Retinoic acid, histone deacetylase inhibitors, and embryonal carcinoma stem cell differentiation

Embryonal carcinomas (EC)---stem cells of teratocarcinomas---have proven beneficial in studying both embryogenesis and tumorgenesis. They provide a model in vitro cell culture system that can be used to study cell growth, differentiation and apoptosis. To exploit the value of the EC cell system, a retroviral mutagenesis strategy was used that produced a mutant EC cell lines with altered cellular properties. Phenotypes associated with the mutant affected morphology, growth in vivo, and cellular differentiation regulated by All-Trans Retinoic Acid (ATRA).; It has long been known that ATRA can rapidly induce differentiation of EC cells in vitro by altering the cellular processes of cell cycle arrest and/or apoptosis thereby making ATRA an obvious candidate as an anti-cancer therapeutic agent. To study RA induced differentiation, the NR1-6 mutant cell line was used to identify the gene (or genes) that mediate its extreme ATRA sensitivity. Unfortunately, the candidate genes in this study did not yield consistent evidence for causative factors (i.e., molecules associated with the NR1-6 ATRA hypersensitivity).; Treatment with an HDAC inhibitor (HDACI), known to act similarly to RA [i.e., Trichostatin A (TSA)] affected both mutant and parental cell lines similarly in terms of the phenotypic parameter of morphology, growth, and apoptosis whereas genotypic comparisons revealed critical differences. Since ATRA and TSA (or other HDACIs) are currently being used combinatorially in several clinical cancer trials and since my preliminary results indicated contrast differences between phenotypic and genotypic alterations, I undertook a broader study, via microarray analyses, to examine the effects of ATRA and/or TSA on global gene expression. My results revealed several differences in gene expression elicited by these compounds while supporting recent studies questioning the proposed model by which HDACI regulate gene expression. The study also indicates that some of these differences may well prove crucial in the clinical management of certain types of cancer as well as other diseases. In particular, two critical pathways were identified that are negatively regulated by TSA and combination (ATRA/TSA) therapy: the cholesterol biosynthesis and pyrimidine metabolism pathways. Besides being implicated in carcinogenesis, the components of these pathways also appear to be essential for regulating diseases such as atherosclerosis and rheumatoid arthritis, and respectively. Therefore, this study identifies HDACI as a potential candidate for therapeutic interventions. The study also identified a new promoter module that regulates expression of one set of genes with differential response to ATRA vs. TSA.