Tumor suppressive effect of lysyl oxidase proenzyme

Lysyl Oxidases: Orchestrators of Cellular Behavior and ECM Remodeling and Homeostasis

Lysyl oxidases have long been considered key secreted extracellular matrix modifying enzymes. As such, their activity has been associated with the crosslinking of collagens and elastin, and as a result, they have been linked to multiple developmental and pathological processes. However, numerous lines of evidence also demonstrated that members of this enzyme family are localized and are active within the cytoplasm or cell nuclei, where they regulate and participate in distinct cellular events. In this review, we focus on a few of these events and highlight the intracellular role these enzymes play. Close examination of these events, suggest that the intracellular activities of lysyl oxidases is mostly observed in processes where concomitant changes in the extracellular matrix takes place. Here, we suggest that the LOX family members act in the relay between changes in the cells’ environment and the intracellular processes that promote them or that follow.

Lysyl oxidase-variant 2 (LOX-v2) colocalizes with promyelocytic leukemia-nuclear bodies in the nucleus

Lysyl oxidase-variant 2 (LOX-v2) is a novel variant of LOX that functions as an amine oxidase for the formation of collagen and elastin fibrils in the extracellular matrix (ECM). LOX-v2 lacks the N-terminal prepropeptide region of LOX but contains the C-terminal domains required for amine oxidase activity. To study the cellular localization of LOX-v2, we generated a recombinant construct of LOX-v2 with an epitope tag at the C-terminus and then transfected the recombinant construct into HEK293 cells. Upon ectopic expression, LOX-v2 showed much higher expression in the nucleus than in the cytoplasm. In coimmunofluorescence staining with subnuclear structures, LOX-v2 colocalized with the promyelocytic leukemia-nuclear bodies (PML-NBs). Further, the ectopic expression of LOX-v2 increased global SUMOylation in the nucleus. PML-NBs have been implicated in various cellular activities, including transcriptional regulation, DNA repair, cell cycle control, anti-viral response, and apoptosis. Our findings strongly indicate that LOX-v2 may be subject to different cellular processing from what LOX undergoes, playing a distinct functional role in the PML-NBs, beyond the cross-linking of the structural proteins.

Decreased lysyl oxidase level protects against development of retinal vascular lesions in diabetic retinopathy

Retinal capillary basement membrane (BM) thickening is closely associated with the development of vascular lesions in diabetic retinopathy. Thickened capillary BM can compromise blood-retinal-barrier characteristics and contribute to retinal vascular permeability, a significant clinical manifestation of diabetic retinopathy. We have previously shown that high glucose increases the expression and activity of lysyl oxidase (LOX), a crosslinking enzyme, in retinal endothelial cells. Additionally, concomitant with overexpression of LOX, increased vascular permeability was observed in diabetic rat retinas. However, it is unknown whether decreasing LOX overexpression may have protective effects against development of retinal vascular lesions in diabetes. To investigate whether reduced LOX level protects against diabetes-induced development of retinal vascular lesions characteristic of diabetic retinopathy, four groups of mice: wild type (WT) control mice, streptozotocin (STZ)-induced diabetic mice, LOX +/- mice, and STZ-induced diabetic LOX +/- mice were used for this study. Diabetes was maintained for 16 weeks; at the end of the study, retinas were assessed for LOX protein level by Western Blot (WB) analysis, and retinal capillary networks were isolated using retinal trypsin digestion and stained with hematoxylin and periodic acid Schiff to identify the number of acellular capillaries (AC) and pericyte loss (PL). In parallel, TUNEL assay was performed on retinal trypsin digests (RTDs) to detect cells undergoing apoptosis in the retinal capillary networks. Retinal vascular permeability was analyzed following FITC-dextran injection in retinal whole mounts. A significant increase in LOX expression was detected in the diabetic retinas compared to those of the WT control retinas, and as expected, a significant decrease in LOX expression in the diabetic LOX +/- retinas was observed compared to those of the diabetic retinas. RTD images showed significantly increased AC and PL counts in the retinas of diabetic mice compared to those of the WT control mice. Importantly, the number of AC and PL was significantly decreased, as was retinal vascular permeability in the retinas of the diabetic LOX +/- mice compared to those of the diabetic mice. Results suggest that decreasing diabetes-induced LOX overexpression may have protective effects against the development of vascular lesions characteristic of diabetic retinopathy. Therefore, LOX overexpression may be a potential target in preventing retinal vascular cell loss and excess permeability associated with diabetic retinopathy.

Divergent roles of lysyl oxidase family members in ornithine decarboxylase- and RAS-transformed mouse fibroblasts and human melanoma cells

We have previously shown that proto-oncoprotein c-Jun is activated in ornithine decarboxylase (ODC)- and RAS-transformed mouse fibroblasts, and that the transformed morphology of these cells can be reversed by expressing the transactivation domain deletion mutant of c-Jun (TAM67). Here, we found that lysyl oxidase (Lox), encoding an extracellular matrix-modifying enzyme, is downregulated in a c-Jun-dependent manner in ODC-transformed fibroblasts (Odc cells). In addition to Lox, the Lox family members Lox-like 1 and 3 (Loxl1 and Loxl3) were found to be downregulated in Odc as well as in RAS-transformed fibroblasts (E4), whereas Lox-like 4 (Loxl4) was upregulated in Odc and downregulated in E4 cells compared to normal N1 fibroblasts. Tetracycline-regulatable LOX re-expression in Odc cells led to inhibition of cell growth and invasion in three-dimensional Matrigel in an activity-independent manner. On the contrary, LOX and especially LOXL2, LOXL3, and LOXL4 were found to be upregulated in several human melanoma cell lines, and LOX inhibitor B-aminopropionitrile inhibited the invasive growth of these cells particularly when co-cultured with fibroblasts in Matrigel. Knocking down the expression of LOX and especially LOXL2 in melanoma cells almost completely abrogated the invasive growth capability. Further, LOXL2 was significantly upregulated in clinical human primary melanomas compared to benign nevi, and high expression of LOXL2 in primary melanomas was associated with formation of metastases and shorter survival of patients. Thus, our studies reveal that inactive pro-LOX (together with Lox propeptide) functions as a tumor suppressor in ODC- and RAS-transformed murine fibroblasts by inhibiting cell growth and invasion, and active LOX and LOXL2 as tumor promoters in human melanoma cells by promoting their invasive growth.

Lysyl oxidase is associated with the epithelial-mesenchymal transition of gastric cancer cells in hypoxia

PURPOSE

It has been reported that lysyl oxidase (LOX) is a hypoxia-responsive factor and is associated with the malignant progression of carcinoma. The aim of this study was to clarify the relationship between the epithelial-mesenchymal transition (EMT) and LOX in gastric cancer cells under hypoxia.

METHODS

Two gastric cancer cell lines, OCUM-2MD3 and OCUM-12, were used in an in vitro study. The effect of LOX small interfering RNA (siRNA) on the EMT and motility of gastric cancer cells under hypoxic condition was analyzed by reverse transcription PCR, Western blot, a wound-healing assay, and an invasion assay. Correlations between LOX expression and the clinicopathological features of 544 patients with gastric carcinoma were examined immunohistochemically.

RESULTS

Hypoxic conditions increased the number of polygonal or spindle-shaped cells resulting from EMT in gastric cancer cells. The EMT of cancer cells induced by hypoxia was inhibited by treatment with LOX siRNA. The number of migrating and invading gastric cancer cells in hypoxia was significantly decreased by LOX knockdown. LOX siRNA significantly increased the E-cadherin level and decreased the vimentin level of gastric cancer cells. LOX expression was significantly associated with invasion depth, tumor differentiation, lymph node metastasis, lymphatic invasion, venous invasion, and peritoneal metastasis. Multivariable analysis revealed that LOX was an independent parameter for overall survival.

CONCLUSION

LOX affects the EMT of gastric cancer cells in hypoxic conditions. LOX expression is a useful prognostic factor for patients with gastric cancer.

Hypoxia and Prostate Cancer Aggressiveness: A Tale With Many Endings

Angiogenesis, increased glycolysis, and cellular adaptation to hypoxic microenvironment are characteristic of solid tumors, including prostate cancer. These representative features are the cornerstone of cancer biology, which are well correlated with invasion, metastasis, and lethality, as well as likely with the success of prostate cancer treatment (eg, tumor hypoxia has been associated with resistance to chemotherapy and radiotherapy). It is well established that prostate cancer cells also metabolically depend on enhanced glucose transport and glycolysis for expansion, whereas growth is contingent with neovascularization to permit diffusion of oxygen and glucose. While hypoxia inducible factor 1 alpha (HIF-1α) remains the central player, the succeeding activated molecules and pathways track distinct branches, all positively correlated with the degree of intratumoral hypoxia. Among these, the vascular endothelial growth factor axis as well as the lysyl oxidase and carbonic anhydrase IX activities are notable in prostate cancer and merit further study. Here, we demonstrate their linkage with HIF-1α as a tentative explanatory mechanism of prostate cancer aggressiveness. Hypoxia drives a tale where HIF-1α-dependent effects lead to many influences in distinct key cancer biology features, rendering targeted therapies toward targets at the endings less efficient. The most appropriate approach will be to inhibit the upstream common driver (HIF-1α) activity. Additional translational and clinical research initiatives in prostate cancer are required to prove its usefulness.

Nuclear translocation of lysyl oxidase is promoted by interaction with transcription repressor p66β

Lysyl oxidase (LOX) is an amine oxidase involved in protein cross-linking of the extracellular matrix. Less well characterized is the role that LOX plays among nuclear proteins, and molecular mechanisms of its transport to the nucleus are currently unknown. Here, we have employed yeast two-hybrid library screening and found that the LOX catalytic domain interacts with the transcription repressor p66β. This interaction has been confirmed in vitro and has been found to be accomplished through the CR2-containing domain of p66β. Moreover, co-expression of p66β and LOX in living tumor cells leads to the nuclear accumulation of LOX. Thus, p66β might be important for the regulation of LOX in the nucleus.

Human copper-dependent amine oxidases

Copper amine oxidases (CAOs) are a class of enzymes that contain Cu(2+) and a tyrosine-derived quinone cofactor, catalyze the conversion of a primary amine functional group to an aldehyde, and generate hydrogen peroxide and ammonia as byproducts. These enzymes can be classified into two non-homologous families: 2,4,5-trihydroxyphenylalanine quinone (TPQ)-dependent CAOs and the lysine tyrosylquinone (LTQ)-dependent lysyl oxidase (LOX) family of proteins. In this review, we will focus on recent developments in the field of research concerning human CAOs and the LOX family of proteins. The aberrant expression of these enzymes is linked to inflammation, fibrosis, tumor metastasis/invasion and other diseases. Consequently, there is a critical need to understand the functions of these proteins at the molecular level, so that strategies targeting these enzymes can be developed to combat human diseases.

Functional aspects of cytidine-guanosine dinucleotides and their locations in genes

Originally, the finding of a particular distribution of cytidine-guanosine dinucleotides (CpGs) in genomic DNA was considered to be an interesting structural feature of eukaryotic genome organization. Despite a global depletion of CpGs, genes are frequently associated with CpG clusters called CpG islands (CGIs). CGIs are prevalently unmethylated but often found methylated in pathologic situations. On the other hand, CpGs outside of CGIs are generally methylated and are found mainly in the heterochromatic fraction of the genome. Hypomethylation of those CpGs is associated with genomic instability in malignancy. Additionally, CpG-rich and CpG-poor regions, as well as CpG-shores, are defined. Usually, the methylation status inversely correlates with gene expression. Methylation of CpGs, as well as demethylation and generation of hydroxmethyl-cytosines, is strictly regulated during development and differentiation. This review deals with the relevance of the organizational features of CpGs and their relation to each other.

Lysyl oxidase propeptide inhibits prostate cancer cell growth by mechanisms that target FGF-2-cell binding and signaling

Enhanced RAS signaling and decreased androgen dependence of prostate cancer cells accompany poor clinical outcomes. Elevated autocrine FGF-2 signaling promotes prostate cancer cell growth and survival. Expression of lysyl oxidase (LOX) inhibits RAS transforming activity. LOX is secreted as 50 kDa pro-lysyl oxidase protein and then undergoes extracellular proteolytic processing to form ~30 kDa lysyl oxidase enzyme and ~18 kDa pro-peptide (LOX-PP). We have previously shown that LOX-PP inhibits breast cancer cell transformation and tumor formation, but mechanisms of action of LOX-PP have not been fully elucidated. Here we report that LOX expression is reduced in prostate cancer cell lines and that recombinant LOX-PP protein inhibits serum-stimulated DNA synthesis and MEK/ERK and PI3K/AKT pathways in DU 145 and PC-3 androgen-independent cell lines. In DU 145 cells, treatment with a pharmacologic FGF-receptor inhibitor or a neutralizing anti-FGFR1 antibody mimicked LOX-PP inhibition of serum-stimulated DNA synthesis. FGF-2-stimulated DNA synthesis, ERK1/2, AKT, and FRS2α activation were found all to be inhibited by LOX-PP in DU 145 cells. LOX-PP reduced specific binding of FGF-2 to DU 145 cells, suggesting that LOX-PP targets FGF signaling at the receptor. Interestingly, PC-3 cells did not respond to FGF-2, consistent with previous reports. We conclude that LOX-PP inhibits proliferation of DU 145 cells by interfering with FGFR(s) binding and signaling, and that LOX-PP has other mechanisms of action in PC-3 cells.