The CRY inhibitor KS15 and REV-ERB antagonist SR8278 promote cell proliferation in cisplatin-treated cells
A schematic of the circadian clock transcription-translation feedback system is provided in Figure 1A along with the two clock-modulating compounds (KS15 and SR8278) that were tested in this study due to their documented ability to alter circadian promoter activity and/or CCG expression22–24. CRY represses CLOCK-BMAL1 transcriptional activity, and the CRY inhibitor KS15 has been shown increase in the expression of CCGs such as Wee1 in cells in vitro22,23. REV-ERB, which negatively regulates BMAL1 expression by binding to its promoter, can be antagonized with SR827824. Because the effectiveness of the anti-cancer drug cisplatin is impacted by cellular processes that are regulated by the circadian clock, such as DNA repair and cell cycle checkpoints18,25, the effect of KS15 and SR8278 on DNA repair, cell cycle progression, and cell viability were tested both alone and in various combinations.
U2OS cells selected for this initial analysis because this cell line expresses circadian proteins and maintains circadian rhythmicity upon stimulation 26. As shown in Figure 1B, treatment of asynchronously growing U2OS cells with increasing concentrations of KS15 and SR8278 (10, 20, and 50 µM) did not dramatically influence cell proliferation either alone or in combination with a fixed concentration (10 µM) of the other compound. Cells were then co-treated with the clock drugs in combination with increasing concentrations of cisplatin. Interestingly, treatment of U2OS cells with 50 µM KS15 partially protected cells against the anti-proliferative effects of cisplatin (Figure 1C). Similar results were observed with 50 µM SR8278 (Figure 1D). Additional cell proliferation assays were carried out with various concentrations of KS15 and SR8278 alone and in combination, and IC50 values for cisplatin were calculated with the different treatment combinations. As shown in Table 1, the highest concentrations of KS15 and SR8278 led to statistically significant changes in IC50 values, which increased from 18 µM in vehicle-treated cells to approximately 40 µM in cells treated with the clock modulating compounds. Similar levels of protection were also observed in cells treated with a combination of lower concentrations of KS15 and SR8278.
To confirm these results in additional cell lines, HaCaT keratinocytes, which also have a functional circadian clock27, were treated with KS15 alone and in combination with SR8278. As shown in Figure 2A, the compounds alone had no significant effect on cell proliferation. However, when HaCaT cells were treated with 50 µM KS15 in combination with 10 µM SR8278 in the presence of increasing concentrations of cisplatin, they displayed significantly higher levels of cell proliferation than vehicle-treated cells (Figure 2B, Table 1). Experiments were then repeated in A549 lung carcinoma cells, which displayed little sensitivity to the clock drugs alone (Figure 2C) but were protected against cisplatin by co-treatment with KS15 and SR8278 (Figure 2D, Table 1). Thus, we conclude from these studies that the clock modulating compounds KS15 and SR8278 can be used to counteract the anti-proliferative effects of the cancer chemotherapy drug cisplatin in cultured human cells in vitro.
KS15 and REV-ERB treatment increases the expression of the CCGs XPA and Wee1
Cellular sensitivity to cisplatin is impacted by many factors, including DNA repair and cell cycle phase3,28,29. Because the NER factor XPA is regulated at the transcriptional level by the circadian clock machinery11,14, XPA mRNA expression was measured by RT-qPCR in U2OS treated with KS15 and SR278 alone and in combination. As shown in Figure 3A, treatment with the combination of KS15 and SR8278 increased XPA expression by approximately 2.9-fold. Expression of Wee1, another well-known target of the circadian cloc 12, was also increased by KS15+SR8278 by 3.7-fold (Figure 3B). To determine if the increased mRNA levels are correlated with changes at the protein level, western blotting was performed using cell lysates from U2OS cells treated with the clock compounds. As shown in Figure 3C, D, protein levels of both XPA and Wee1 were increased by nearly 2-fold in cells treated with the combination of KS15 and SR8278.
KS15 and REV-ERB promote cisplatin-DNA adduct removal
XPA is a rate-limiting factor in NER30, and thus the increased expression of XPA observed in Figure 3 may facilitate the removal of cisplatin-induced intra-strand DNA adducts. U2OS cells were treated with vehicle or KS15 and SR8278 in the presence of increasing concentrations of cisplatin, and then genomic DNA was purified for analysis by DNA immunoblotting with an anti-cisplatin-DNA adduct antibody. As shown in Figure 4A, cisplatin treatment for 2 hr induced a dose-dependent induction of adduct formation. Cell culture media was then replaced with fresh medium containing the clock drugs but lacking cisplatin to determine how the clock drugs impact the repair of these DNA adducts after additional 22 hr of incubation. Consistent with XPA’s role in promoting NER and its up-regulation by treatment of cells with KS15+SR8278, fewer remaining cisplatin-DNA adducts were observed in the cells treated with KS15 and SR8278 at the 24 hr time point than in the vehicle-treated cells (Figure 4A, B).
KS15 and REV-ERB promote G1 cell cycle arrest
Because the Wee1 kinase prevents the entry of cells into mitosis, the increased expression of Wee1 observed in Figure 3 may be expected to lead to an enrichment of cells in the G2 phase of the cell cycle. However, when U2OS cells were treated with KS15 and SR8278 (alone and in combination) and analyzed for DNA content by flow cytometry, a modest reduction in cells in late S/2 phase was observed (Figure 5A). Rather, quantitation of cell cycle distribution from 3 independent experiments showed there to a modest but statistically significant increase in cells in the G1 phase of the cell cycle (Figure 5B) when cells were treated with the combination of KS15 and SR8278. To identify a potential mechanism for this increased fraction of cells in G1 phase, expression of the cyclin-dependent kinase inhibitory protein p2131, which was previously reported to be a target of the circadian clock machinery32, was examined. Western blotting showed that p21 protein levels were increased by treatment of SR8278 alone or in combination with KS15 (Figure 5C). Thus, cell cycle progression is impacted by treatment with small molecules that target circadian clock proteins in a similar manner as for DNA repair.