Cancer stem cells (CSCs) comprise a subset of hierarchically organized, rare cancer cells with the ability to initiate cancer in xenografts of genetically modified murine models. CSCs are thought to be responsible for tumor onset, self-renewal/maintenance, mutation accumulation, and metastasis. The existence of CSCs could explain the high frequency of neoplasia relapse and resistance to all of currently available therapies, including chemotherapy. The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling pathway is a key regulator of physiological cell processes which include proliferation, differentiation, apoptosis, motility, metabolism, and autophagy. Nevertheless, aberrantly upregulated PI3K/Akt/mTOR signaling characterizes many types of cancers where it negatively influences prognosis. Several lines of evidence indicate that this signaling system plays a key role also in CSC biology. Of note, CSCs are more sensitive to pathway inhibition with small molecules when compared to healthy stem cells. This observation provides the proof-of-principle that functional differences in signaling transduction pathways between CSCs and healthy stem cells can be identified. Here, we review the evidence which links the signals deriving from the PI3K/Akt/mTOR network with CSC biology, both in hematological and solid tumors. We then highlight how therapeutic targeting of PI3K/Akt/mTOR signaling with small molecule inhibitors could improve cancer patient outcome, by eliminating CSCs.

Lipid signaling pathways are involved in cell growth, differentiation, and apoptosis, and could have a role in the progression of myelodysplastic syndromes (MDS) into acute myeloid leukemia (AML). Indeed, recent studies showed that phosphoinositide-phospholipase (PI-PL)Cbeta1 mono-allelic deletion correlates with a higher risk of AML evolution. Also, a single patient treated with azacitidine, a DNA methyltransferase inhibitor currently used in MDS, displayed a direct correlation between PI-PLCbeta1 gene expression and drug responsiveness. Consequently, we hypothesized that PI-PLCbeta1 could be a target for demethylating therapy. First, we analyzed the structure of PI-PLCbeta1 gene promoter, then quantified the degree of PI-PLCbeta1 promoter methylation and gene expression in MDS patients at baseline and during azacitidine administration. Indeed, PI-PLCbeta1 mRNA increased in responder patients, along with a reduction of PI-PLCbeta1 promoter methylation. Also, the molecular response correlated to and anticipated the clinical outcome, thus suggesting that PI-PLCbeta1 gene reactivation could predict azacitidine responsiveness. Our results demonstrate not only that PI-PLCbeta1 promoter is hypermethylated in high-risk MDS patients, but also that the amount of PI-PLCbeta1 mRNA could predict the clinical response to azacitidine, therefore indicating a promising new therapeutic approach.

Phosphoinositides (PI) form just a minor portion of the total phospholipid content in cells but are significantly involved in cancer development and progression. In several cancer types, phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P₃] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] play significant roles in regulating survival, proliferation, invasion, and growth of cancer cells. Phosphoinositide-specific phospholipase C (PLC) catalyze the generation of the essential second messengers diacylglycerol (DAG) and inositol 1,4,5 trisphosphate (InsP₃) by hydrolyzing PtdIns(4,5)P₂. DAG and InsP₃ regulate Protein Kinase C (PKC) activation and the release of calcium ions (Ca²⁺) into the cytosol, respectively. This event leads to the control of several important biological processes implicated in cancer. PLCs have been extensively studied in cancer but their regulatory roles in the oncogenic process are not fully understood. This review aims to provide up-to-date knowledge on the involvement of PLCs in cancer. We focus specifically on PLCβ, PLCγ, PLCδ, and PLCε isoforms due to the numerous evidence of their involvement in various cancer types.

Inositide‐specific phospholipase Cβ1 (PLCβ1) signaling in cell proliferation has been investigated thoroughly in the G ₁ cell cycle phase. However, little is known about its involvement in G ₂ / M progression. We used murine erythroleukemia cells to investigate the role of PLCβ1 in G ₂ /M cell cycle progression and screened a number of candidate intermediate players, particularly mitogen‐activated protein kinase (MAPK) and protein kinase C (PKC), which can, potentially, transduce serum mitogenic stimulus and induce lamin B1 phosphorylation, leading to G ₂ /M progression. We report that PLCβ1 colocalizes and physically interacts with lamin B1. Studies of the effects of inhibitors and selective si‐RNA mediated silencing showed a role of JNK, PKCα, PKCβI, and the β1 isoform of PI‐PLC in cell accumulation in G ₂ / M [as observed by fluorescence‐activated cell sorter (FACS)]. To shed light on the mechanism, we considered that the final signaling target was lamin B1 phosphorylation. When JNK, PKCα,orPLCβ1 were silenced, lamin B1 exhibited a lower extent of phosphorylation, as compared to control. The salient features to emerge from these studies are a common pathway in which JNK is likely to represent a link between mitogenic stimulus and activation of PLCβ1, and, foremost, the finding that the PLCβ1‐mediated pathway represents a functional nuclear inositide signaling in the G ₂ /M transition.— Fiume, R., Ramazzotti, G., Teti, G., Chiarini, F., Faenza, I., Mazzotti, G., Billi, A. M., Cocco, L., Involvement of nuclear PLCβl in lamin B1 phosphorylation and G2/M cell cycle progression. FASEB J. 23, 957–966 (2009)

Our main goal in this study was to investigate the role of phospholipase C (PLC) β₁ and PLCγ₁ in skeletal muscle differentiation and the existence of potential downstream targets of their signaling activity. To examine whether PLC signaling can modulate the expression of cyclin D3, a target of PLCβ₁ in erythroleukemia cells, we transfected C2C12 cells with expression vectors containing PLCβ₁ or PLCγ₁ cDNA and with small interfering RNAs from regions of the PLCβ₁ or PLCγ₁ gene and followed myogenic differentiation in this well-established cell system. Intriguingly, overexpressed PLCβ₁ and PLCγ₁ were able to mimic insulin induction of both cyclin D3 and muscle differentiation. By knocking down PLCβ₁ or PLCγ₁ expression, C2C12 cells almost completely lost the increase in cyclin D3, and the differentiation program was down-regulated. To explore the induction of the cyclin D3 gene promoter during this process, we used a series of 5′-deletions of the 1.68-kb promoter linked to a reporter gene and noted a 5-fold augmentation of promoter activity upon insulin stimulation. These constructs were also cotransfected with PLCβ₁ or PLCγ₁ cDNAs and small interfering RNAs, respectively. Our data indicate that PLCβ₁ or PLCγ₁ signaling is capable of acting like insulin in regard to both the myogenic differentiation program and cyclin D3 up-regulation. Taken together, this is the first study that hints at cyclin D3 as a target of PLCβ₁ and PLCγ₁ during myogenic differentiation in vitro and implies that up-regulation of these enzymes is sufficient to mimic the actions of insulin in this process.