CELLULAR-BINDING PROTEINS FOR FATTY-ACIDS AND RETINOIDS - SIMILAR OR SPECIALIZED FUNCTIONS

The cellular fatty acid-binding proteins (FABP) and cellular retinoid (retinol, retinoic acid)-binding proteins (CRtBP) are structurally and functionally-defined groups within an evolutionarily conserved gene family. CRtBP are expressed in both fully differentiated and developing tissues in a manner that supports a relationship to the action of retinoic acid in morphogenesis and cellular differentiation. The FABP are, by contrast, expressed only in fully differentiated tissues in a manner compatible with a major function in the metabolism of long-chain fatty acids (LCFA) for energy production or storage. The precise function(s) of FABP and CRtBP remain imperfectly understood, while subspecialization of function(s) within the two groups is suggested by the complex diversity in both of structurally distinct members that display striking tissue and temporal specificity of expression in addition to ligand specificity. Notwithstanding this considerable apparent functional diversity among the FABP and CRtBP, available evidence supports a dual set of generic functions for both protein groups in a) promoting cellular flux of poorly water-soluble ligands and their subsequent metabolic utilization or transformation, and b) sequestration of ligands in a manner that limits their association with alternative binding sites within the cell, of which members of the steroid hormone nuclear receptor superfamily (HNR) are a potentially important category. Theoretical as well as experimental models probing diffusional fluxes of LCFA in vitro and in living cells have provided support for a function for FABP in intracellular LCFA transport. Protein-bound ligand also appears to provide the substrate for metabolic transformation of retinoids bound to CRtBP, but convincing evidence is lacking for an analogous mechanism in the direct facilitation of fatty acid utilization by FABP. An emerging relationship between FABP and CRtBP function centers on their binding of, and induction by, ligands which activate or transform specific HNR - the retinoic acid receptors and the peroxisome proliferator activated receptor in the case of CRtBP and FABP, respectively. Evidence consistent with both a 'promotive' role (provision of ligands for HNR) and a 'protective' role (limiting availability of free ligand for HNR association) has been advanced for CRtBP. Available data supports a 'protective' function for cellular retinoic acid-binding proteins (CRABP) and liver FABP (L-FABP) and points to the existence of ligand-defined, lipid-binding-protein - HNR relationships in which CRABP serve to attenuate the induction of gene expression by retinoic acid, and in which L-FABP may modulate a cellular adaptive multigene response to increased LCFA flux or compromised LCFA utilization. Furthermore, the emerging role of LCFA in the regulation of gene expression combined with the complex interplay between heterologous HNR-ligand associations and gene cross-regulation implies an important potential interaction between FABP, CRtBP, and their respective ligands in gene regulation.