Lipid droplets (also known as lipid bodies) are lipid-rich, cytoplasmic organelles that play important roles in cell signaling, lipid metabolism, membrane trafficking, and the production of inflammatory mediators. Lipid droplet biogenesis is a regulated process, and accumulation of these organelles within leukocytes, epithelial cells, hepatocytes, and other nonadipocyte cells is a frequently observed phenotype in several physiologic or pathogenic situations and is thoroughly described during inflammatory conditions. Moreover, in recent years, several studies have described an increase in intracellular lipid accumulation in different neoplastic processes, although it is not clear whether lipid droplet accumulation is directly involved in the establishment of these different types of malignancies. This review discusses current evidence related to the biogenesis, composition and functions of lipid droplets related to the hallmarks of cancer: inflammation, cell metabolism, increased proliferation, escape from cell death, and hypoxia. Moreover, the potential of lipid droplets as markers of disease and targets for novel anti-inflammatory and antineoplastic therapies will be discussed.

Nuclear factor of activated T cells (NFAT) was first described as an activation and differentiation transcription factor in lymphocytes. Several in vitro studies suggest that NFAT family members are redundant proteins. However, analysis of mice deficient for NFAT proteins suggested different roles for the NFAT family of transcription factors in the regulation of cell proliferation and apoptosis. NFAT may also regulate several cell cycle and survival factors influencing tumor growth and survival. Here, we demonstrate that two constitutively active forms of NFAT proteins (CA-NFAT1 and CA-NFAT2 short isoform) induce distinct phenotypes in NIH 3T3 cells. Whereas CA-NFAT1 expression induces cell cycle arrest and apoptosis in NIH 3T3 fibroblasts, CA-NFAT2 short isoform leads to increased proliferation capacity and induction of cell transformation. Furthermore, NFAT1-deficient mice showed an increased propensity for chemical carcinogen-induced tumor formation, and CA-NFAT1 expression subverted the transformation of NIH 3T3 cells induced by the H-rasV12 oncogene. The differential roles for NFAT1 are at least partially due to the protein C-terminal domain. These results suggest that the NFAT1 gene acts as a tumor suppressor gene and the NFAT2 short isoform acts gene as an oncogene, supporting different roles for the two transcription factors in tumor development.

Accumulating evidence suggests that obesity and enhanced inflammatory reactions are predisposing conditions for developing colon cancer. Obesity is associated with high levels of circulating leptin. Leptin is an adipocytokine that is secreted by adipose tissue and modulates immune response and inflammation. Lipid droplets (LD) are organelles involved in lipid metabolism and production of inflammatory mediators, and increased numbers of LD were observed in human colon cancer. Leptin induces the formation of LD in macrophages in a PI3K/mTOR pathway-dependent manner. Moreover, the mTOR is a serine/threonine kinase that plays a key role in cellular growth and is frequently altered in tumors. We therefore investigated the role of leptin in the modulation of mTOR pathway and regulation of lipid metabolism and inflammatory phenotype in intestinal epithelial cells (IEC-6 cells). We show that leptin promotes a dose- and time-dependent enhancement of LD formation. The biogenesis of LD was accompanied by enhanced CXCL1/CINC-1, CCL2/MCP-1 and TGF-β production and increased COX-2 expression in these cells. We demonstrated that leptin-induced increased phosphorylation of STAT3 and AKT and a dose and time-dependent mTORC activation with enhanced phosphorilation of the downstream protein P70S6K protein. Pre-treatment with rapamycin significantly inhibited leptin effects in LD formation, COX-2 and TGF-β production in IEC-6 cells. Moreover, leptin was able to stimulate the proliferation of epithelial cells on a mTOR-dependent manner. We conclude that leptin regulates lipid metabolism, cytokine production and proliferation of intestinal cells through a mechanism largely dependent on activation of the mTOR pathway, thus suggesting that leptin-induced mTOR activation may contribute to the obesity-related enhanced susceptibility to colon carcinoma.

Multidrug resistance (MDR) is considered a multifactorial event that favors cancer cells becoming resistant to several chemotherapeutic agents. Numerous mechanisms contribute to MDR, such as P-glycoprotein (Pgp/ABCB1) activity that promotes drug efflux, overexpression of inhibitors of apoptosis proteins (IAP) that contribute to evasion of apoptosis, and oncogenic pathway activation that favors cancer cell survival. MDR molecules have been identified in membrane microparticles (MP) and can be transferred to sensitive cancer cells. By co-culturing MP derived from MDR-positive cells with recipient cells, we showed that sensitive cells accumulated Pgp, IAP proteins and mRNA. In addition, MP promoted microRNA transfer and NFκB and Yb-1 activation. Therefore, our results indicate that MP can induce a multifactorial phenotype in sensitive cancer cells.

Intracellular lipid accumulation has been associated with a poor prognosis in cancer. We have previously reported the involvement of lipid droplets in cell proliferation in colon cancer cells, suggesting a role for these organelles in cancer development. In this study, we evaluate the role of lipid droplets in cell cycle regulation and cellular transformation. Cell cycle synchronization of NIH 3T3 cells revealed increased numbers and dispersed distribution of lipid droplets specifically during S phase. Also, the transformed cell lineage NIH 3T3-H-rasV12showed an accumulation of both lipid droplets and PLIN2 protein above the levels in NIH 3T3 cells.PLIN2gene overexpression, however, was not able to induce NIH 3T3 cell transformation, disproving the hypothesis thatPLIN2is an oncogene. Furthermore, positive PLIN2 staining was strongly associated with highly proliferative Ki-67-positive areas in human colon adenocarcinoma tissue samples. Taken together, these results indicate that cell cycle progression is associated with tight regulation of lipid droplets, a process that is altered in transformed cells, suggesting the existence of a mechanism that connects cell cycle progression and cell proliferation with lipid accumulation.

Abstract. Lithium is a specific inhibitor of GSK3-β, and hence, an activator of the Wnt/β-catenin pathway, whereas the epidermal growth factor (EGF) has been linked to malignant transformation in epithelial cancer cells. Both pathways are aberrantly activated in most colorectal cancers (CRCs). However, the relationship between them in modulating events related to the progression of this cancer type remains to be defined. In this study, we investigated whether the Wnt/β-catenin and EGFR pathways converge to modulate the malignant potential of CRC. We used Caco-2 cells, a well-established model used to study CRC, and treatments with lithium chloride, as a modulator of the Wnt/β-catenin pathway, and with EGF as an inducer of EGFR signaling. We found that both agents altered the subcellular distribution of claudin-1 and β-catenin, two important proteins of the apical junctional complex, but not their abundance in the cell. Nuclear stabilization of β-catenin, a marker of Wnt pathway activation, was observed after treatment with both compounds. However, lithium, but not EGF, inhibited GSK3-β, indicating that these agents modulate this enzyme in a differential fashion. Furthermore, EGF treatment increased the proliferative and migratory capacity but did not alter the colony formation potential of these cells. Surprisingly, lithium, known to activate the Wnt/β-catenin pathway, inhibited the increased proliferation by arresting cells in the G 2 /M phase as well as the cell migration promoted by EGF, as demonstrated by the combined treatment with these agents. Lithium treatment alone reduced the cell colony formation. Thus, our findings suggest that lithium plays an important role in regulating cellular events related to tumor progression in CRC.

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NFAT transcription factors are highly phosphorylated proteins residing in the cytoplasm of resting cells. Upon dephosphorylation by the phosphatase calcineurin, NFAT translocates to the nucleus, where it orchestrates developmental and activation programs in diverse cell types. NFAT is rephosphorylated and inactivated through the concerted action of at least three different kinases: CK1, GSK-3 and DYRK. NFAT signalling is further implicated in lymphocyte homeostasis and its deregulation has been suggested to be involved in the pathogenesis of different malignancies. Previous studies have primarily concentrated on NFAT2, which was found to be overexpressed and constitutively activated in a majority of biopsies from Diffuse Large B-Cell lymphomas and Burkitt’s lymphomas, presumably reflecting activation of the Ca/NFAT signalling pathway as part of their pathogenesis. Other recent studies have demonstrated that NFAT2 activation can lead to increased expression of different cell survival factors (CD154, BLyS) in several NHL subtypes. Here, we analyzed the role of NFAT1 in malignant transformation and in the pathogenesis of T-ALL using a transgenic mouse model for the disease. We show that sustained activity of NFAT1 and NFAT2 induces opposite phenotypes in NIH 3T3 cells. While NIH 3T3 cells infected with empty retrovirus showed normal viability and stopped growing upon reaching confluence, cells infected with a constitutively active version of NFAT2 overgrew the monolayer and continued to proliferate beyond confluence. NIH 3T3 cells expressing a constitutively active form of NFAT1 on the contrary exhibited a remarkable reduction in cell proliferation and never reached confluence, suggesting opposite roles for NFAT1 and NFAT2 in the control of cell growth and proliferation. To directly test whether NFAT1 has the capability to suppress tumor growth and to subvert a transformed cell type, we generated NIH 3T3 cell clones harboring the oncogene H-rasV12, which showed a typical transformed phenotype. Expression of constitutively active NFAT1 in the transformed NIH 3T3-HrasV12 cells leads to a significant reduction in cell proliferation which was accomanied by a large decrease in DNA replication and an accumulation in the G1 phase of the cell cycle. To test if NFAT1 has tumor suppressor characteristics in vivo, we generated transgenic mice conditionally expressing a hyperactivable form of NFAT1 from the ROSA26 locus. These mice were subsequently bred to Tel-Jak2 transgenic mice, which develop T-ALL at 8–20 wks of age. Mice that expressed hyperactivable NFAT1 in their T cells in addition to the disease inducing Tel-Jak2 transgene exhibited a significantly attenuated phenotype of the disease. Infiltration of vital organs such as lung, liver and bone marrow was significantly delayed and overall survival was almost twice as long in the animals that expressed hyperactivable NFAT1 in their T cell compartment (88 days vs. 159 days). To summarize, our data identify NFAT1 as a novel tumor suppressor gene in T-ALL and emphasize the importance of the Ca/NFAT signalling pathway in the pathogenesis of hematologic malignancies.