Zebrafish as a model for the study of human cancer

Zebrafish provide an exciting animal model system for the study of human cancers. During the last few years many zebrafish models of cancer have been generated that recapitulate human hematologic malignancies and solid tumors. Concurrent technological advances have significantly improved the genetic tractability and unique advantage of in vivo imaging in zebrafish, providing a means to dissect the molecular pathways underlying tumor initiation, progression and metastasis. Comparisons of cancer-associated gene expression profiles have demonstrated a high degree of similarity in the gene signatures of specific types of tumor cells in fish and humans, indicating that the contributing genetic pathways leading to cancer are evolutionarily conserved. Furthermore, the high fecundity, optical clarity and small embryo size of zebrafish continue to make it particularly amenable to performing whole-organism small molecule screens to identify targets for therapeutic development. This chapter reviews a wide array of these zebrafish cancer models and illustrates the advantages of the zebrafish system for exploring the molecular mechanisms governing cancer-related cellular processes.

p21WAF1 expression induced by MEK/ERK pathway activation or inhibition correlates with growth arrest, myogenic differentiation and onco-phenotype reversal in rhabdomyosarcoma cells

Background

p21^WAF1, implicated in the cell cycle control of both normal and malignant cells, can be induced by p53-dependent and independent mechanisms. In some cells, MEKs/ERKs regulate p21^WAF1 transcriptionally, while in others they also affect the post-transcriptional processes. In myogenic differentiation, p21^WAF1 expression is also controlled by the myogenic transcription factor MyoD. We have previously demonstrated that the embryonal rhabdomyosarcoma cell line undergoes growth arrest and myogenic differentiation following treatments with TPA and the MEK inhibitor U0126, which respectively activate and inhibit the ERK pathway.

In this paper we attempt to clarify the mechanism of ERK-mediated and ERK-independent growth arrest and myogenic differentiation of embryonal and alveolar rhabdomyosarcoma cell lines, particularly as regards the expression of the cell cycle inhibitor p21^WAF1.

Results

p21^WAF1 expression and growth arrest are induced in both embryonal (RD) and alveolar (RH30) rhabdomyosarcoma cell lines following TPA or MEK/ERK inhibitor (U0126) treatments, whereas myogenic differentiation is induced in RD cells alone. Furthermore, the TPA-mediated post-transcriptional mechanism of p21^WAF1-enhanced expression in RD cells is due to activation of the MEK/ERK pathway, as shown by transfections with constitutively active MEK1 or MEK2, which induces p21^WAF1 expression, and with ERK1 and ERK2 siRNA, which prevents p21^WAF1 expression. By contrast, U0126-mediated p21^WAF1 expression is controlled transcriptionally by the p38 pathway. Similarly, myogenin and MyoD expression is induced both by U0126 and TPA and is prevented by p38 inhibition. Although MyoD and myogenin depletion by siRNA prevents U0126-mediated p21^WAF1 expression, the over-expression of these two transcription factors is insufficient to induce p21^WAF1. These data suggest that the transcriptional mechanism of p21^WAF1 expression in RD cells is rescued when MEK/ERK inhibition relieves the functions of myogenic transcription factors. Notably, the forced expression of p21^WAF1 in RD cells causes growth arrest and the reversion of anchorage-independent growth.

Conclusion

Our data provide evidence of the key role played by the MEK/ERK pathway in the growth arrest of Rhabdomyosarcoma cells. The results of this study suggest that the targeting of MEK/ERKs to rescue p21^WAF1 expression and myogenic transcription factor functions leads to the reversal of the Rhabdomyosarcoma phenotype.

cAMP effects on myogenic gene expression in rhabdomyosarcoma cells

Rhabdomyosarcoma is a tumor of skeletal muscle origin affecting children and young adults. Although relatively undifferentiated, cell lines derived from this tumor express myogenic regulatory factors and so may be useful models of abortive myogenic differentiation. In the present studies, we have determined the effect of increased intracellular cAMP on proliferation, morphologic differentiation, and expression of myogenic genes in the prototypic embryonal rhabdomyosarcoma cell line, RD. Whereas growth in dibutyryl cAMP (dbcAMP), forskolin, or butyrate led to morphologic differentiation, growth in dbcAMP inhibited proliferation, while growth in butyrate slowed but did not stop cell division. Expression of the genes for myogenin and myosin light chain was inhibited by dbcAMP, while butyrate decreased myogenin and increased myosin light chain transcription. MyoD and MRF4 expression was not altered under either condition and no myf5 expression was detected. We also determined the effects of dbcAMP and butyrate on total protein expression, as well as on a panel of muscle- and neural-specific proteins using functional assays, immunohistochemistry, and immunoprecipitation. The total protein levels of cells treated with either agent were double those of untreated cells. DbcAMP increased the activity of acetylcholinesterase (AChE) up to 10-fold compared to untreated cells, while butyrate had a substantially lesser effect. These increases were due to increased AChE protein synthesis and stability in dbcAMP treated cells, compared to butyrate or untreated cells. Finally, cells under all conditions expressed MAP2, a neural-specific microtubule associated protein. Together, these data suggest that intracellular cAMP levels modulate distinct subsets of the myogenic differentiation pathway in rhabdomyosarcoma cells. Moreover, they also indicate that RD cells are able to express markers of different cell lineages, which may help explain some of the paradoxical features of these tumors.

Broad cytolytic specificity of myotoxin II, a lysine-49 phospholipase A2 of Bothrops asper snake venom

The cytotoxic activity of Bothrops asper myotoxin II, a lysine-49 phospholipase A2 isoform, on different cell types in culture, was investigated. Myotoxin II caused a dose-dependent cytolytic effect on all cell types tested, characterized by rapid release of cytoplasmic lactic dehydrogenase and drastic morphological cell alterations. Quantitative differences in the susceptibility to myotoxin II among cell types fell within a relatively narrow range, and in general, the toxin was cytolytic at concentrations of 50-100 micrograms/ml (3-7 microM), when assays were performed using culture medium as a diluent. Toxin activity was markedly enhanced if phosphate-buffered saline was utilized instead of medium. The cytotoxic activity of myotoxin III, an aspartate-49 isoform from the same venom, on both endothelial cells and skeletal muscle myoblasts was higher than that of myotoxin II, suggesting that, although phospholipase A2 activity is clearly not required for the induction of cell damage, it may have an enhancing role. In contrast to B. asper myotoxins, other basic phospholipases A2 with myotoxic activity in vivo (notexin from Notechis scutatus, and two enzymes isolated from Vipera russelli venom) did not affect endothelial cells and myoblasts. Pretreatment of cells with neuraminidase, tunicamycin, or protamine, did not alter their susceptibility to myotoxin II. At low temperatures (2-4 degrees C) myotoxin II was devoid of cytolytic effect. Washing and neutralization experiments using heparin with low affinity for antithrombin or mouse monoclonal antibody MAb-3 suggest that at low temperatures myotoxin II binds very weakly to the cells, and that its normal interaction with the putative target is probably not only based on charge, but that a membrane penetration event may be required.

Consistent cytogenetic aberrations in hepatoblastoma: a common pathway of genetic alterations in embryonal liver and skeletal muscle malignancies?

Cytogenetic analyses of four consecutive hepatoblastomas revealed near-diploid stemline karyotypes with relatively simple chromosome aberrations. Cytogenetic abnormalities shared by each tumor included trisomy for all of part of chromosome 2 and trisomy for chromosome 20. In two cases, partial trisomy for chromosome 2 resulted from direct duplication of long arm material with the shortest region of overlap being 2q23-2q35. In one tumor, each metaphase also contained a variable number of double minute chromosomes found not to derive from NMYC amplification. Interestingly, trisomy for 2q and trisomy 20 are also characteristic events in pediatric embryonal rhabdomyosarcomas. Furthermore, others have reported loss of heterozygosity for the short arm of chromosome 11 in both hepatoblastoma and childhood embryonal rhabdomyosarcoma, and both these malignant diseases are associated with Beckwith-Wiedemann syndrome. These cytogenetic and molecular findings suggest a parallel pathway of genetic steps in the initiation and/or progression of tumors of embryonal liver and skeletal muscle.

[Embryonal rhabdomyosarcoma diagnosed antenatally]

A case of congenital embryonal rhabdomyosarcoma of the right shoulder is described. The patient was the first child born to a 24 year-old woman who had previously been treated for sterility. The diagnosis was made by echography during the 36th week of gestation. This full term white boy was born by cesarean section because of the tumor size. Surgical treatment was completed by chemotherapy. Despite 4 courses of VAC, local recurrence was noted that led to a second surgical excision followed by a new cyclic chemotherapy (IVA). The treatment was fairly well tolerated. The child is well 24 months later.

Rhabdomyosarcoma of the ovary: ultrastructural study of a case and review of literature

Pure rhabdomyosarcomas are extremely rare and lethal primary neoplasms of the ovary. The available literature revealed 14 such cases. This report describes the light and electron microscopic findings of another case. At the electron microscopic level, rhabdomyogenesis recapitulates skeletal muscle differentiation during early fetal life. Rhabdomyoblasts in the ovary may arise from either the uncommitted stromal fibroblasts or from fibroblasts of endometriotic stroma. The age of patients at diagnosis ranged between 13 months and 86 years with 60% of women aged 40 years and older. About 50% of the cases were diagnosed with disease beyond the ovary, and survival ranged between 18 days and 15 months after diagnosis. Aggressive combination therapy including surgery, radiation, and chemotherapy appears to be the only hope for improved survival.

Primary rhabdomyosarcoma of the central nervous system

A case of primary rhabdomyosarcoma in the brain stem is described in an 8 year old girl. The clinical data showed a right side hemiplegia then a total paralysis of the left sixth cranial nerve and a paraplegia which became lethal in 3 months. The necrospy revealed a tumoral nodule in the left medulla oblongata and pons with diffuse subarachnoidal extension from the cranial nerves to the cauda equine roots. Histologically the tumor appeared to be polymorph with numerous rhabdomyoblasts which had a clear cross striation and which were sometimes less differentiated without any neuronal or glial elements. Perivascular tumoral cells and blood vessels were closely linked, the Virchow-Robin spaces were clearly involved. The electron microscopic study confirmed that the less differentiated cells were of a rhabdomyoblastic nature. A review of the litterature indicates that these malignant neoplasias are highly exceptional, and can be classified within the group of primary tumors of the neuraxis with muscular elements. The histogenetic origin of these tumors appears to be the ectomesenchyme of neural crests.