il\VOLVEMENT OF RETII\OIC ACID II{ - MSpace at the University of ...

et al. 1994; Napoli 1996). The function of retinoid binding proteins does not stop there,
since RBp are found to be involved in the transportation of retinol through the cellular
membranes. By binding to retinol, RBP protect the celiular membrane from amphypathic
and intercalative effects of this lipid molecule Q.{apoli 1996). RBP also mediates the
transport of retinol in the cytosol by preventing a free flotation of its lipid-soluble
molecule in the aqueous cytosolic compartment. CRABP has an even more complex
function. The CRABP is also found to protect the cells from an excessive RA levels,
acting as a sponge mechanism soaking up RA. This is supported by the fact that the cells
that overexpress the CRABP are found to be less responsive to retinoic acid therapy
{Fiorella & Napoli 1991 400 /id}. Another function of CRABP is that, in its holo form,
CRABP is found to increase the metabolism of RA (Boylan and Gudas 1991; Boylan and
Gudas 1992;Fíorella and Napoli 1991; Williams and Napoli 1985). CRABP may also
play a role in the RA signaling mechanism by controlling the amount of free retinoic
acid. Free retinoic acid, through its binding to a number of intracellular receptors, has a
potential to regulate transcription processes. All of this data indicates that CRABP may
have a dual role. In specific conditions, these retinoid binding proteins may promote the
catabolism of retinoic acid. At the same time, CRABP may be involved in the
transportation of RA to nucleus for its binding with nuclear receptors which will result in
regulation of transcription. A new emerging concept indicates that CRABP II has a
potential to directly interact with nuclear receptors RAR alpha and RXR alpha, which
may represent a novel pathway in retinoic acid signaling cascade (Bastie et al' 2001;
Budhu and Noy 2002).
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