes, including osteoblast differentiation. Since miR-214 is a regulator of Osterix (Osx) which is an osteoblast-specific transcription factor that is needful for osteoblast differentiation and bone formation, we further investigated whether PLCβ1 could be a potential target of miR-214 in the control of osteogenic differentiation by gain- and loss- of function experiment. The results indicated that inhibition of miR-214 in C2C12 cells significantly enhances the protein level of PLCβ1 and promotes C2C12 BMP-2-induced osteogenesis by targeting PLCβ1.

Inositide-dependent signaling pathways as new therapeutic targets in myelodysplastic syndromes

INTRODUCTION

Nuclear inositide signaling pathways specifically regulate cell proliferation and differentiation. Interestingly, the modulation of nuclear inositides in hematological malignancies can differentially affect erythropoiesis or myelopoiesis. This is particularly important in patients with myelodysplastic syndromes (MDS), who show both defective erythroid and myeloid differentiation, as well as an increased risk of evolution into acute myeloid leukemia (AML).

AREAS COVERED

This review focuses on the structure and function of specific nuclear inositide enzymes, whose impairment could be linked with disease pathogenesis and cancer. The authors, stemming from literature and published data, discuss and describe the role of nuclear inositides, focusing on specific enzymes and demonstrating that targeting these molecules could be important to develop innovative therapeutic approaches, with particular reference to MDS treatment.

EXPERT OPINION

Demethylating therapy, alone or in combination with other drugs, is the most common and current therapy for MDS patients. Nuclear inositide signaling molecules have been demonstrated to be important in hematopoietic differentiation and are promising new targets for developing a personalized MDS therapy. Indeed, these enzymes can be ideal targets for drug design and their modulation can have several important downstream effects to regulate MDS pathogenesis and prevent MDS progression to AML.

Phosphoinositide-specific phospholipase C in health and disease

Phospholipases are widely occurring and can be found in several different organisms, including bacteria, yeast, plants, animals, and viruses. Phospholipase C (PLC) is a class of phospholipases that cleaves phospholipids on the diacylglycerol (DAG) side of the phosphodiester bond producing DAGs and phosphomonoesters. Among PLCs, phosphoinositide-specific PLC (PI-PLC) constitutes an important step in the inositide signaling pathways. The structures of PI-PLC isozymes show conserved domains as well as regulatory specific domains. This is important, as most PI-PLCs share a common mechanism, but each of them has a peculiar role and can have a specific cell distribution that is linked to a specific function. More importantly, the regulation of PLC isozymes is fundamental in health and disease, as there are several PLC-dependent molecular mechanisms that are associated with the activation or inhibition of important physiopathological processes. Moreover, PI-PLC alternative splicing variants can play important roles in complex signaling networks, not only in cancer but also in other diseases. That is why PI-PLC isozymes are now considered as important molecules that are essential for better understanding the molecular mechanisms underlying both physiology and pathogenesis, and are also potential molecular targets useful for the development of innovative therapeutic strategies.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

PLC and PI3K/Akt/mTOR signalling in disease and cancer

Cancer cell metabolism is deregulated, and signalling pathways can be involved. For instance, PI3K/Akt/mTOR is associated with normal proliferation and differentiation, and its alteration is detectable in cancer cells, that exploit the normal mechanisms to overcome apoptosis. On the other hand, also the family of Phospholipase C (PLC) enzymes play a critical role in cell growth, and any change concerning these enzymes or their downstream targets can be associated with neoplastic transformation. Here, we review the role of PLC and PI3K/Akt/mTOR signal transduction pathways in pathophysiology.