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

This is indeed a unique characteristic among receptors involved in signaling
pathways which supports the retinoic acid's exceptional role in the processes of
vertebrate development and homeostasis (Chambon 1996). Another characteristic of
retinoic acid receptors is the fact that these receptors may be evolutionary related to
another group of life-essential receptors{hyroid hormone receptors. This is supported by
the fact that both genes encoding for thyroid hormone receptor a (TR-a) and RAR a are
located on the same human chromosome. The same can be said for the TR-P and RAR 0
receptors (human chromosome 3). Furthennore, both receptor proteins have a potential to
bind to same RAREs and can act syrergistically to induce the transcription of rat growth
hormone genes (Wan 1993).
Regarding the specific tissue distribution, RAR s is found to be ubiquitously
expressed in both adult and embryonic tissues in rodents (f{apoli 1996). The expression
of RAR B is more limited to the adult tissues such as the brain, liver, kidney, heart and
lungs. Its expression during the embryonic development follows spatial and temporal
pattems that coincide with ontogenesis of the nervous system. The RAR ^y receptors are
predominantly found in skin and lungs, and these receptors play an important role in the
process of morphogenesis and chondrogenesis in embryonic tissues. The RXR a
receptors are found to be abundant in the liver, lung, kidney, muscle and spleen tissue'
RXR P distribution is ubiquitous, while RXR"y receptors are found in tissues such as
muscle, heart, liver, kidney and adrenal glands (Napoli 1996). Heart tissue is found to
express all of the RAR and RXR receptors.
III.c.4. Signating Pathway: Retinoic acid signaling pathway and gene reguiation
mechanisms have bqen described (Dilworth and Chambon 2001)'
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