Andy Yen, College of Veterinary Medicine

The Yen laboratory is interested in the cellular molecular mechanisms of action and the effects on gene expression of the vitamin A metabolite, retinoic acid. Retinoic acid is the biologically active form of vitamin A and is known to regulate both cell growth and differentiation. Intracellularly retinoic acid is converted to a variety of retinoid metabolites. These act as ligands for both RAR (retinoic acid receptor) and RXR (retinoid X receptor) subclasses of receptors which can homo and heterodimerize (also, see Noy laboratory). Such ligand activated receptor complexes can transcriptionally activate or repress genes by binding cis-acting RARE’s (retinoic acid response elements).

This motivates two basic questions about the mechanism of action of retinoic acid. (1) Which of the RAR's or RXR's are essential for retinoic acid to control cell proliferation or cell differentiation. (2) What genes are transcriptionally regulated by these essential receptors. Much of the work in the Yen laboratory is carried out using a retinoic acid responsive cultured human myeloblast, HL-60, which are uncommitted hematopoietic precursor cells that undergo G0 cell cycle arrest and myeloid differentiation in response to retinoic acid. The experimental strategy is to first experimentally ablate expression of specific RAR's and RXR's or combinations thereof to determine which receptors are essential for retinoic acid to elicit cell cycle arrest or cell differentiation. A second objective is to overexpress putative essential receptors to determine if they will enhance the rate of retinoic acid induced cell cycle arrest and differentiation. A third objective is to analyze expression of novel mRNA’s induced by retinoic acid in HL-60 stable transfectants which express only the RAR's and/or RXR's essential for eliciting G0 arrest and differentiation. A fourth line of study extends preliminary data that indicates that retinoic acid probably induces the expression of less than approximately 20 early genes.

The Yen laboratory has set out to determine the identity of a small ensemble of early differentiation and growth regulatory genes whose expression is induced by retinoic acid. They also plan to establish whether the induced expression of each specific gene depends on signaling through RARs, RXRs or via ERK2 (MAP kinase) activated transcription factors, which genes are essential for retinoic acid to induce either differentiation, retinoblastoma (RB) tumor suppressor hypophosphorylation, or G0 growth arrest, and which genes might act as central regulators seminal to inducing either differentiation, RB hypophosphorylation, or G0 arrest. This will determine what early regulatory genes retinoic acid induces, the signaling routes they are responding to, and whether a gene differentially regulates cell differentiation, RB hypophosphorylation, or G0 growth arrest.

As the Noy and Clardy groups obtain molecular information rgarding the key residues important for tetinoic acid receptor hours- and hetero-oligomerization, the Yen laboratory will examine the effects of receptor mutants defective for regulated oligomerization on the different signaling outcomes listed above. Overall the combined data from cellular and structure-function studies should provide a comprehensive picture of the control of cell proliferation and differentiation by retinoic acid.