Diverse Activities of Carotenoid Cleavage Oxygenases - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 407

(Maier et al. 1995, Fester et al. 1999, Vierheilig et al. 2000). Isotope trace experiments determined
that the cyclohexenones are derived from the 2-C-methylerythritol phosphate or 1-deoxy-d-xylulose
5-phosphate (DXP) isoprenoid precursor pathway, which supports the observation that colonized
roots exhibit elevated transcript levels of two rate-limiting DXP biosynthetic genes ( dxs and dxr )
(Maier et al. 1998, Walter et al. 2000, Strack et al. 2003). The CCO enzyme responsible for the
formation of mycorradicin and blumenin has not been identii ed, but a CCO paralog in Medicago
trunculata was found to be upregulated in mycorrhizal roots (Lohse et al. 2005). The details of
apocarotenoid formation and function in AM symbiosis are still being worked out.
The signaling aspects of apocarotenoids are the least well understood and potentially the most
promising areas of new research. In plants, ABA has an immensely important role in drought tol-
erance and seed development. The discovery of the indirect route to ABA formation was a major
milestone in plant research. Discoveries such as the role CCD7 and CCD8 play in regulating lateral
branching are at the forefront of current CCO research (see above). The uncharacterized phytohor-
mone generated by CCD7 and CCD8 holds potential for new roles of apocarotenoids in signaling.
The biological activities of apocarotenoids in microorganisms are currently largely unknown, but
may be involved in previously undetected signaling pathways.
In animals, retinal's involvement with the visual cycle is well established, but the signaling func-
tions of other retinoids are not as well known. Retinal (15-apo-b-carotenal; C 20 ) is the chromophore
of rhodopsin in the vertebrate visual cycle (Spudich et al. 2000). Retinal can also be converted to
other retinoids with potent biological activities in metazoans through oxidation to RA and then
isomerization to 9- cis -RA. 9- cis -RA has been identii ed as an important signaling molecule in the
immune system (Szondy et al. 1998, Wang et al. 2007), in development (Lampert et al. 2003), and
in cancer prevention (Altucci et al. 2007). The RA derivatives signal by binding to nuclear retinoic
acid receptor (RAR) and retinoid X receptor (RXR) (Altucci et al. 2007). RAs have also been exam-
ined as potential cancer therapies (Patel et al. 2007) and vitamin A as well as 9- cis -RA are thought
to be involved in transcriptional regulation (Bachmann et al. 2002).
The recent construction of a knockout BCO1 mouse should provide more insight into the role
of retinoids in metabolism and the specii c role that carotenoid cleavage enzymes play in signaling
in animals (Hessel et al. 2007). BCO1 dei cient mice showed serious impairments in b,b-carotene
metabolism suggesting that BCO1 is the key enzyme involved in vitamin A production (Hessel et al.
2007). BCO1 knockout mice accumulated b-carotene in adipose tissues and exhibited increased
lipid accumulation in the liver suggesting that RA inl uences liver fatty acid metabolism (Hessel et
al. 2007). Involvement of BCO1 in lipid metabolism is also supported by its transcriptional regula-
tion by the peroxisome proliferator-activated receptor g (PPAR-g) which dimerizes with RXR to
control BCO1 gene expression (Boulanger et al. 2003).
Signii cantly less is known about the function of the second CCO (BCO2) present in animals.
Studies in rat found that BCO1 and BCO2 were differentially expressed and that lycopene, the sub-
strate of BCO2, may modulate b-carotene or lipid metabolism (Zaripheh et al. 2006). BCO1 knock-
out mice showed increased hepatic BCO2 mRNA levels (fourfold) and lowered (i vefold) lycopene
levels probably as the result of increased lycopene cleavage by BCO2 (Lindshield et al. 2007).
Animals that have been fed lycopene accumulate lycopene cleavage products (referred to as lyco-
penoids) such as apo-8 ′ -lycopenal (found at levels of ∼ 600 pmol g −1 in rat liver), apo-10 ′ -lycopenal

(found at levels of -lycopenal, and other polar products (Gajic

et al. 2006, Hu et al. 2006). Lycopenoid levels in tissues are equivalent or greater than RA levels
in similar tissues, and it is hypothesized that they may be agonists or antagonists for nuclear recep-
tors such as RARs/RXRs/PPARs that are known to interact directly or indirectly with retinoids
(Lindshield et al. 2007). It is known that lycopene metabolites transactivate antioxidant response
element genes (Ben-Dor et al. 2005) and enhance gap junction communication in rat liver cells
(Aust et al. 2003). However, further studies are needed to understand the effects of both retinoids
and lycopenoids in animals and to understand their bioactivity and unravel their complex signaling
activities.

∼ 8 pmol g −1 in ferret lung), apo-12 ′