Acitretin decreases tumor cell-induced angiogenesis

The Use of Retinoids for the Prevention and Treatment of Skin Cancers: An Updated Review

Retinoids are natural and synthetic vitamin A derivatives that are effective for the prevention and the treatment of non-melanoma skin cancers (NMSC). NMSCs constitute a heterogenous group of non-melanocyte-derived skin cancers that impose substantial burdens on patients and healthcare systems. They include entities such as basal cell carcinoma and cutaneous squamous cell carcinoma (collectively called keratinocyte carcinomas), cutaneous lymphomas and Kaposi’s sarcoma among others. The retinoid signaling pathway plays influential roles in skin physiology and pathology. These compounds regulate diverse biological processes within the skin, including proliferation, differentiation, angiogenesis and immune regulation. Collectively, retinoids can suppress skin carcinogenesis. Both topical and systemic retinoids have been investigated in clinical trials as NMSC prophylactics and treatments. Desirable efficacy and tolerability in clinical trials have prompted health regulatory bodies to approve the use of retinoids for NMSC management. Acceptable off-label uses of these compounds as drugs for skin cancers are also described. This review is a comprehensive outline on the biochemistry of retinoids, their activities in the skin, their effects on cancer cells and their adoption in clinical practice.

Tumor angiogenesis as a therapeutic target

Angiogenesis - the formation of new blood vessels within a tumor (or many other tissue types) - has become a hotbed of pharmacological research as well as industrial drug discovery. This is the result of the efforts of a generation of scientists elucidating the complex (patho)physiological, biochemical and molecular events accompanying angiogenesis. It is estimated that >300 drug candidates are currently in various stages of testing, and it is, therefore, impossible to capture all of this in a brief review. Therefore, the emphasis here is on relatively advanced projects that are either in preclinical or clinical development, thus neglecting, to a large extent, the many exciting avenues being pursued in both academic and biotechnology laboratories. Although the potential of the approaches described cannot be overestimated, it is also important to note that there is still no drug on the market that achieves clinical benefit based on a selective modulation or inhibition of angiogenesis.

New therapeutic strategies for the treatment and prevention of head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) is an aggressive epithelial malignancy that is now the sixth most common neoplasm in the world today. Approximately 50,000 cases in the United States and more than 500,000 cases worldwide will be diagnosed in 2000 [1]. Despite numerous advances in treatment utilising the most recent protocols for surgery, radiation and chemotherapy, the long-term survival has remained at less than 50% over the past 40 years [2]. This poor long-term survival is due to a number of variables including delayed diagnosis as well as the frequent development of multiple primary tumours. Therefore, in addition to early detection, continued emphasis must be placed on preventing the development of new primaries as well as establishing more effective treatments for individuals who present with advanced disease. This review will summarise some of the recent advances in the realms of chemotherapy and radiation therapy. In addition, it will discuss the present status of chemoprevention in HNSCC. Finally, we will discuss the rationale for the use of anti-angiogenic agents as one possible means of developing new chemopreventive protocols that result in reduced toxicity while maintaining similar clinical efficacies.

Angiogenesis in the development of head and neck cancer and its inhibition by chemopreventive agents

Squamous cell carcinoma is an aggressive malignancy that often develops as multiple independent lesions throughout the mucosa of the upper aerodigestive tract. Therefore, the comprehensive treatment of this disease must not only address the initial primary neoplasm, but also prevent the progression of the premalignant lesions lurking throughout the rest of the mucosal surfaces. The need to treat these lesions has resulted in a search for chemopreventive agents that can halt or even reverse their malignant progression. The biologic and molecular mechanisms by which most chemopreventive agents act have remained unclear and controversial. Recent work from several laboratories has demonstrated that some drugs may act in part by inhibiting the ability of tumors to induce blood vessel growth. Angiogenesis, the growth of new blood vessels from pre-existing ones, is absolutely required for solid neoplasms to grow beyond 2-3 mm in diameter. Therefore, chemopreventive agents that act to inhibit angiogenesis may provide a very powerful modality by which one may limit the growth of both pre-malignant lesions and small nests of tumor cells. This review will outline the basic changes that occur in tumor cells that result in the switch from an anti-angiogenic to an angiogenic phenotype. In addition, it will discuss the mechanisms by which some chemopreventive agents, presently under clinical investigation, inhibit tumor angiogenesis. Finally, this paper will present a rationale for the use of multiple anti-angiogenic agents as a means of developing new chemopreventive protocols that result in reduced patient toxicity while maintaining similar clinical efficacies.

Modulation of endothelial cell proliferation by retinoid x receptor agonists

One feature that contraindicates the wide therapeutic use of natural retinoids is their adverse effects during systemic use and the lack of receptor selectivity. In contrast, synthetic retinoids are distinguishable from each other on the basis of their partial or exclusive preference in binding and activation of selective retinoid receptors. We examined the inhibitory activities of natural and synthetic retinoids for their ability to reverse basic fibroblast growth factor-induced endothelial cell proliferation. Both the naturally occurring retinoids at nanomolar concentrations reversed basic fibroblast growth factor-induced endothelial cell proliferation. Among the synthetic retinoids tested, retinoic acid receptor/retinoid x receptor pan-agonist AGN 191659 [(E)-5-[2-(5,6,7, 8-tetrahydro-3, 5,5,8,8-pentamethyl-2-naphtyl) propen-1-yl]-2-thiophenecarboxylic acid] and retinoid x receptor pan-agonist AGN 191701 [(E)-2-[2-(5,6,7,8-tetrahydro-3, 5,5,8, 8-pentamethyl-2-naphthyl) propen-1-yl]-4-thiophenecarboxylic acid] at nanomolar concentrations reversed the basic fibroblast growth factor-induced endothelial cell proliferation. Since none of the retinoic acid receptor agonists tested had any effect, the inhibitory effect of AGN 191659 could be attributed to its retinoid x receptor receptor activity. These results suggest that retinoid x receptor agonists may be more selective anti-angiogenic agents due to their ability to inhibit endothelial cell proliferation.

Human papillomavirus-associated tumors of the skin and mucosa

This review discusses diseases of the lower genital tract in which the presence of human papillomavirus (HPV) DNA is well documented. We discuss epidemiologic, clinical, and experimental data supporting a causative role for HPV in mucosal malignancies, with emphasis on the role of viral and host factors in their pathogenesis. Of special interest is the recently discovered association of cutaneous tumors with HPVs, previously known only for tumors in epidermodysplasia verruciformis (EV). The frequent detection of EV-specific or EV-related HPVs in immunosuppressed persons and in cutaneous tumors in the general population supports the importance of EV as a model of cutaneous oncogenesis. We also discuss recent serologic findings based on enzyme-linked immunosorbent assay results with the use of viruslike particles. This is important both for detection of present or past HPV infection and for epidemiologic and immunologic studies. Novel therapeutic modalities for HPV tumors and prospects for prophylactic and therapeutic vaccination are presented.

Experimental basis of cancer combination chemotherapy with retinoids, cytokines, 1,25-dihydroxyvitamin D3, and analogs

Retinoids, cytokines as well as 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and analogs possess properties known to contribute potentially to cancer chemopreventive and chemotherapeutic effects. They induce cell differentiation, inhibit cell proliferation, suppress expression of viral oncogenes, and inhibit angiogenesis necessary for tumor growth. Since clinical combination chemotherapy of cytotoxic agents has proven superior to monotherapy, this modality might also be useful for other classes of antitumor drugs. A series of retinoids, such as all-trans-, 13-cis-, 9-cis retinoic acid, and acitretin, cytokines, 1,25(OH)2D3, and analogs have been investigated in model systems of differentiation, proliferation, viral oncogenes, and angiogenesis. The three classes of compounds have common effects but nevertheless show a variance depending on the particular representative of each class. Combination of compounds of the different classes led in the various models to a higher efficacy compared with the compounds given alone. Cytokines such as IFN alpha, IFN gamma, G-CSF, TNF alpha, IL-1, and IL-4 markedly potentiate the differentiation-inducing effect of retinoids. Cytokines as well as retinoids combined with 1,25(OH)2D3 and analogs synergistically enhanced differentiation induction in human transformed hemopoietic cell lines. On a series of human transformed epithelial cell lines a panel of cytokines, such as IFN alpha, IFN gamma, TNF alpha, TGF beta, and EGF acted synergistically with retinoids on inhibition of proliferation. This was also observed by combining retinoids with 1,25(OH)2D3 and analogs. Retinoids as well as interferons alpha and gamma have the capacity to suppress the oncogene expression of human papilloma viruses which are involved in induction and growth of certain malignancies such as cervical cancer.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic effect of retinoids and interferon alpha on tumor-induced angiogenesis: anti-angiogenic effect on HPV-harboring tumor-cell lines

Various retinoids and interferons exert anti-tumor effects both in experimental studies and in clinical trials. Recent reports indicate that they have a synergistic antineoplastic activity. Our study aimed to determine whether these synergistic anti-tumor effects are related to inhibition of tumor-cell-induced angiogenesis. A further aim was to compare the anti-angiogenic activity of various retinoids including 9-cis retinoic acid, a ligand for nuclear retinoic acid receptor RXR, given alone and in combination with interferon alpha-2a (IFN alpha-2a). An in vivo experimental model of cutaneous angiogenesis in the mouse was used. Angiogenesis was induced by intradermal injection of HPV16- or HPV18 DNA-harboring tumor-cell lines. All-trans retinoic acid (all-trans RA), 13-cis retinoic acid (13-cis RA) and 9-cis retinoic acid (9-cis RA) as well as IFN alpha-2a applied to mice intraperitoneally for 5 consecutive days before induction of angiogenesis resulted in significant inhibition of angiogenesis. Combination of retinoids with IFN alpha-2a led to a synergistic inhibition of angiogenesis, as compared to the effects of the drugs given alone. Similar results were obtained when tumor cells were preincubated in vitro with the compounds, before injection into untreated mice. Our findings on synergistic anti-angiogenic effects of retinoids and IFN alpha-2a could explain, at least partially, the anti-tumor efficacy of combined therapy with these agents, and provide support for the role of angiogenesis in tumor growth.

Inhibition of tumor cell-induced angiogenesis by retinoids, 1,25-dihydroxyvitamin D3 and their combination

Tumor-induced angiogenesis (TIA), i.e., the ability of transformed cells to stimulate new blood vessel formation is an important factor contributing to tumor growth and invasiveness. The antiangiogenic effect of the retinoids, all-trans retinoic acid, 13-cis retinoic and 9-cis retinoic acid, of 1,25-dihydroxyvitamin D3, and of their combinations were studied using an experimental system in vivo. TIA was induced in immunosuppressed mice by intradermal injection of the two human transformed keratinocyte lines, Skv-e2, harboring DNA of human papillomavirus (HPV) type 16, and HeLa, harboring HPV18 DNA. The three retinoids and 1,25-dihydroxyvitamin D3, when administered systemically to mice, before the angiogenesis assay significantly decreased TIA. Their combination led to a synergistic inhibition of TIA. These results provide the basis for the use of combination of retinoids and 1,25-dihydroxyvitamin D3 in treatment of neoplastic diseases.

Tumour angiogenesis

The progressive emergence of a close relationship between the formation of blood vessels in the vicinity of tumour cells and the development and spreading of tumours, strongly suggests that angiogenesis might be a prerequisite for tumour development. Angiogenesis starts and develops in response to two sets of extracellular signals: soluble angiogenic factors and extracellular matrix. Different experimental models have been used to study angiogenesis in vivo, but they have numerous limitations. Three-dimensional culture systems reconstitute normal interactions between endothelial cells and the surrounding extracellular matrix. Numerous parameters including angiogenic growth factors and cytokines, cell-to-cell interactions and cell-to-extracellular matrix adhesion influence the growth and differentiation of endothelial cells in vitro as well as in vivo. Angiogenesis plays a major role not only in tumour growth but also in metastasis development. Mechanisms of switching to angiogenic phenotype have been recently described and onset of angiogenic activity is now recognized as another discrete step in tumorigenesis. Tumour cells can induce b-FGF expression and exportation, VEGF and VEGF receptor expression and inactivation of the cancer suppressor gene encoding for a fragment of thrombospondin. A controlled net proteolytic balance produced by tumour cells or endothelial cells is required to favour migration and invasion of endothelial cells and angiogenesis. The hypothesis that assessment of tumour angiogenesis might predict tumour aggressiveness in human cancer has recently gained support from several clinical studies. This has been shown for cutaneous melanoma, breast carcinoma, and non-small-cell lung cancer by quantitation of microvessels in human biopsies using von Willebrand factor or CD3 antigen labelling with specific antibodies. However, more specific and sensitive markers are needed to improve this approach for predicting tumour aggressiveness. Folkman proposed twenty years ago that inhibition of angiogenesis might represent a suitable complementary strategy for the treatment of various forms of cancer. Since then numerous angiostatic compounds have been identified but very few of them fit the required criteria of a potential drug. Fumagillin and particularly its synthetic analogue AGM 1470 might be developed for use in humans in the near future.