Heterogeneous nuclear ribonuclearproteins (hnRNPs) are nucleic acid-binding proteins and have critical roles in DNA repair, telomere regulation, and transcriptional gene regulation. Previously, we showed that hnRNP G has tumor-suppressive activity in human oral squamous cell carcinoma cells. Therefore, the identification of hnRNP G target genes is important for understanding the function of hnRNP G and its tumor suppressive activity. In this study, we identify a known tumor suppressor gene, thioredoxin-interacting protein (Txnip) gene as a novel target of hnRNP G. Expression of Txnip is upregulated by wild-type (wt) hnRNP G but not by a suppression-defective mutant hnRNP G (K22R) in human squamous cell carcinoma. Wt hnRNP G binds and transactivates the Txnip promoter in vivo, whereas the K22R mutant does not. Furthermore, overexpression of Txnip alone in cancer cells leads to the inhibition of anchorage-independent growth and in vivo tumorigenicity in immunocompromised mice, suggesting a reversion of the transformation phenotype. These studies indicate that hnRNP G promotes the expression of Txnip and mediates its tumor suppressive effect.

Many studies have suggested the involvement of wild-type (wt) p53 in the repair of DNA double-strand breaks (DSBs) via DNA end-joining (EJ) process. To investigate this possibility, we compared the capacity and fidelity of DNA EJ in RKO cells containing wt p53 and RKO cells containing no p53 (RKO cells with p53 knockdown). The p53 knockdown cells showed lower fidelity of DNA EJ compared to the control RKO cells. The DNA end-protection assay revealed the association of a protein complex including heterogeneous nuclear ribonucleoprotein G (hnRNP G) with the DNA ends in RKO cells containing wt p53, but not with the DNA ends in RKO cells with p53 knockdown. Depletion of endogenous hnRNP G notably diminished the fidelity of EJ in RKO cells expressing wt p53. Moreover, an ectopic expression of hnRNP G significantly enhanced the fidelity of DNA EJ and the protection of DNA ends in human cancer cells lacking hnRNP G protein or containing mutant hnRNP G. Finally, using recombinant hnRNP G proteins, we demonstrated the hnRNP G protein is able to bind to and protect DNA ends from degradation of nucleases. Our results suggest that wt p53 modulates DNA DSB repair by, in part, inducing hnRNP G, and the ability of hnRNP G to bind and protect DNA ends may contribute its ability to promote the fidelity of DNA EJ.