Takitani Kimitaka, Zhu Chang-Lin, Inoue Akiko, Tamai Hiroshi
Department of Pediatrics, Osaka Medical College, 2-7 Daigakumachi, Takatsuki, Osaka, 569-8686, Japan.
Eur J Nutr. 2006 Sep;45(6):320-6. doi: 10.1007/s00394-006-0601-3. Epub 2006 May 13.
beta-Carotene exhibits biological activity as provitamin A. Key step in vitamin A formation is the cleavage of beta-carotene to retinal by an enzyme designated as beta-carotene 15,15'-monooxygenase (BCM). Recently, it is reported that expression of BCM gene in the intestine is under feedback regulation by retinoic acid (RA). However, the regulation of BCM gene expression in various other tissues is still unknown.
In the present study, we identified the full-length cDNA encoding the rat BCM gene and investigated the regulation of its expression in several tissues by RA in the presence of vitamin A deficiency.
We cloned the full-length cDNA encoding BCM gene from a rat intestinal cDNA library by hybridization screening. The BCM gene expression was examined using Northern blotting and reverse transcription-PCR analysis. We also investigated whether BCM gene expression was regulated by retinoids in several tissues of vitamin A-deficient rats.
Sequence analysis of this clone revealed an open reading frame of 1,701 bases encoding a protein of 566 amino acids. The predicted polypeptide showed 94%, 81%, and 66% identity with mouse, human, and chicken BCM, respectively. Rat BCM mRNA was highly expressed in the intestine and liver, while there was weak expression in the testes, kidneys, and lungs. Immunoblotting revealed that rat BCM is a 64-kDa protein. BCM gene expression was increased in the small intestine by vitamin A deficiency compared with that in rats on a control diet, while this upregulation was suppressed by all-trans RA (ATRA) or 9-cis RA (9-cis RA). BCM gene expression in the lungs and testes was also suppressed by ATRA or 9-cis RA in rats with vitamin A deficiency. However, hepatic BCM gene expression was only decreased by ATRA and renal expression was not affected by either retinoid. As the small intestine is the major site of beta-carotene conversion, intestinal BCM gene expression may be more tightly regulated.
These data suggest that BCM gene expression in several tissues may be down-regulated by RA at the level of conversion of beta-carotene to retinal. To prevent an excess of retinol, homeostasis may occur at the level of conversion of beta-carotene to retinal in several tissues.
β-胡萝卜素作为维生素A原具有生物活性。维生素A形成的关键步骤是由一种名为β-胡萝卜素15,15'-单加氧酶（BCM）的酶将β-胡萝卜素裂解为视黄醛。最近，有报道称肠道中BCM基因的表达受视黄酸（RA）的反馈调节。然而，BCM基因在其他各种组织中的表达调控仍不清楚。
在本研究中，我们鉴定了编码大鼠BCM基因的全长cDNA，并在维生素A缺乏的情况下研究了RA对其在多个组织中表达的调控。
我们通过杂交筛选从大鼠肠道cDNA文库中克隆了编码BCM基因的全长cDNA。使用Northern印迹和逆转录PCR分析检测BCM基因的表达。我们还研究了维生素A缺乏大鼠的多个组织中BCM基因表达是否受类视黄醇调控。
该克隆的序列分析显示一个1701个碱基的开放阅读框，编码一个566个氨基酸的蛋白质。预测的多肽与小鼠、人类和鸡的BCM分别具有94%、81%和66%的同一性。大鼠BCM mRNA在肠道和肝脏中高表达，而在睾丸、肾脏和肺中表达较弱。免疫印迹显示大鼠BCM是一种64 kDa的蛋白质。与对照饮食的大鼠相比，维生素A缺乏使小肠中的BCM基因表达增加，而全反式视黄酸（ATRA）或9-顺式视黄酸（9-cis RA）可抑制这种上调。维生素A缺乏的大鼠中，ATRA或9-cis RA也抑制了肺和睾丸中的BCM基因表达。然而，肝BCM基因表达仅被ATRA降低，肾表达不受任何一种类视黄醇的影响。由于小肠是β-胡萝卜素转化的主要部位，肠道BCM基因表达可能受到更严格的调控。
这些数据表明，在多个组织中，BCM基因的表达可能在β-胡萝卜素转化为视黄醛的水平上受到RA的下调。为防止视黄醇过量，在多个组织中，β-胡萝卜素转化为视黄醛的水平可能会出现稳态。
