Cellular resistance to platinum anticancer compounds is governed by no less than two molecular processes; DNA repair and cellular accumulation of drug. Gli1 is an upstream regulator of nucleotide excision repair, effecting this process through c-jun. We, therefore, investigated whether Gli1 plays a role in cellular accumulation of cisplatin. Using a Gli1-specific shRNA, we explored the role of Gli1 in the cellular accumulation and efflux of cisplatin, in cisplatin-resistant A2780-CP70 human ovarian cancer cells. When Gli1 is inhibited, cellular uptake of cisplatin was approximately 33% of the level of uptake under control conditions. When Gli1 is inhibited, cellular efflux of cisplatin was completely abrogated, over a 12-h period of observation. We assayed nuclear lysates from these cells, for the ability to bind the DNA sequence that is the Gli-binding site (GBS) in the 5′UTR for each of five known cisplatin transmembrane transporters. Four of these transporters are active in cisplatin uptake; and, one is active in cisplatin efflux. In each case, nuclear lysate from A2780-CP70 cells binds the GBS of the respective cisplatin transport gene. We conclude that Gli1 plays a strong role in total cellular accumulation of cisplatin in these cells; and, that the combined effects on cellular accumulation of drug and on DNA repair may indicate a role for Gli1 in protecting cellular DNA from lethal types of DNA damage.
The transcription factor Activator Protein 1 (AP1), is an important regulator in cisplatin resistance. AP1 is comprised of the two proteins, c-jun and c-fos. The transcription factor Gli1, a member of the hedgehog signaling pathway, is also an important factor in cisplatin resistance and transcriptionally regulated c-jun. The c-jun interface with Gli1 is mediated by a 130 kDa isoform of Gli1. We have investigated whether Gli1 may regulate c-fos by selectively knocking down Gli1 expression using anti-Gli1 shRNA or by treating cells with a Hedgehog pathway inhibitor, cyclopamine. A2780-CP70 cells were treated with an IC50 dose of anti-Gli1 shRNA and the changes in RNA and protein levels were analyzed over a 72 hour time course. No changes in the expression of c-jun or c-fos RNA transcripts were observed versus control. Western blot analyses showed Gli1 protein expression was reduced at 24 hours and undetectable at 48 and 72 hours. Sonic hedgehog (Shh) protein levels were dramatically reduced by 24 hours, low but measureable at 48 hours, and undetectable at 72 hours. Indian hedgehog (Ihh) expression increased at 24 hours and remained stable for the remainder of the 72 hour time course. Treatment of A2780-CP70 cells with an IC50 dose of cyclopamine resulted in a different intracellular response than treatment with Gli1 shRNA. Cyclopamine specifically targets the cell membrane receptor Smoothened. Experiments were performed analyzing RNA and protein expression over a 72 hour time period after treatment with cyclopamine. C-jun and c-fos RNA transcript levels were higher than control 6 hours after treatment and remained high for the duration of the 72 hour experiment. Western blot analyses after cyclopamine treatment showed Gli1 protein expression gradually decreasing during the 72 hours, but were never fully depleted. Expression of Shh increased over 72 hours, while Ihh levels remained stable. The identical Gli-Binding-Site (GBS) in the c-jun promoter was found in the promoter of c-fos. We investigated whether the Gli1 isoform that binds c-jun, also binds c-fos. Simultaneous Western and Southwestern blots show only one of the known Gli1 isoform binds the promoter of c-fos. Further studies with Chromatin Immunoprecipitation (ChIP) assays confirmed the 130 kDa isoform of Gli1 binds the GBS of the c-fos promoter. We conclude that the 130 kDa Gli1 isoform binds both components of AP1, c-jun and c-fos, and modulates the cellular resistance to cisplatin. Citation Format: Lauren Amable, Kenji Kudo, Elaine Gavin, Jason Fain, Eddie Reed. A specific Isoform of Gli1 binds the promoter region of c-fos in A2780-CP70 human ovarian cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-223. doi:10.1158/1538-7445.AM2013-LB-223
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