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Despite advances in the field, colorectal cancer (CRC) remains a leading cause of cancer-related mortality worldwide. Research into bioactive sphingolipids over the past two decades has played an important role in increasing our understanding of the pathogenesis and therapeutics of CRC. In the complex metabolic network of sphingolipids, ceramidases (CDases) have a key function. These enzymes hydrolyze ceramides into sphingosine (SPH) which in turn is phosphorylated by sphingosine kinases (SK) 1 and 2 to generate sphingosine-1 phosphate (S1P). Importantly, we have recently shown that inhibition of neutral CDase (nCDase) induces an increase of ceramide in colon cancer cells which decreases cellular growth, increases apoptosis and modulates the WNT/β-catenin pathway. We have also shown that the deletion of nCDase protected mice from the onset and progression of colorectal cancer in the AOM carcinogen model. Here, we demonstrate that AKT is a key target for the growth suppressing functions of ceramide. The results show that inhibition of nCDase activates GSK3β through dephosphorylation, and thus is required for the subsequent phosphorylation and degradation of β-catenin. Our findings show that inhibition of nCDase also inhibits the basal activation status of AKT, and we further establish that a constitutively active AKT (AKT T308D, S473D; AKTDD) reverses the effect of nCDase on β-catenin degradation. Functionally, the AKTDD mutant is able to overcome the growth suppressive effects of nCDase inhibition in CRC cells. Moreover, nCDase inhibition induces a growth delay of xenograft tumors from control cells, whereas xenograft tumors from constitutively active AKT cells become resistant to nCDase inhibition. Taken together, these results provide important mechanistic insight into how nCDase regulates cell proliferation. These findings demonstrate a heretofore unappreciated, but critical, role for nCDase in enabling/maintaining basal activation of AKT and also suggest that nCDase is a suitable novel target for colon cancer therapy.

Sphingosine-1-phosphate (S1P) plays important roles in cardiovascular development and immunity. S1P is abundant in plasma because erythrocytes-the major source of S1P-lack any S1P-degrading activity; however, much remains unclear about the source of the plasma S1P precursor, sphingosine (SPH), derived mainly from the hydrolysis of ceramides by the action of ceramidases that are encoded by 5 distinct genes, acid ceramidase 1 ( ASAH1)/ Asah1, ASAH2/ Asah2, alkaline ceramidase 1 ( ACER1)/ Acer1, ACER2/ Acer2, and ACER3/ Acer3, in humans/mice. Previous studies have reported that knocking out Asah1 or Asah2 failed to reduce plasma SPH and S1P levels in mice. In this study, we show that knocking out Acer1 or Acer3 also failed to reduce the blood levels of SPH or S1P in mice. In contrast, knocking out Acer2 from either whole-body or the hematopoietic lineage markedly decreased the blood levels of SPH and S1P in mice. Of interest, knocking out Acer2 from whole-body or the hematopoietic lineage also markedly decreased the levels of dihydrosphingosine (dhSPH) and dihydrosphingosine-1-phosphate (dhS1P) in blood. Taken together, these results suggest that ACER2 plays a key role in the maintenance of high plasma levels of sphingoid base-1-phosphates-S1P and dhS1P-by controlling the generation of sphingoid bases-SPH and dhSPH-in hematopoietic cells.-Li, F., Xu, R., Low, B. E., Lin, C.-L., Garcia-Barros, M., Schrandt, J., Mileva, I., Snider, A., Luo, C. K., Jiang, X.-C., Li, M.-S., Hannun, Y. A., Obeid, L. M., Wiles, M. V., Mao, C. Alkaline ceramidase 2 is essential for the homeostasis of plasma sphingoid bases and their phosphates.

Traditional methods of cancer treatment are limited in their efficacy due to both inherent and acquired factors. Many different studies have shown that the generation of ceramide in response to cytotoxic therapy is generally an important step leading to cell death. Cancer cells employ different methods to both limit ceramide generation and to remove ceramide in order to become resistant to treatment. Furthermore, sphingosine kinase activity, which phosphorylates sphingosine the product of ceramide hydrolysis, has been linked to multidrug resistance, and can act as a strong survival factor. This review will examine several of the most frequently used cancer therapies and their effect on both ceramide generation and the mechanisms employed to remove it. The development and use of inhibitors of sphingosine kinase will be focused upon as an example of how targeting sphingolipid metabolism may provide an effective means to improve treatment response rates and reduce associated treatment toxicity. This article is part of a Special Issue entitled Tools to study lipid functions.

Colorectal cancer is one of the major causes of death in the western world. Despite increasing knowledge of the molecular signaling pathways implicated in colon cancer, therapeutic outcomes are still only moderately successful. Sphingolipids, a family of N-acyl linked lipids, have not only structural functions but are also implicated in important biological functions. Ceramide, sphingosine and sphingosine-1-phosphate are the most important bioactive lipids, and they regulate several key cellular functions. Accumulating evidence suggests that many cancers present alterations in sphingolipids and their metabolizing enzymes. The aim of this review is to discuss the emerging roles of sphingolipids, both endogenous and dietary, in colon cancer and the interaction of sphingolipids with WNT/β-catenin pathway, one of the most important signaling cascades that regulate development and homeostasis in intestine. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Colorectal cancer is one of the leading causes of cancer mortality in Western civilization. Studies have shown that colorectal cancer arises as a consequence of the modification of genes that regulate important cellular functions. Deregulation of the WNT and RAS/MAPK/PI3K signaling pathways has been shown to be important in the early stages of colorectal cancer development and progression. Krüppel-like factor 5 (KLF5) is a transcription factor that is highly expressed in the proliferating intestinal crypt epithelial cells. Previously, we showed that KLF5 is a mediator of RAS/MAPK and WNT signaling pathways under homeostatic conditions and that it promotes their tumorigenic functions during the development and progression of intestinal adenomas. Recently, using an ultrahigh-throughput screening approach we identified a number of novel small molecules that have the potential to provide therapeutic benefits for colorectal cancer by targeting KLF5 expression. In the current study, we show that an improved analogue of one of these screening hits, ML264, potently inhibits proliferation of colorectal cancer cells in vitro through modifications of the cell-cycle profile. Moreover, in an established xenograft mouse model of colon cancer, we demonstrate that ML264 efficiently inhibits growth of the tumor within 5 days of treatment. We show that this effect is caused by a significant reduction in proliferation and that ML264 potently inhibits the expression of KLF5 and EGR1, a transcriptional activator of KLF5. These findings demonstrate that ML264, or an analogue, may hold a promise as a novel therapeutic agent to curb the development and progression of colorectal cancer. Mol Cancer Ther; 15(1); 72–83. ©2015 AACR.

Glucocorticoids (GCs) play a key role in the physiology of the hypothalamic-somatotroph axis, since these steroids enhance growth hormone (GH) gene transcription and increase GHRH receptor synthesis. However, GC excess inhibits normal growth in all species studied. This is mainly due to the impaired GH secretion observed during hypercortisolism, a situation in which GH responses to a number of stimuli, including GHRH, are blunted. The inhibitory effect of GCs on GH secretion seems to be dependent on enhanced hypothalamic SS secretion. Since SS release is stimulated by beta-adrenergic agonism we tested the possibility that GC inhibition of GH secretion would depend on increased beta-adrenoceptor activity in SS-producing neurons. The experimental design consisted in evaluating the GH response to GHRH in normal subjects after having induced hypercortisolism, with DEX, and blocking beta-adrenoceptors with propranolol (PRO). Moreover, to investigate the specificity of this mechanism, GHRH-induced GH release was tested after inducing hypercortisolism and enhancing alpha 2-adrenergic or muscarinic cholinergic tone, by giving clonidine (CLO) or pyridostigmine (PD), respectively. As expected, nocturnal DEX administration inhibited the GH response to GHRH. In this situation of hypercortisolism, both PRO and CLO, but not PD, were able to reverse the inhibitory effect of DEX on GHRH-elicited release. However, the potentiating effect of these drugs on the GHRH-induced GH secretion was only observed for PRO. These data confirm that GC excess inhibits GH release by increasing hypothalamic SS secretion, and that the mechanism is mediated by GC-induced enhanced beta-adrenergic responsiveness. Therefore, the defective GHRH secretion observed in chronic hypercortisolism must be a consequence of the continuous blockade that SS excess exerts on GHRH-producing neurons. Our postulate agrees with other data in the literature showing that GCs modulate the secretion of some hypothalamic peptides by changing the responsiveness of the producing neurons from alpha 2-adrenoceptors to that of beta-adrenoceptors.