Evidence for A1 and A2B adenosine receptors in baby hamster kidney (BHK) cells

Adenosine, type 1 receptors: role in proximal tubule Na+ reabsorption

Adenosine type 1 receptor (A1 -AR) antagonists induce diuresis and natriuresis in experimental animals and humans. Much of this effect is due to inhibition of A1 -ARs in the proximal tubule, which is responsible for 60-70% of the reabsorption of filtered Na(+) and fluid. Intratubular application of receptor antagonists indicates that A1 -AR mediates a portion of Na(+) uptake in PT and PT cells, via multiple transport systems, including Na(+) /H(+) exchanger-3 (NHE3), Na(+) /PO4(-) co-transporter and Na(+) -dependent glucose transporter, SGLT. Renal microperfusion and recollection studies have shown that fluid reabsorption is reduced by A1 -AR antagonists and is lower in A1 -AR KO mice, compared to WT mice. Absolute proximal reabsorption (APR) measured by free-flow micropuncture is equivocal, with studies that show either lower APR or similar APR in A1 -AR KO mice, compared to WT mice. Inhibition of A1 -ARs lowers elevated blood pressure in models of salt-sensitive hypertension, partially due to their effects in the proximal tubule.

Adenosine A₁ receptors do not play a major role in the regulation of lipogenic gene expression in hepatocytes

Activation of adenosine A₁ receptors was reported to promote fatty acid synthesis in AML-12 cells, by increasing the expression of SREBP-(1c) (sterol regulatory binding protein 1c) and FAS (fatty acid synthase). Since these findings have important therapeutic implications for the discovery of adenosine A₁ receptor agonists, further studies were undertaken to determine the expression and functional relevance of adenosine A₁ receptor in the liver. To that end, we used two classes of distinct adenosine A₁ receptor agonists: CPA (N⁶-cyclopentyl-adenosine), a full agonist and GS-9667 (2-{6-[((1R,2R)-2-hydroxycyclopentyl)-amino]purin-9-yl}(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]-oxolane-3,4-diol), a partial agonist. Treatment of AML-12 cells, HepG2 cells and primary human hepatocytes with either CPA or GS-9667 did not increase the gene expression of SREBP-(1c) or FAS. Furthermore, in AML-12 and HepG2 cells, CPA did not antagonize forskolin-stimulated cAMP production, a characteristic of adenosine A₁ receptor activation, indicating that these cells lack adenosine A₁ receptor function. Consistent with this finding, adenosine A₁ receptor gene expression was found to be very low and adenosine A₁ receptor protein levels were hardly detectable by radioligand binding assays in hepatic cell lines such as AML-12 and HepG2 as well as in both mouse and human liver tissues. Finally, acute treatment with adenosine A₁ receptor agonist GS-9667 had no significant effect on gene expression of both SREBP-(1c) and FAS in livers of Sprague Dawley rats. Taken together, our data suggest that the expression of adenosine A₁ receptor is too low to play a major role in the regulation of lipogenic gene expression in hepatocytes.

Adenosine and Kidney Function

In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.

Evidence against adenosine analogues being agonists at the growth hormone secretagogue receptor

Adenosine and adenosine analogues have been reported to act as agonists or partial agonists at the growth hormone secretagogue receptor 1a (GHSR1a). We have re-examined this question. A concentration-dependent increase in intracellular calcium concentration ([Ca(2+)](i)) was observed in GHSR1a transfected HEK 293-EBNA cells stimulated with adenosine (EC50: 0.2 microM) or 2-chloroadenosine (EC50: 1.1 microM) but also in untransfected HEK 293-EBNA cells stimulated with 2-chloroadenosine (EC50: 0.67 microM) or 5'-N-ethylcarboxamidoadenosine (NECA) (EC50: 0.045 microM). These findings support endogenous expression of adenosine receptors, presumably A(2B) receptors in HEK 293-EBNA cells. In GHSR1a transfected CHO cells, lacking adenosine receptors, the GHSR1a agonist hGhrelin (EC50: 2.4 nM) increased [Ca(2+)](i), but no effects of adenosine, 2-chloroadenosine or NECA were detected. An inverse agonist of GHSR1a, [d-Arg-1, d-Phe-5, d-Trp-7, 9, Leu-11] substance P, reduced hGhrelin effects but adenosine, 2-chloroadenosine or 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) did not. NECA increased the [Ca(2+)](i) in co-transfected (GHSR1a and A(2B) receptor) CHO cells (EC50: 0.053 microM), but no additive or synergistic effects on [Ca(2+)](i) or cAMP formation were observed after stimulation with NECA in the absence or in the presence of hGhrelin. In binding studies on GHSR1a transfected CHO cell membranes, [(125)I]-hGhrelin binding could be displaced by the GHSR1a agonist MK-0677 (IC50: 0.34 nM), hGhrelin (IC50: 1.5 nM), and the substance P analogue (IC50: 0.64 microM) but not by adenosine or 2-chloroadenosine. We conclude that adenosine and analogues do not act as agonists or partial agonists at the GHSR1a and that cross-talk between the GHSR1a and A(2B) receptors is limited.

Adenosine type 1 receptor antagonists in fluid retaining disorders

Adenosine is a vasoactive hormone whose action is mediated through at least four receptors. The most prevalent receptors are type 1, which promote vasoconstriction, and type 2, comprised of 2 subtypes (a,b) that promote vasodilation. In the kidney, type 1 receptors located on preglomerular vessels and in the tubule are involved in the regulation of glomerular filtration. Whole body fluid balance is strongly dependent on the ability of the kidney to maintain stable glomerular filtration. Several antagonists to adenosine type 1 receptors have been developed. These agents generate excess fluid (diuresis) and sodium (natriuresis) excretion in control animals and animal models of fluid retention, as well as in normal and oedematous humans. In both animals and humans, these effects are generally achieved without major changes in glomerular filtration. Animal studies have confirmed the location of adenosine type 1 receptors in relevant tissue sites in the kidney. More highly selective antagonists for adenosine type 1 receptors are regularly developed, improving their use in fluid retaining disorders. Clinical trials with these agents have commenced for the treatment of hypertension, renal failure and congestive heart failure, all disorders that include varying levels of fluid retention. The clinical trial results have been mixed. The early results with congestive heart failure suggest great promise for these agents, whereas trials in hypertension and renal failure have been equivocal.

Adenosine modulates cell growth in baby hamster kidney (BHK) cells

Adenosine is known to modulate cell growth in a variety of mammalian cells either via the activation of receptors or through metabolism. We investigated the effect of adenosine on Baby Hamster Kidney (BHK) cell growth and attempted to determine its mechanism of modulation. In wild-type BHK cells, adenosine evoked a biphasic response in which a low concentration of adenosine (1-5 microM) produced an inhibition of colony formation but at higher concentrations (up to 50 microM) this inhibition was progressively reversed. However, no biphasic response was observed in an "adenosine kinase" deficient BHK mutant, "5a", which suggests that adenosine kinase plays an important role in the modulation of growth response to adenosine. Adenosine receptors did not appear to have a role in regulating cell growth of BHK cells. Specific A1 and A2 receptor antagonists were unable to reverse the effect of adenosine on cell growth. Even though a specific A3 adenosine receptor antagonist MRS-1220 partly reversed the inhibition in colony formation at 1 microM adenosine, it also affected the transport of adenosine. Thus adenosine transport and metabolism appears to play the major role in this modulation of cell growth as 5'-amino-5'-deoxyadenosine, an adenosine kinase inhibitor, reversed the inhibition of cell growth observed at 1 microM adenosine. These results, taken together, would suggest that adenosine modulates cell growth in BHK mainly through its transport and metabolism to adenine nucleotides.