Flavokawains A and B from kava (Piper methysticum) activate heat shock and antioxidant responses and protect against hydrogen peroxide-induced cell death in HepG2 hepatocytes

Context Flavokawains are secondary metabolites from the kava plant (Piper methysticum Forst. f., Piperaceae) that have anticancer properties and demonstrated oral efficacy in murine cancer models. However, flavokawains also have suspected roles in rare cases of kava-induced hepatotoxicity. Objective To compare the toxicity flavokawains A and B (FKA, FKB) and monitor the resulting transcriptional responses and cellular adaptation in the human hepatocyte cell line, HepG2. Materials and methods HepG2 were treated with 2-100 μM FKA or FKB for 24-48 h. Cellular viability was measured with calcein-AM and changes in signalling and gene expression were monitored by luciferase reporter assay, real-time PCR and Western blot of both total and nuclear protein extracts. To test for subsequent resistance to oxidative stress, cells were pretreated with 50 μM FKA, 10 μM FKB or 10 μM sulphoraphane (SFN) for 24 h, followed by 0.4-2.8 mM H2O2 for 48 h, and then viability was assessed. Results FKA (≤100 μM) was not toxic to HepG2, whereas FKB caused significant cell death (IC50=23.2 ± 0.8 μM). Both flavokawains activated Nrf2, increasing HMOX1 and GCLC expression and enhancing total glutathione levels over 2-fold (p < 0.05). FKA and FKB also activated HSF1, increasing HSPA1A and DNAJA4 expression. Also, flavokawain pretreatment mitigated cell death after a subsequent challenge with H2O2, with FKA being more effective than FKB, and similar to SFN. Conclusions Flavokawains promote an adaptive cellular response that protects hepatocytes against oxidative stress. We propose that FKA has potential as a chemopreventative or chemotherapeutic agent.

Abstract 681: Heat shock-mediated suppression of ODC protein expression in colorectal cancer cells

Polyamines are small, positively charged metabolites that have important roles in several cancer hallmarks, including cell proliferation, invasion, and angiogenesis. Difluoromethylornithine (DFMO) is an FDA-approved drug that is in clinical trials for treatment of colorectal cancer. DFMO functions by inhibiting ornithine decarboxylase (ODC), which catalyzes the rate-limiting step in polyamine biosynthesis. By reducing polyamine levels, DFMO has shown much promise as an anti-cancer chemotherapeutic. One potential limitation to DFMO is that it causes a profound increase in ODC expression, which we suspect may counteract the intended use of the drug. However, strategies to suppress ODC expression would theoretically increase DFMO efficacy.

We have discovered that transient hyperthermia (heat shock, 42°C) causes a profound and rapid suppression in ODC protein levels in RKO colorectal cancer cells. To investigate the responsible mechanism, we began by examining ODC mRNA expression and stability. Hyperthermia caused only a moderate reduction in ODC transcript levels. Also, mRNA stability, which was assessed using actinomycin D, was not reduced by heat shock. We next examined the effect of hyperthermia on the control of ODC protein turnover, which is known to be mediated by ODC antizyme (OAZ1). We performed real-time PCR to assess the effect of heat shock on OAZ1 expression. Data reveal a significant increase in the level of OAZ1 transcript. Since heat shock is known to activate the transcription factor HSF1, we next investigated the role of HSF1 in OAZ1 induction. Using siRNA to silence HSF1, we found that the induction of OAZ1 by heat shock is fully dependent on HSF1 expression. In addition to heat shock, HSF1 is also activated by various chemotherapeutics, but most notably by Hsp90 inhibitors. We therefore tested if Hsp90 inhibitors, geldanamycin and 17-AAG could be used to induce OAZ1 and promote ODC turnover. These results show that both Hsp90 inhibitors caused a significant activation of HSF1, an induction in OAZ1 expression, and decrease in ODC protein levels. We therefore propose that Hsp90 inhibitors may be effective in attenuating ODC expression and polyamine metabolism. We suggest that they may be used in combination with DFMO to prevent the compensatory increase in ODC levels and increase anti-cancer effects.

Citation Format: Christina T.K. Wales, Aaron T. Jacobs. Heat shock-mediated suppression of ODC protein expression in colorectal cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 681. doi:10.1158/1538-7445.AM2014-681

Abstract 763: Hyperthermia promotes c-myc turnover and reduces ODC expression in colorectal cancer cells

Hyperthermia therapy is an FDA-approved procedure that is used to improve the clinical response to radiation and chemotherapy in a variety of solid malignancies. This method involves raising the temperature of tumor-bearing tissues to 41-43°C, which is commonly achieved by using microwave or radiofrequency (RF)-emitting devices. Despite a multitude of treatment centers that are now providing this type of therapy, the cellular and molecular processes are not fully understood. Here, we report that that the oncogenic transcription factor, c-myc has a potentially important role in the anti-cancer mechanism of hyperthermia. By treating colorectal cancer cells (RKO cell line) with heat (incubation at 42°C), we observe a rapid decrease in c-myc levels that is not seen with other transcription factors. We have also observed a similar effect in other cell types, including lung (A549) and neuroblastoma (SH-SY5Y). It is already known that the turnover of c-myc protein is enhanced by phosphorylation at Thr58. Our Western blot data reveal that hyperthermia causes a significant increase in Thr58 phosphorylation, which is accompanied by a reduction in total c-myc levels. c-Myc is frequently deregulated in human cancers, where it promotes the expression of genes that drive tumor cell proliferation. To assess the role of c-myc target genes in the response to hyperthermia, we looked at one such target gene, ornithine decarboxylase (ODC), which catalyzes the rate-limiting step in polyamine biosynthesis. We see that the hyperthermia-induced degradation of c-myc is accompanied by a drastic reduction in ODC mRNA and protein levels. Using actinomycin D, we observe that heat has no effect on ODC mRNA stability, suggesting that the decrease in ODC is due to a reduction in gene transcription. In support of this, we find that silencing c-myc expression with siRNA inhibits ODC mRNA and protein levels by a similar amount as hyperthermia. We propose that the down-regulation of ODC caused by c-myc degradation may be an important part of the clinical response to hyperthermia, and future work will evaluate the effect of heat on cellular polyamine levels.

Citation Format: Christina T.K. Wales, Aaron T. Jacobs. Hyperthermia promotes c-myc turnover and reduces ODC expression in colorectal 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 763. doi:10.1158/1538-7445.AM2013-763