Page 126 - 69_04

B. B. FREDHOLM ANAL. REAL ACAD. NAL. FARM.

Dumont, Vassart and Schiffmann; 1990, Libert, Schiffmann, Lefort,
Parmentier, Gerard, Dumont, Vanderhaeghen and Vassart; 1991). Soon
the same receptors were cloned from rat and humans (Mahan, McVittie,
Smyk-Randall, Nakata, Monsma Jr, Gerfen and Sibley; 1991, Furlong,
Pierce, Selbie and Shine; 1992), and a related receptor, the A2B receptor,
was cloned from rat brain (Stehle, Rivkees, Lee, Weaver, Deeds and
Reppert; 1992). These receptors had all been predicted from extensive
pharmacological studies. The fourth receptor, A3, was more unexpected.
By now these four adenosine receptors have been cloned from several
mammalian and non-mammalian species. A1, A2A and A2B receptors are
well conserved among mammals, but A3 receptors show considerable
structural variability.

For all four adenosine receptors the coding region is split up by an
intron in a region corresponding to the second intracellular loop
(Fredholm, Arslan, Halldner, Kull, Schulte and Wasserman; 2000).
Already when the structure of the A1 receptor was first reported the
presence of two major transcripts was noted. Transcripts containing three
exons, called exons 4, 5 and 6 were found in all tissues expressing the
receptor, whereas transcripts containing exons 3, 5 and 6 are in addition
found in tissues such as brain, testis and kidney, that express high levels
of the receptor (Ren and Stiles; 1994, 1995). There are two promoters, a
proximal one denoted promoter A, and a distal one denoted promoter B,
which are about 600 bp apart. The 5’-untranslated region of the adenosine
A2A receptor gene also displays two alternative promoters, although it
lacks the diversity of exons found in the A1 receptor gene (Chu, Tu, Lee,
Kuo, Lai and Chern; 1996, Lee, Chang, Su, Lin, Sun, Lai and Chern;
1999). The A2A receptor shows one hybridizing transcript in most tissues
examined (Stehle et al.; 1992, Ren and Stiles; 1994, 1995). The rat A2B
receptor shows two hybridizing transcripts of 1.8 and 2.2 kb, where the
latter is the dominant one (Stehle et al.; 1992). This could, in analogy
with the above, suggest the presence of multiple promoters. The human
A3 receptor shows two transcripts: the most abundant is approximately 2
kb in size, and the less abundant one about 5 kb (Atkinson, Townsend-
Nicholson, Nicholl, Sutherland and Schofield; 1997), perhaps indicating
similarities with the A1 receptor gene.

140