Specific growth suppression of human cancer cells by targeted delivery of Dictyostelium mitochondrial ribosomal protein S4

Abstract

BackgroundIn general, growth and differentiation are mutually exclusive but are cooperatively regulated throughout development. Thus, the process of a cell’s switching from growth to differentiation is of great importance not only for the development of organisms but also for malignant transformation, in which this process is reversed. We have previously demonstrated using a Dictyostelium model system that the Dictyostelium mitochondrial ribosomal protein S4 (Dd-mrp4) gene expression is essential for the initiation of cell differentiation: Dd-mrp4-null cells fail to initiate differentiation, while the initial step of cell differentiation and the subsequent morphogenesis are markedly enhanced in mrp4OE cells overexpressing the Dd-mrp4 in the extramitochondrial cytoplasm. This raised a possibility that the ectopically enforced expression of the Dd-mrp4 in human cells might inhibit their growth, particularly of malignant tumor cells, by inducing cell differentiation.MethodsFour kinds of human tumor cell lines were transfected by three kind of vector constructs (the empty vector: pcDNA3.1 (Mock); pcDNA3.1-rps4 bearing Dictyostelium cytoplasmic ribosomal protein S4; pcDNA3.1-mrp4 bearing Dictyostelium mitochondrial ribosomal protein S4). As controls, four kinds of human primary cultured cells were similarly transfected by the above vector constructs. After transfection, growth kinetics of cells was analyzed using cell viability assay, and also the TUNEL method was used for evaluation of apoptotic cells.ResultsEctopically expressed Dd-mrp4 suppressed cell proliferation through inducing apoptotic cell death specifically in the human lung adenocarcinoma (A549), epithelial cervical cancer (HeLa), hepatocellular carcinoma (HepG2) and colonic carcinoma (Caco-2), but not in primary cultured normal cells, such as human brain microvascular endothelial cells (HBMECs); human umbilical vein endothelial cells (HUVECs) and human normal hepatocytes (hHeps™), with one exception (human cardiac fibloblasts (HCF)).ConclusionThe present finding that the ectopically enforced expression of Dd-mrp4 in human several tumor cell lines specifically suppresses their proliferation suggests strongly that the Dd-mrp4 gene derived from Dictyostelium mitochondria may provide a new promising therapeutic strategy for disrupting cell viability pathways in human cancers.

Cell Motility and Cancer

“…For this reason, many studies have analyzed cell motility in non-mammalian model organisms, like amoebae, worms, flies and others. Elegant studies with Batrachochytrium dendrobatidis [2][3][4], Dictyostelium discoideum [5][6][7][8], Caenorhabditis elegans [9][10][11] and Drosophila melanogaster [12][13][14][15][16], among others, have provided useful information to understand the fundamental mechanisms of cell motility.…”

A Long-Range Foresight for the Medical Application of Apoptosis Specifically Induced by Dd-MRP4, Dictyostelium Mitochondrial Ribosomal Protein S4, to Cancer Therapy

“…Dd-MRP4 is highly basic (pI: about 11.5), just like histone, and easily imagined to bind readily with DNA and/or RNA. Homology of Dd-MRP4 to Dd-RPS4 (cytoplasmic Dictyostelium ribosomal protein) and HRPS4 (human ribosomal protein) is quite low, as previously described [ 4 ], though it has a S4 RNA binding domain that probably mediates binding to RNA.…”

“…First of all, the most active and shortest regions of Dd-mrp4 gene, which have extremely high specificity to cancer cell, should be determined. For this, deletion experiments of Dd-mrp4 gene, using cancer cell lines such as hepatocellular carcinoma (HepG2), will be helpful, because HepG2 cells exhibited the most striking response to heterogeneously expressed Dd-mrp4 [ 4 ]. The protein produced from the most active region of Dd-mrp4 gene is referred to as “MRP4-MC” for convenience.…”

“…Mitochondria exert a central role in many pathways leading to programmed cell death, and several apoptosis-inducing substances have been reported ( Table 1 ) [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. Although the precise mechanisms underlying the role in apoptosis remain to be elucidated and seem to be somewhat different from one species to another, it is mainly controlled by mitochondrial proteins.…”

“…Coupled with this finding, increasing evidence indicates that mitochondria have novel, essential, and multiple functions as the regulatory machinery of the initiation of differentiation, cell-type determination, cell movement and pattern formation, most strikingly evidenced in Dicyostelium development [ 18 ]. As suggested, we have recently found that an ectopically expressed Dd-MRP4 is able to suppress specifically proliferation of human cancer cells, by means of apoptosis induction [ 4 ]. Dictyostelium cells are soil microorganism belonging to a kingdom different far from human, which produce many pharmacologically active compounds, including polyketides such as DIFs (differentiation-inducing factors; DIF-1 and DIF-3, etc. )…”