Retinoids (an analogue of Vitamin-A (all-trans-Retinol)) and Retinoic Acid (a metabolite of Vitamin-A), regulate expression of target genes through binding and activation of the nuclear receptors like RARs (Retinoic Acid Receptors) and the RXRs (Retinoid X Receptors). all-trans-Retinoic Acid, the Carboxylic Acid form of Vitamin-A is of biological significance since it is predominant under most physiological situations. Biologically active ligands for the RARs and RXRs include all-trans-Retinoic Acid, 9-cis-Retinoic Acid, Retinoids among others, yet circulating levels of 9-cis-Retinoic Acid are much lower than those of all-trans-Retinoic Acid and the physiological significance of the isomerization all-trans-Retinoic Acid to 9-cis-Retinoic Acid and vice versa is yet to be ascertained (Ref.1 & 2). The RARs are expressed at higher levels than the RXRs. Vitamin-A and Retinoids are obtainted from dietary sources is converted to all-trans-Retinoic Acid in the liver, and then it diffuses easily to the target tissues through cellular membranes and is translocated to the RAR/RXR hetrodimer. all-trans-Retinoic Acid is translocated to the nucleus through CRABP (Cellular Retinoic Acid Binding Protein) (Ref.3).
The RARs and RXRs are transcription factors whose ligands promote the recruitment of co-activators and co-repressors. Among the nuclear receptor co-activators the best characterized are p160 family of proteins comprising of SRC1 (Steroid Receptor Coactivator-1)/NCOA1 (Nuclear Receptor Coactivator-1); GRIP1 (Glucocorticoid Receptor-Interacting Protein-1)/NCOA2; and RAC-3 (Receptor-Associated Coactivator-3)/NCOA3. The p160 proteins participate in transcriptional activation by RAR/RXRs heterodimer through AF2 and interact with the heterodimers in a ligand-dependent manner. These interactions are mediated by two distinct regions on the p160s; the central NR (Nuclear Receptor) Boxes bind to the RAR/RXRs AF2 Domains, whereas the p160 C-terminus interacts with the RAR/RXRs N-terminal AF1 Domains (Ref.4). The p160s enhance RAR/RXRs transactivation by recruiting other transcriptional regulatory factors through two Activation Domains, AD1 and AD2. AD1 interacts with CBP (CREB-Binding Protein) and p300, and AD2 associates with CARM1 (Coactivator-Associated Arginine Methyltransferase-1) and PRMT1 (Protein Arginine N-Methyltransferase-1). The RAR and RXRs heterodimer bind to the DNA on specific regions known as RARE (Retinoic Acid Response Element) and RXRE (Retinoid X Response Element), respectively. These nuclear complexes along with their co-activators act as chromatin remodeling factors, which controls a variety of biological processes such as cellular differentiation, development, survival and growth (Ref.5).
The mechanisms cell survival and chromatin remodeling is dependent upon direct phosphorylation of transcription regulators and its associated proteins like CREB (cAMP Response Element-Binding Protein), CBP (CREB-Binding Protein), p300 and PCAF (p300/CBP-Associated Factor). PKA has served as the central model for signal transduction and cellular regulation of such transcription regulators through phosphorylation, upon being activated by with Growth Factors and their receptors. The HAT (Histone Acetyltransferase) activity of CBP, p300 and HAT1 (Histone Acetyltransferase-1) acetylates Histones of the Nucleosome complexes. They act synergistically with Acetyl-CoA (Acetyl-Coenzyme-A), and their targets include the acetlyation of core Histones-H3 and Histones-H4 on Lys (Lysine) positions. Apart from PKA, CBP and p300 are also the targets of CARM1 (Ref.6). The HMT (Histone Methyltransferase) activity of CARM1 methylates of CBP and p300 and this mechanism blocks CBP and p300 from acting as a co-activator to other proteins like CREB, p160s, and PCAF. CARM1 along with S-AdoMet (S-Adenosylmethionine) redirects the limited CBP and p300 pool in the cell to be available for RAR/RXRs heterodimers. Methylated CBP and p300 fail to mediate transcription and Histone acetylation. Other forms of post-translational protein modification such as phosphorylation are reversible in nature, but as of yet a protein Demethylase is not known. Attachment of p160s, CBP, p300 to RAR/RXRs heterodimer increases its ligand-independent and ligand-dependent transactivation and also enhances p160 transcriptional activity when p160s are tethered to DNA. Several forms of post-translational modification regulate protein activities. PRMT1 and CARM1 methylates Histones-H4 and Histones-H3 on Arg3 (Arginine 3) and Arg17 (Arginine 17) positions, respectively (Ref.7 & 8).
The methylation of Arginines and acetylation of Lysines within Histones represents an activating step in mammalian gene transcription. These stimulate RAR/RXRs transactivation by interacting with components of the Basal Transcriptional Complex such as RNA Pol II (RNA Polymerase-II), TBP (TATA Box-Binding Protein), TFIIA (Transcription Factor-IIA), TFIIB, TFIIE, TFIIF, TFIIH (General Transcription Factor IIH Polypeptide) and TAFs (TBP Associated Factors). These GTFs (General Transcription Factors) promotes the expression of Retinoic Acid dependent genes as well as proteins like Bcl2 (B-Cell CLL/Lymphoma-2) and Sst (Somatostatin) (Ref.9 & 10). These genes play an important role in controlling the pathogenesis of B-cell malignancies, AMLs (Acute Myeloid Leukemias), Acromegaly, Vasoactive Intestinal Peptide Tumors, etc in humans. Targeting of nuclear complexes to chromatin and their activation by Vitamin-A vitamers represents a complex signal transduction by RAR/RXRs heterodimers that appears to rely on a collaboration of multiple factors for regulation of gene expression (Ref.11).