Crossing the barrier: STRA6 in epidermal differentiation

Vitamin A Transport Mechanism of the Multitransmembrane Cell-Surface Receptor STRA6

Vitamin A has biological functions as diverse as sensing light for vision, regulating stem cell differentiation, maintaining epithelial integrity, promoting immune competency, regulating learning and memory, and acting as a key developmental morphogen. Vitamin A derivatives have also been used in treating human diseases. If vitamin A is considered a drug that everyone needs to take to survive, evolution has come up with a natural drug delivery system that combines sustained release with precise and controlled delivery to the cells or tissues that depend on it. This "drug delivery system" is mediated by plasma retinol binding protein (RBP), the principle and specific vitamin A carrier protein in the blood, and STRA6, the cell-surface receptor for RBP that mediates cellular vitamin A uptake. The mechanism by which the RBP receptor absorbs vitamin A from the blood is distinct from other known cellular uptake mechanisms. This review summarizes recent progress in elucidating the fundamental molecular mechanism mediated by the RBP receptor and multiple newly discovered catalytic activities of this receptor, and compares this transport system with retinoid transport independent of RBP/STRA6. How to target this new type of transmembrane receptor using small molecules in treating diseases is also discussed.

Retinol encapsulated into amorphous Ca(2+) polyphosphate nanospheres acts synergistically in MC3T3-E1 cells

Both the quality and quantity of collagen, the major structural component of the skin, decrease in aging skin. We succeeded to encapsulate retinol into amorphous inorganic polyphosphate (polyP) nanoparticles together with calcium ions ("aCa-polyP-NP"), under formation of amorphous Ca-polyP/retinol nanospheres ("retinol/aCa-polyP-NS"). The globular nanospheres are not cytotoxic, show an almost uniform size of ≈ 45 nm and have a retinol content of around 25%. Both components of those nanospheres, retinol and the aCa-polyP-NP, if administered together, caused a strong increase in proliferation of mouse calvaria MC3T3 cells. The expressions of collagen types I, II and III genes, but not the expression of collagen type V gene, were significantly enhanced if retinol is added together with aCa-polyP-NP. This synergistic effect was especially pronounced for the expression of the collagen type III gene. We propose that the synergistic effect of the retinol/aCa-polyP-NS on cell growth and collagen type III expression is induced via two routes, first through cellular uptake of the 45 nm nanospheres by clathrin-mediated endocytosis and second through extracellular disintegration of the nanospheres resulting in the release of retinol which is then taken up into the cells after binding to the retinal binding protein receptor.