Studies utilizing experimental animals, epidemiological approaches, cellular models, and clinical trials all provide evidence that retinoic acid and some of its synthetic derivatives (retinoids) are useful pharmacological agents in cancer therapy and prevention. In this chapter, we first review the current knowledge of retinoic acid receptors (RARs) and their role in mediating the actions of retinoic acid. We then focus on a discussion of RARalpha and acute promyelocytic leukemia followed by a discussion of the role of RARs, in particular RARbeta expression, in other cancer types. Loss of normal RAR function in the presence of physiological levels of RA (either due to alterations in the protein structure or level of expression) is associated with a variety of different cancers. In some cases treatment with pharmacological doses of RA can be effective.

The human histone H4 gene FO108 is regulated during the cell cycle with a peak in transcription during early S phase. The cell-cycle element (CCE) required for H4 histone activation is a sequence of 11 base pairs that binds a protein factor in electrophoretic mobility shift assays that has been designated histone nuclear factor M (HiNF-M). Here we report the purification of HiNF-M, and show it to be a protein of relative molecular mass (M(r)) 48K that is identical to interferon (IFN) regulatory factor 2 (IRF-2), a negative transcriptional regulator of the IFN response. Recombinant IRF-2 (as well as the related protein IRF-1 (ref. 5)) binds the CCE specifically and activates transcription of this H4 histone gene. IRF-2 has been shown to have oncogenic potential, and our results demonstrate a link between IRF-2 and a gene that is functionally coupled to DNA replication and cell-cycle progression at the G1/S phase transition.

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Cell cycle-regulated gene expression is essential for normal cell growth and development and loss of stringent growth control is associated with the acquisition of the transformed phenotype. The selective synthesis of histone proteins during the S phase of the cell cycle is required to render cells competent for the ordered packaging of replicating DNA into chromatin. Regulation of H4 histone gene transcription requires the proliferation-specific promoter binding factor HiNF-D. In normal diploid cells, HiNF-D binding activity is regulated during the cell cycle; nuclear protein extracts prepared from normal cells in S phase contain distinct and measurable HiNF-D binding activity, while this activity is barely detectable in G1 phase cells. In contrast, in tumor-derived or transformed cell lines, HiNF-D binding activity is constitutively elevated throughout the cell cycle and declines only with the onset of differentiation. The change from cell cycle-mediated to constitutive interaction of HiNF-D with the promoter of a cell growth-controlled gene is consistent with, and may be functionally related to, the loss of stringent cell growth regulation associated with neoplastic transformation.

Retinoids have great promise in the area of cancer therapy and chemoprevention. Although some tumor cells are sensitive to the growth inhibitory effect of alltrans-retinoic acid (ATRA), many ovarian tumor cells are not. 6-((1-Admantyl)-4-hydroxyphenyl)-2-naphthalenecarboxylic acid (CD437) is a conformationally restricted synthetic retinoid that induces growth arrest and apoptosis in both ATRA-sensitive and ATRA-resistant ovarian tumor cell lines. To better understand the mechanism by which CD437 induces apoptosis in ovarian tumor cell lines, we prepared a cell line, CA-CD437R, from the ATRA-sensitive ovarian cell line, CA-OV-3, which was resistant to CD437. We found that the CD437-resistant cell line was also resistant to the induction of apoptosis by tumor necrosis factor-␣ but not resistant to the induction of apoptosis by another synthetic retinoid, fenretinide N-(4-hydroxyphenyl)retinamide. We also show that this cell line remains ATRA-sensitive and exhibits no deficiencies in RAR function. Analysis of this CD437-resistant cell line suggests that the pathway for induction of apoptosis by CD437 is similar to the pathway utilized by tumor necrosis factor-␣ and different from the pathway induced by the synthetic retinoid, fenretinide N-(4-hydroxyphenyl)retinamide. The CA-CD437R cell line is a valuable tool, permitting us to further elucidate the molecular events that mediate apoptosis induced by CD437 and other synthetic retinoids. Results of experiments utilizing this cell line suggest that the alteration responsible for resistance of CA-CD437R cells to CD437 induced event maps after the activation of p38 and TR3 expression, prior to mitochondrial depolarization, subsequent release of cytochrome c and activation of caspase-9 and caspase-3.

The green tea polyphenol epigallocatechin-3-gallate (EGCG) has cancer chemopreventive properties against various types of cancers. The compound is known to attack various targets in transformed cells. In this report, we examined the action of EGCG on ovarian cancer cells. Eight ovarian cancer cell lines were tested (SKOV3, CAOV3, OVCAR3, OVCAR10, A2780, CP70, C30, and C200) and showed IC50s for EGCG at the micromolar range, including ones that are resistant to the chemotherapeutic drug cisplatin. The ovarian cancer cells were sensitive to H2O2 at similar concentrations, and EGCG treatment led to enhanced intracellular H2O2. Neutralization with pyruvate, a scavenger of H2O2, suggests that the toxicity of EGCG may be mediated by oxidative stress from the free radical. Addition of Tempol, a superoxide dismutase mimetic, demonstrates that H2O2 might be generated endogenously from superoxide. The toxicity of cisplatin and the development of cisplatin resistance are major obstacles in treatment of ovarian cancer. We found that addition of EGCG amplified the toxicity of cisplatin. EGCG increased cisplatin potency by three to six-fold in SKOV3, CAOV3, and C200 cells, the latter being a cell line induced to have several hundred fold resistant to cisplatin above the parental line. Our findings suggest that EGCG may accentuate oxidative stress to inhibit growth of ovarian cancer cells and sensitize them to cisplatin.

All-trans-retinoic acid (ATRA) has been shown to inhibit the growth of a number of ovarian tumor cell lines while others have been found to be resistant to retinoid suppression of growth. Interestingly, two synthetic retinoids, CD437 and 4-HPR, inhibit the growth of both ATRA-sensitive (CA-OV-3) and ATRA-resistant (SK-OV-3) ovarian tumor cells. However, in contrast to ATRA, both induce apoptosis. Our goal was to elucidate the mechanism by which these two synthetic retinoids induce apoptosis in ovarian tumor cells. Since it has been documented that apoptosis induction is often mediated by the activation of a cascade of proteases known as caspases, we initially studied the role of caspases in induction of apoptosis by CD437 and 4-HPR. We found that both retinoids induced caspase-3 and caspase-9 enzyme activity. Furthermore, using caspase specific inhibitors we determined that caspase-3 and caspase-9 activity was essential for the induction of apoptosis by these synthetic retinoids since these inhibitors completely blocked CD437 and 4-HPR induced apoptosis. Interestingly, we found that treatment with bongkriekic acid (BA), a mitochondrial membrane depolarization inhibitor, blocked apoptosis, caspase-9 activation and caspase-3 activation induced by both retinoids. Finally, we were able to determine that CD437 treatment induced the translocation of TR3, a nuclear orphan receptor, whereas, 4-HPR did not. Our results suggest that CD437 and 4-HPR initially activate separate pathways to induce mitochondrial depolarization but both utilize mitochondrial depolarization, caspase-9 activation, and caspase-3 activation in the later stages of apoptosis induction.

The polyphenolic compounds curcumin and quercetin increased sensitivity of ovarian cancer cells (CAOV3 and SKOV3) to cisplatin. The effect was obtained when the compounds were added simultaneously with cisplatin, as well as when they were added 24 h before. High serum levels of certain cytokines, for example interleukin-6 (IL-6), have been associated with poor prognosis and cisplatin resistance in various forms of cancer. Furthermore, it has been hypothesized that cytokines may increase proliferation, metastasis, and stimulate production of detoxification enzymes and multi-drug resistant proteins. Curcumin inhibits the production of many cytokines. The two ovarian cell lines differ significantly in IL-6 production, and correspondingly the high producer, CAOV3, was less susceptible to cisplatin. Curcumin inhibited the production of IL-6 in this cell suggesting that one of the mechanisms for synergy between cisplatin and curcumin was by reducing the autologous production of IL-6. However, the synergy was also observed in the low IL-6 producer, SKOV3, indicating that the action was most probably a result of multiple targeting. In sum, this study suggests that the compounds, curcumin and quercetin, potentially may be useful for enhancing drug sensitivity in certain cancer.