Drosophila caliban, a nuclear export mediator, can function as a tumor suppressor in human lung cancer cells

Abstract

We previously showed that the Drosophila DNA binding homeodomain of Prospero included a 28 amino-acid sequence (HDA) that functions as a nuclear export signal. We describe here the identification of a protein we named Caliban, which can directly interact with the HDA. Caliban is homologous to human Sdccag1, which has been implicated in colon and lung cancer. Here we show that Caliban and Sdccag1 are mediators of nuclear export in fly and human cells, as interference RNA abrogates export of EYFP-HDA in normal fly and human lung cells. Caliban functions as a bipartite mediator nuclear export as the carboxy terminus binds HDA and the amino terminus itself functions as an NES, which directly binds the NES receptor Exportin. Finally, while non-cancerous lung cells have functional Sdccag1, five human lung carcinoma cell lines do not, even though Exportin still functions in these cells. Expression of fly Caliban in these human lung cancer cells restores EYFP-HDA nuclear export, reduces a cell's ability to form colonies on soft agar and reduces cell invasiveness. We suggest that Sdccag1 inactivation contributes to the transformed state of human lung cancer cells and that Caliban should be considered a candidate for use in lung cancer gene therapy.

The tumor suppressor Caliban regulates DNA damage-induced apoptosis through p53-dependent and -independent activity

“…20 While clbn was shown to be upregulated in a p53-dependent manner after irradiation, 24 its role in DNA damage response and tumor suppression was obscure. Here, we report the generation of knockout clbn flies and use them to investigate its functions in DNA damage response.…”

“…This expression also reduced their ability to form colonies on soft agar and invasiveness, but did not affect other growth parameters. 20 Here we compared the ability of the human lung adenocarcinoma cell line A549 and two of its derivatives, A549-Clbn-M and A549-Clbn-S, which ectopically express a Myc-tagged and untagged wt fly clbn þ gene, respectively, to form tumors when injected into athymic nude mice. Subcutaneous injections of the NSCLC line A549 into mice produced fast-growing tumors that obtained a large size (B85 mm 2 ).…”

“…The stable integration and ectopic expression of the fly clbn þ gene in two different human adenocarcinoma cell lines, A549 and EKVX, greatly reduced their ability to form colonies on soft agar and invasiveness indices. 20 Finally, it was shown that more than half of mismatch repair-deficient lymphoma cells examined in one study had frame shift mutation in the sdccag1 gene. 23 While studies have connected Sdccag1/Clbn with cancer, none have addressed a mechanism or pathway for its mode of action.…”

A highly conserved family of domains related to the DNA-glycosylase fold helps predict multiple novel pathways for RNA modifications

“…Hence, it is conceivable that the proposed modification mediated by NFACT proteins might also intersect with IRES-mediated translation under stress. The Drosophila NFACT protein caliban has been proposed to be part of a network including p53, caspase, and Hid proteins during DNA damage-induced apoptosis 13 , 14 . Here again the modification could play a role as part of a stress response ensuing from DNA damage, as many key oncogenes and apoptosis factors are translated via IRES.…”

“…These include fibronectin binding in certain pathogenic bacteria, 5 - 10 a core component of the ribosome-associating, co-translational degradation complex RQC in yeast, 11 , 12 regulation of the DNA-damage response in Drosophila , 13 and mediation of nuclear export in Drosophila and human 14 . Given the disparate nature of these findings, we decided to revisit this gene family using state-of-the-art techniques in sequence analysis and comparative genomics while tapping the wealth of new information that has accumulated in the years since its initial characterization.…”