Phorbol ester-mediated induction of cyclooxygenase-2 gene expression is inhibited by retinoids

Arachidonic Acid-Derived Bioactive Lipids: Their Role and the Role for Their Inhibitors in Dermatology

Background:

In addition to corticosteroids, there are increasing numbers of anti-inflammatory agents that specifically target bioactive lipids generated from arachidonic acid. Knowledge of the diverse mechanisms of action of these different bioactive lipids holds promise in the therapy of a wide spectrum of cutaneous and systemic disorders.

Objective:

Therapeutic manipulations of these lipid molecules through inhibition, stimulation, or direct replacement have broad physiologic effects. These therapeutic strategies not only modulate inflammation, pain, and hemostatic parameters, they also play a role in cardiac, respiratory, renal, and gastrointestinal function and disease, as well as in angiogenesis and in factors that control cell growth and apoptosis important in carcinogenesis.

Conclusion:

Newer drug discovery methods, including combinatorial chemistry with molecular modeling, have made it possible to develop inhibitors and analogs with increasing specificity and bioactivity and decreasing toxicity. Although the application of these analogs and inhibitors for cutaneous disease is limited today, either as primary agents or adjuvant therapy, these drugs will have a place in our therapeutic regimes of the future. We present a review of the therapeutic agents now available from manipulation of these bioactive lipids, and their role and future in dermatology.

Characterization of cellular retinoid-binding proteins in human myometrium during pregnancy

Many complementary or competing signalling pathways bear an influence on the myometrium at any one time, and because the retinoic acid signalling pathway influences differentiation of a wide array of human tissues, this may be one of the determinants of myometrial differentiation during pregnancy. We have explored the novel hypothesis that the retinoids may act as important regulators in controlling the differentiated state of the human myometrium during pregnancy by characterizing the expression profiles for cellular retinoid-binding proteins CRBPI, CRABPI and CRABPII in non-pregnant, pregnant (non-labouring) and labouring human myometrium taken from the functionally distinct upper and lower uterine segments. In addition, we have investigated the effect of all-trans retinoic acid (ATRA) on the expression of several retinoic acid response genes including cyclooxygenase-2 (COX-2) and connexin-43 (Cx-43). Different spatial and temporal patterns of expression were observed for CRBPI, CRABPI and CRABPII within the upper and lower uterine segments through the three trimesters of pregnancy and in labour. Furthermore, the expression of COX-2, Cx-43, CRABPI, the transcription factor c-Jun and the retinoic acid receptor RARbeta altered in response to different concentrations of ATRA, suggesting that the differential expression of cellular retinoid-binding proteins may lead to different levels of retinoic acid being delivered to its nuclear targets, leading to the differential expression of specific target genes within the myometrium during pregnancy.

Induction of retinoic acid receptor-beta suppresses cyclooxygenase-2 expression in esophageal cancer cells

Since retinoic acid receptor (RAR)-beta mRNA is frequently lost during esophageal carcinogenesis and esophageal cancer cells that do not express RAR-beta are resistant to retinoic acid (RA), we stably transfected RAR-beta expression vector into an esophageal cancer cell line TE-8 and an antisense RAR-beta into TE-3 cells. Transfection of RAR-beta decreased cell growth and colony formation and induced apoptosis in TE-8 cells. Antisense RAR-beta-transfected TE-3 cells had a shorter doubling time and became resistant to RA. Induction of RAR-beta decreased COX-2 expression in RAR-beta transfected TE-8 cells, whereas antisense RAR-beta transfected TE-3 cells increased COX-2 expression. The inhibitory effect of RAR-beta on COX-2 expression was further enhanced in the presence of RA, which was blocked by an RAR antagonist. The synthetic retinoid N-(4-hydroxyphenyl)retinamide, which does not bind effectively to RAR-beta, had no effect on COX-2 suppression. Furthermore, RA blocked bile acid-induced COX-2 expression and prostaglandin E(2) production only in the RAR-beta positive cells. Our data demonstrated that anticancer effect of RAR-beta may be related to its ability to suppress COX-2 expression and support that the loss of RAR-beta expression may contribute to esophageal carcinogenesis.

Magnetic resonance imaging of ethyl-nitrosourea-induced rat gliomas: a model for experimental therapeutics of low-grade gliomas

Human low-grade gliomas represent a population of brain tumors that remain a therapeutic challenge. Preclinical evaluation of agents, to test their preventive or therapeutic efficacy in these tumors, requires the use of animal models. Spontaneous gliomas develop in models of chemically induced carcinogenesis, such as in the transplacental N-ethyl-N-nitrosourea (ENU) rat model. However, without the ability to detect initial tumor formation, multiplicity or to measure growth rates, it is difficult to test compounds for their interventional or preventional capabilities. In this study Fisher-334 rats, treated transplacentally with ENU, underwent magnetic resonance imaging (MRI) examination in order to evaluate this approach for detection of tumor formation and growth. ENU-induced intracranial cerebral tumors were first observable in T2-weighted images beginning at 4 months of age and grew with a mean doubling time of 0.487 +/- 0.112 months. These tumors were found histologically to be predominately mixed gliomas. Two therapeutic interventions were evaluated using MRI, vitamin A (all-trans retinol palmitate, RP), as a chemopreventative agent and the anti-angiogenic drug SU-5416. RP was found to significantly delay the time to first tumor observation by one month (P = 0.05). No differences in rates of tumor formation or growth rates were observed between control and RP-treated groups. MRI studies of rats treated with SU-5416 resulted in reduction in tumor growth rates compared to matched controls. These results show that MRI can be used to provide novel information relating to the therapeutic efficacy of agents against the ENU-induced tumor model.

Expression of cyclo-oxygenase-2 in naturally occurring squamous cell carcinomas in dogs

Squamous cell carcinoma is one of the most common cancers in humans and is also a frequently diagnosed neoplasm in dogs. Induction of cyclo-oxygenase-2 (COX-2), a key rate-limiting enzyme in prostaglandin biosynthesis, has been implicated in the oncogenesis of various cancers in humans, including squamous cell carcinomas. However, expression of COX-2 has not been reported in spontaneous squamous cell carcinomas of non-human species. Canine squamous cell carcinomas share several similarities with the human disease. Therefore, the objective of this study was to determine whether COX isoenzymes were expressed in naturally occurring cases of squamous cell carcinomas in dogs. Canine normal skin (n=4) and squamous cell carcinomas (n=40) were studied by immunohistochemistry and immunoblotting analysis using polyclonal antibodies selective for COX-1 or COX-2. COX-2 was strongly expressed by neoplastic keratinocytes in all cases of squamous cell carcinomas, whereas no COX-2 was detected in normal skin and in the non-neoplastic skin and oral mucosa included in the tumor tissue samples (p<0.01). Immunoblotting analysis confirmed the restricted expression of COX-2 (72,000--74,000 molecular weight doublet) in squamous cell carcinomas only. In contrast, faint COX-1 staining was found in normal skin and in squamous cell carcinomas. This study demonstrates for the first time that COX-2 is induced in canine squamous cell carcinomas, and provides a new model to investigate the role and regulation of COX-2 gene expression in naturally occurring squamous cell carcinomas. (J Histochem Cytochem 49:867-875, 2001)