TI - VRK2 isoforms are also downregulated by p53 and the effect is prevented by p300 or chloroquine . AB - The human VRK2 gene generates by alternative splicing two isoforms of VRK2 of 508 ( VRK2A ) and 397 ( VRK2B ) aminoacids respectively [31] . The two isoforms are identical in their first 396 aminoacids , thus VK2B is a variant that lacks the C-terminal domain , which contains the membrane anchor of VRK2A [31] , [32] . Both VRK2 isoforms have the conserved endosomal-lysosomal target sequence , therefore it is highly likely that they should also be downregulated by the same mechanism as VRK1 [12] . Furthermore VRK2 also PHOSphorylates p53 in the same residue as VRK1 [31] . Therefore it was tested if the two VRK2 isoforms , A ( bound to the endoplasmic reticulum ) and B ( mostly nuclear ) , were also downregulated by high levels of p53 . H1299 cells were transfected with each of the VRK2 isoforms and high levels of p53 . The amount of p53 that induced downregulation of VRK1 was also able to downregulate both isoforms of VRK2 ( first two lanes in Fig 8A ) . In order to determine if the underlying mechanism was the same , two experiments were performed . First it was established if the level of p300 was also able to protect VRK2 isoforms from downregulation . Cells were transfected with increasing amounts of p300 in the presence of p53 at a level that downregulated VRK2 isoforms . P300 was able to protect both VRK2 isoforms from p53 -induced downregulation in a p300 dose dependent manner (Fig 8A) . Next it was determined if VRK2 downregulation was also sensitive to inhibitors of endosome-lysosome vesicular transport , such as chloroquine . For this experiment cells were transfected with a fixed amount of p53 that induces an almost complete downregulation of both VRK2 isoforms , and the effect of an increasing concentration of chloroquine was determined . Chloroquine inhibited in a dose -dependent manner the downregulation of VRK2A and VRK2B induced by p53 (Fig 8B) . Therefore it was concluded that VRK2A and B , like VRK1 , protein levels were down regulated by a similar mechanism that requires p53 and that is sensitive to the level of p300 , and to inhibitors of the endosome-lysosome pathway .