Cytokine and apoptosis gene networks that control development and cancer

MicroRNA-1 inhibits proliferation of hepatocarcinoma cells by targeting endothelin-1

AIMS

MicroRNA-1 (miR-1) has been demonstrated as a tumor-suppressive miRNA, which shows a down-regulated pattern in several human malignancies including hepatocellular carcinoma (HCC). However, the pathophysiologic roles of miR-1 and their mechanisms in HCC tumorigenesis are still not totally elucidated.

MAIN METHODS

Pre-miR-1 was cloned into pSuper plasmid to overexpress the miR-1 in hepatoma cells. Real-time PCR and Western blot were applied to detect miR-1, ET-1 mRNA and protein levels respectively. Dual luciferase reporter assay was conducted to investigate the binding site of miR-1 on 3'UTR of ET-1 mRNA. Proliferation of hepatoma cells was evaluated by MTT assay.

KEY FINDINGS

We observed that over-expression of miR-1 by miRNA-expressing plasmid transfection in HepG2 and Hep3B cells significantly reduced the proliferation of these cells. To explore the mechanism, we examined the potential target genes of miR-1 by bioinformatics. A potent mitogen, Endothelin-1 (ET-1), attracted our attention. Elevated expression of ET-1 but reduced miR-1 level was detected both in human liver cancer tissues and in hepatoma cell lines using Western Blot and miRNA real-time PCR respectively. By the over-expression and inhibition of miR-1 in HepG2 and Hep3B, we confirmed that miR-1 negatively regulated ET-1 expression in hepatoma cells. A luciferase reporter assay showed that miR-1 regulation was established by pairing to a complementary binding site within the ET-1 3'UTR. Finally, attenuated proliferation of hepatoma cells by over-expression of miR-1 could be partially restored by exogenous ET-1 treatment.

SIGNIFICANCE

Our findings demonstrate that miR-1 could inhibit ET-1 expression to attenuate the proliferation of hepatoma cells.

Mitochondrial concept of leukemogenesis: key role of oxygen-peroxide effects

Background and hypothesis

The high sensitivity of hematopoietic cells, especially stem cells, to radiation and to pro-oxidative and other leukemogenic agents is related to certain of their morphological and metabolic features. It is attributable to the low (minimal) number of active mitochondria and the consequently slow utilization of O₂ entering the cell. This results in an increased intracellular partial pressure of O₂ (pO₂) and increased levels of reactive oxygen (ROS) and nitrogen (RNS) species, and a Δ(PO – AO) imbalance between the pro-oxidative (PO) and antioxidative (AO) constituents.

Proposed mechanism

Because excessive O₂ is toxic, we suggest that hematopoietic cells exist in a kind of unstable dynamic balance. This suggestion is based on the idea that mitochondria not only consume O₂ in the process of ATP production but also constitute the main anti-oxygenic stage in the cell's protective antioxidative system. Variations in the mitochondrial base capacity (quantity and quality of mitochondria) constitute an important and highly efficient channel for regulating the oxidative stress level within a cell.

The primary target for leukemogenic agents is the few mitochondria within the hematopoietic stem cell. Disturbance and weakening of their respiratory function further enhances the initial pro-oxidative state of the cell. This readily results in peroxygenation stress, creating the necessary condition for inducing leukemogenesis. We propose that this is the main cause of all related genetic and other disorders in the cell. ROS, RNS and peroxides act as signal molecules affecting redox-sensitive transcription factors, enzymes, oncogenes and other effectors. Thereby, they influence the expression and suppression of many genes, as well as the course and direction of proliferation, differentiation, leukemogenesis and apoptosis.

Differentiation of leukemic cells is blocked at the precursor stage. While the transformation of non-hematopoietic cells into tumor cells starts during proliferation, hematopoietic cells become leukemic at one of the interim stages in differentiation, and differentiation does not continue beyond that point. Proliferation is switched to differentiation and back according to a trigger principle, again involving ROS and RNS. When the leukemogenic Δ_L(PO – AO) imbalance decreases in an under-differentiated leukemia cell to the differentiation level Δ_D(PO – AO), the cell may continue to differentiate to the terminal stage.

Conclusion

The argument described in this article is used to explain the causes of congenital and children's leukemia, and the induction of leukemia by certain agents (vitamin K3, benzene, etc.). Specific research is required to validate the proposals made in this article. This will require accurate and accessible methods for measuring and assessing oxidative stress in different types of cells in general, and in hematopoietic cells in particular, in their different functional states.

Cyclic AMP and the reverse transformation reaction

Traditional methods for cancer treatment have been aimed at killing the cancer cells. Unfortunately this approach all too often is accompanied by harmful killing of normal cells. The present paper describes an experimental program in our laboratory in which cancer cells are treated so as to revert to normal cell behavior. This process, which we have named reverse transformation, appears to offer considerable hope in the treatment of a large number of malignancies.

Phosphate ions mediate chondrocyte apoptosis through a plasma membrane transporter mechanism

In a previous investigation we showed that phosphate ions (Pi) induced apoptosis of terminally differentiated hypertrophic chondrocytes. To explore the mechanism by which Pi induces cell death, we asked the following two questions. First, can we prevent Pi-induced apoptosis by inhibiting plasma membrane Na-Pi cotransport? Second, which specific Na-Pi transporters are expressed in chondrocytes and are they developmentally regulated? Terminally differentiated hypertrophic chondrocytes were isolated from chick tibial cartilage and cell death was measured in the presence of 3-7 mmol/L Pi. To ascertain whether apoptosis was linked to a rise in cellular Pi loading, we examined the effect of phosphonoformic acid (PFA), a competitive inhibitor of Na-Pi cotransport on Pi-induced apoptosis in chondrocytes. We found that 1 mmol/L PFA blocked anion-induced cell death and prevented an increase in the cell Pi content. In a parallel study, we determined that the bisphosphonate, alendronate, also protected chondrocytes from death, albeit at a lower concentration than PFA. Using a DNA end-labeling procedure, we showed that the Pi-treated cells were apoptotic and, as might be predicted, the presence of PFA blocked induction of the death sequence. Next, we examined the expression of two Pi transporters in relation to chondrocyte maturation and anion treatment. We noted that there was expression of the constitutive transporter, Glvr-1, and a type II cotransporter in chick growth plate cells. Although these transport systems are active in terminally differentiated cells, it is probable that the initiation of apoptosis may require the induction of other Pi-transport systems. It is concluded that, at the mineralization front, cell death is linked directly to the elevation in environmental anion concentration and the concomitant rise in intracellular Pi levels.

Extracellular phosphate ions cause apoptosis of terminally differentiated epiphyseal chondrocytes

Epiphyseal chondrocytes end their life cycle through apoptosis. While this event provides a mechanism for the removal of terminally differentiated cells from cartilage, agents that promote this physiological process have not been defined. To address this issue, using a cell culture technique that models events that take place in the growth plate, we asked the following questions: Can agents that promote chondrocyte maturation and cartilage mineralization serve as specific triggers for cell death? Are chondrocytes susceptible to apoptogens at a singular developmental stage? Treatment of embryonic tibial chondrocytes with inorganic phosphate (Pi) induced death in a dose- and time-dependent manner. Within 48 hr, 3 mM Pi increased chondrocyte death by 30%; lower concentrations of Pi induced death after 48 hr. To ascertain if death was due to apoptosis, we evaluated Pi-induced death by a number of different methods and compared the results to those induced by the apoptogen, staurosporine. Analysis of the death process indicated that cartilage cells shared many of the common biological features of the apoptotic process. Thus, there was DNA fragmentation, terminal deoxynucleotidyl transferase (TUNEL) labeling, an increase in cells in the sub-G1 fraction of the cell cycle, and morphological evidence of apoptosis. To explore the specificity of the Pi effect, the experiment was repeated using embryonic sternal cephalic and caudal chondrocytes, cells that are at an earlier developmental stage than the terminally differentiated tibial cells. We noted that these cells remained vital despite a major increase in the medium Pi content. Results of this study suggest that Pi is a stage-specific inducer of apoptosis in maturing chondrocytes and that this role may be linked to chondrocyte maturation and mineralization of the extracellular matrix.