The effect of sulforaphane on histone deacetylase activity in keratinocytes: Differences between in vitro and in vivo analyses

Sulforaphane is a natural product found in broccoli, which is known to exert many different molecular effects in the cell, including inhibition of histone deacetylase (HDAC) enzymes. Here, we examine for the first time the potential for sulforaphane to inhibit HDACs in HaCaT keratinocytes and compare our results with those found using HCT116 colon cancer cells. Significant inhibition of HDAC activity in HCT116 nuclear extracts required prolonged exposure to sulforaphane in the presence of serum. Under the same conditions HaCaT nuclear extracts did not exhibit reduced HDAC activity with sulforaphane treatment. Both cell types displayed down-regulation of HDAC protein levels by sulforaphane treatment. Despite these reductions in HDAC family member protein levels, acetylation of marker proteins (acetylated Histone H3, H4, and tubulin) was decreased by sulforaphane treatment. Time-course analysis revealed that HDAC6, HDAC3, and acetylated histone H3 protein levels are significantly inhibited as early as 6 h into sulforaphane treatment. Transcript levels of HDAC6 are also suppressed after 48 h of treatment. These results suggest that HDAC activity noted in nuclear extracts is not always translated as expected to target protein acetylation patterns, despite dramatic inhibition of some HDAC protein levels. In addition, our data suggest that keratinocytes are at least partially resistant to the nuclear HDAC inhibitory effects of sulforaphane, which is exhibited in HCT116 and other cells.

A novel chemopreventive mechanism for a traditional medicine: East Indian sandalwood oil induces autophagy and cell death in proliferating keratinocytes

One of the primary components of the East Indian sandalwood oil (EISO) is α-santalol, a molecule that has been investigated for its potential use as a chemopreventive agent in skin cancer. Although there is some evidence that α-santalol could be an effective chemopreventive agent, to date, purified EISO has not been extensively investigated even though it is widely used in cultures around the world for its health benefits as well as for its fragrance and as a cosmetic. In the current study, we show for the first time that EISO-treatment of HaCaT keratinocytes results in a blockade of cell cycle progression as well as a concentration-dependent inhibition of UV-induced AP-1 activity, two major cellular effects known to drive skin carcinogenesis. Unlike many chemopreventive agents, these effects were not mediated through an inhibition of signaling upstream of AP-1, as EISO treatment did not inhibit UV-induced Akt or MAPK activity. Low concentrations of EISO were found to induce HaCaT cell death, although not through apoptosis as annexin V and PARP cleavage were not found to increase with EISO treatment. However, plasma membrane integrity was severely compromised in EISO-treated cells, which may have led to cleavage of LC3 and the induction of autophagy. These effects were more pronounced in cells stimulated to proliferate with bovine pituitary extract and EGF prior to receiving EISO. Together, these effects suggest that EISO may exert beneficial effects upon skin, reducing the likelihood of promotion of pre-cancerous cells to actinic keratosis (AK) and skin cancer.

A novel chemopreventive mechanism for a traditional medicine: East Indian sandalwood oil induces autophagy and cell death in proliferating keratinocytes

One of the primary components of the East Indian sandalwood oil (EISO) is α-santalol, a molecule that has been investigated for its potential use as a chemopreventive agent in skin cancer. Although there is some evidence that α-santalol could be an effective chemopreventive agent, to date, purified EISO has not been extensively investigated even though it is widely used in cultures around the world for its health benefits as well as for its fragrance and as a cosmetic. In the current study, we show for the first time that EISO-treatment of HaCaT keratinocytes results in a blockade of cell cycle progression as well as a concentration-dependent inhibition of UV-induced AP-1 activity, two major cellular effects known to drive skin carcinogenesis. Unlike many chemopreventive agents, these effects were not mediated through an inhibition of signaling upstream of AP-1, as EISO treatment did not inhibit UV-induced Akt or MAPK activity. Low concentrations of EISO were found to induce HaCaT cell death, although not through apoptosis as annexin V and PARP cleavage were not found to increase with EISO treatment. However, plasma membrane integrity was severely compromised in EISO-treated cells, which may have led to cleavage of LC3 and the induction of autophagy. These effects were more pronounced in cells stimulated to proliferate with bovine pituitary extract and EGF prior to receiving EISO. Together, these effects suggest that EISO may exert beneficial effects upon skin, reducing the likelihood of promotion of pre-cancerous cells to actinic keratosis (AK) and skin cancer.

Abstract 1077: Dominant negative PI3-Kinase inhibits UV-induced squamous cell carcinoma in vivo

Phosphoinositide-3 Kinase (PI3-Kinase) is an upstream regulator of cellular proliferation and anti-apoptotic signaling that has been implicated in cancer induction in many tissues, including skin. Pharmacological inhibition of PI3-Kinase is effective at inhibiting both chemical and UV-driven skin carcinogenesis. However, the specificity of these pharmacological inhibitors is often questioned. Therefore, specific genetic inhibition of PI3-Kinase in the epidermis needs to be carried out in mice. We report here on the generation of a new transgenic mouse which constitutively expresses the dominant negative mutant form of the regulatory subunit of the PI3-Kinase dimer, p85, in the epidermis under control of the keratin-5 promoter. These p85DN transgenic mice were derived on the SKH-1 hairless genetic background. A chronic UVB exposure experiment showed significant inhibition of both tumor size (p=0.0012) and tumor burden (p=0.0003) for mice carrying the transgene compared to wildtype littermates. The time to first tumor was also delayed in transgenic mice, but with only marginal significance (p=0.07). p85DN mice also showed significant inhibition of epidermal Activator Protein-1 (AP-1) transcription factor activity after acute UVB treatment (p=0.046). UVB induced activation of AP-1 in the epidermis has been shown to play a functional role in UVB mouse skin carcinogenesis. Further analysis of protein expression after acute treatment with UVB suggests that Akt signaling and COX-2 activation are both downregulated in p85DN transgenic mice. To our knowledge, this is the first report of genetic inhibition of PI3-Kinase in mouse skin carcinogenesis studies. Together, these data suggest a functional role of PI3-Kinase in UVB-induced squamous cell carcinoma and support the assertion that inhibition of this pathway is beneficial in chemoprevention of non-melanoma skin cancer. This work was supported in part by the following NIH grants: K07CA132956, K07CA132956-02S1, P01CA27502 and P30CA23074.

Citation Format: Sally E. Dickinson, Erik R. Olson, Jadrian J. Rusche, P. Jane Criswell, Jack Zhang, Zagong Dong, Chengcheng Hu, Kathylynn Saboda, David S. Alberts, G. Timothy Bowden. Dominant negative PI3-Kinase inhibits UV-induced squamous cell carcinoma in vivo. [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 1077. doi:10.1158/1538-7445.AM2013-1077

The proximal promoter region of the human vascular endothelial growth factor gene has a G-quadruplex structure that can be targeted by G-quadruplex-interactive agents

Previous studies on the functional analysis of the human vascular endothelial growth factor (VEGF) promoter using the full-length VEGF promoter reporter revealed that the proximal 36-bp region (-85 to -50 relative to transcription initiation site) is essential for basal or inducible VEGF promoter activity in several human cancer cells. This region consists of a polypurine (guanine) tract that contains four runs of at least three contiguous guanines separated by one or more bases, thus conforming to a general motif capable of forming an intramolecular G-quadruplex. Here, we show that the G-rich strand in this region is able to form an intramolecular propeller-type parallel-stranded G-quadruplex structure in vitro by using the electrophoretic mobility shift assay, dimethyl sulfate footprinting technique, the DNA polymerase stop assay, circular dichroism spectroscopy, and computer-aided molecular modeling. Two well-known G-quadruplex-interactive agents, TMPyP4 and Se2SAP, stabilize G-quadruplex structures formed by this sequence in the presence of a potassium ion, although Se2SAP is at least 10-fold more effective in binding to the G-quadruplex than TMPyP4. Between these two agents, Se2SAP better suppresses VEGF transcription in different cancer cell lines, including HEC1A and MDA-MB-231. Collectively, our results provide evidence that specific G-quadruplex structures can be formed in the VEGF promoter region, and that the transcription of this gene can be controlled by ligand-mediated G-quadruplex stabilization. Our results also provide further support for the idea that G-quadruplex structures may play structural roles in vivo and therefore might provide insight into novel methodologies for rational drug design.

Facilitation of a structural transition in the polypurine/polypyrimidine tract within the proximal promoter region of the human VEGF gene by the presence of potassium and G-quadruplex-interactive agents

The proximal promoter region of the human vascular endothelial growth factor (VEGF) gene contains a polypurine/polypyrimidine tract that serves as a multiple binding site for Sp1 and Egr-1 transcription factors. This tract contains a guanine-rich sequence consisting of four runs of three or more contiguous guanines separated by one or more bases, corresponding to a general motif for the formation of an intramolecular G-quadruplex. In this study, we observed the progressive unwinding of the oligomer duplex DNA containing this region into single-stranded forms in the presence of KCl and the G-quadruplex-interactive agents TMPyP4 and telomestatin, suggesting the dynamic nature of this tract under conditions which favor the formation of the G-quadruplex structures. Subsequent footprinting studies with DNase I and S1 nucleases using a supercoiled plasmid DNA containing the human VEGF promoter region also revealed a long protected region, including the guanine-rich sequences, in the presence of KCl and telomestatin. Significantly, a striking hypersensitivity to both nucleases was observed at the 3′-side residue of the predicted G-quadruplex-forming region in the presence of KCl and telomestatin, indicating altered conformation of the human VEGF proximal promoter region surrounding the guanine-rich sequence. In contrast, when specific point mutations were introduced into specific guanine residues within the G-quadruplex-forming region (Sp1 binding sites) to abolish G-quadruplex-forming ability, the reactivity of both nucleases toward the mutated human VEGF proximal promoter region was almost identical, even in the presence of telomestatin with KCl. This comparison of wild-type and mutant sequences strongly suggests that the formation of highly organized secondary structures such as G-quadruplexes within the G-rich region of the human VEGF promoter region is responsible for observed changes in the reactivity of both nucleases within the polypurine/polypyrimidine tract of the human VEGF gene. The formation of the G-quadruplex structures from this G-rich sequence in the human VEGF promoter is further confirmed by the CD experiments. Collectively, our results provide strong evidence that specific G-quadruplex structures can naturally be formed by the G-rich sequence within the polypurine/polypyrimidine tract of the human VEGF promoter region, raising the possibility that the transcriptional control of the VEGF gene can be modulated by G-quadruplex-interactive agents.