Table IV.
Summary of the Involvement of the Various RARs and RXRs in the Transduction of the Retinoid Signal in F9 Cells, as Deduced from the Present and Previous Studies of RAR and RXR Mutant Cells and the Use of Receptor-specific Retinoids
| Retinoid-induced events and RXR/RAR pairs capable of transducing the signal | Role of RXRs | Role of RARs | ||
|---|---|---|---|---|
| Primitive endodermal differentiation | RARγ | |||
| RARγ ligand active on its own at saturating | ||||
| RXR ligand inactive on its own (Roy et al., 1995) | concentration (Taneja et al., 1996) | |||
| RARγ−/− cells differentiate very poorly (Boylan et al., 1993) | ||||
| RXRα/RARγ in all instances | RXRα−/− cells differentiate very poorly (Clifford et al., 1996) | synergizes with RXRα at suboptimal ligand concentration (Clifford et al., 1996) | ||
| weakly hindered by RARα in WT cells (Taneja et al., 1996) | ||||
| RXR ligand is required at suboptimal concentration | RARα | |||
| of RARγ ligand (Roy et al., 1995; Taneja et al., 1996) | RARα ligand inactive on its own (Taneja et al., 1996) | |||
| RARα−/− cells differentiate normally (Boylan et al., 1995) | ||||
| can poorly replace RARγ provided RXRα is activated | ||||
| RXRα can be poorly replaced by RXR(β+γ) | RARβ | |||
| provided that RARγ is present | inactive or very poorly active (Taneja et al., 1996) | |||
| Visceral endodermal differentiation | RARγ | |||
| RXR ligand inactive on its own RXR ligand is required at suboptimal concentration of a RAR ligand RXRα can be efficiently replaced by RXR(β+γ) provided that RARγ is present RXRα prevents efficient synergism between RXR(β + γ) and RARγ | indispensable in WT cells | |||
| RARγ ligand active on its own at saturating concentration | ||||
| RXRα/RARγ in WT cells | synergizes with RXRs at suboptimal ligand concentration | |||
| hindered by RARα in WT cells | ||||
| RARα | ||||
| RXR(β+γ)/RARγ in absence | RARα ligand inactive on its own or very poorly active | |||
| of RXRα (efficiently) | RARα−/− cells differentiate normally | |||
| synergizes with RXRα in the absence of RARγ | ||||
| RXRα/RARα or RARβ in | hindered by RARγ in WT cells | |||
| absence of RARγ (efficiently) | RARβ | |||
| RARβ ligand inactive on its own or very poorly active | ||||
| synergizes with RXRα in the absence of RARγ | ||||
| blocked by RARγ in WT cells | ||||
| Inhibition of proliferation | RARγ | |||
| not indispensable in WT cells | ||||
| RXRα/RARγ or RARα in WT cells | RARγ ligand active on its own at saturating concentration | |||
| RXR ligand inactive on its own | synergizes with RXRs at suboptimal ligand concentration | |||
| partially hindered by RARα in WT cells | ||||
| RXRα/RARβ in absence of RARγ | RXR ligand is required at limiting concentration of RAR ligand | RARα | ||
| RARα ligand inactive on its own | ||||
| synergizes with RXRα in WT cells | ||||
| RXR(β+γ)/RARγ in absence of RXRα | RXRα can be partially replaced by RXR(β+γ) provided that RARγ is present | partially hindered by RARγ in WT cellsRARβ | ||
| RARβ ligand inactive on its own | ||||
| synergizes with RXRα in the absence of RARγ | ||||
| blocked by RARγ in WT cells | ||||
| Induction of proliferation in | RARα | |||
| RXRα−/−/RARγ−/−cells | RARα ligand active on its own | |||
| RXR ligand inactive on its own | RARβ | |||
| RXR(β+γ)/RARα | RARβ ligand active on its own | |||
| RXR(β+γ)/RARβ | synergizes with RXR(β+γ) | |||
| Apoptosis | RARγ | |||
| indispensable in WT cells | ||||
| RXRα/RARγ in WT cells | RXR ligand inactive on its own | RARα ligand active on its own at saturating ligand concentration | ||
| RXR(β+γ)/RARγ in absence of RXRα, but very inefficiently | RXRα ligand is required at suboptimal ligand concentration of RAR ligand | synergizes with RXRα at suboptimal ligand concentration | ||
| hindered by RARα in WT cells | ||||
| RARα | ||||
| RXRα/RARα or RARβ in absence of RARγ, but inefficiently | RXRα can be poorly replaced by RXR(β+γ) provided that RARγ is present | RARα ligand inactive on its own weakly synergizes with RXRα in the absence of RARγ blocked by RARγ in WT cells | ||
| RARβ | ||||
| RARβ ligand inactive on its own | ||||
| weakly synergizes with RXRα in the absence of RARγ | ||||
| blocked by RARγ in WT cells |