Functional interaction between SIK2 and PP2A
SIK2是屬於AMPK家族，SIK次家族的一員。AMPK家族的成員主要功能在於偵測細胞能量狀態，對於細胞凋亡以及自噬作用的逆境反應。在AMPK家族中，SIK2是唯一可以與PP2A及VCP/P97有交互作用的成員。然而，對於SIK2-PP2A複合體的功能仍然不清楚。在癌細胞中，SIK2的降解在泛素蛋白質體降解系統及自噬作用中扮演重要角色。SIK2 knockdown會造成PP2Ac量的下降，並且會增加Bcl-2/S70的磷酸化。我發現PME-1/S15的磷酸化在SIK2 knockdown時會上升。在之前的研究中，調節PP2A次單元B的招募對於 PP2A受質特異性及活性是相當重要的步驟。LCMT-1對於PP2Ac次單元的羧基末端白胺酸的羧基甲基化和PP2A之活化是必需的酵素。PME-1可和PP2A結和並抑制其活性，是移除PP2Ac/L309甲基的羧基甲基化酯酶。我發現PME-1會被排除於SIK2-PP2A複合體之外，進而能保存PP2A的活性。同時在SIK2 knockdown下CaMK1會被活化並且直接磷酸化PME-1/S15。正常狀態下，SIK2會藉由PP2A負調控 CaMK1的活性。意外的是，SIK2-KD同樣可以形成SIK2-PP2A複合體，因此可以保存PP2A的活性。綜合以上結果，SIK2可能扮演adaptor的角色，藉由排除PME-1，因而保存PP2A的活性。在泛素蛋白質體降解系統及自噬作用進行的過程中，鈣離子依賴活化的CaMK1以及磷酸化PME-1/S15會受到PP2A的回饋抑制。以上結果顯示，SIK2透過維持PP2A活性及Bcl-2的量在ER恆定的過程中扮演重要的角色。
Salt-inducible kinase 2 (SIK2) is a member of AMPK family, SIK subfamily. Members of AMPK family play wide array of functions ranging from sensing energy status, stress response to apoptosis and autophagy. SIK2 is the only member of AMPK family that can interact with PP2A and VCP/P97. However, the function of this SIK2-PP2A complex is largely unknown. It was demonstrated in cancer cells that SIK2 is important protein degradation by both UPS and autophagy. SIK2 knockdown resulted in decreasing both levels of PP2Ac and carboxymethylated PP2Ac (PP2Ac/L309Me). SIK2 knockdown also resulted in increasing the phosphorylation of Bcl-2/S70. I identified the phosphorylation of PME-1/S15 was positively regulated by knockdown of SIK2. The enzymatic activity of PP2A may be regulated by heterotrimer formation in which recruiting the substrate-specific regulatory B subunit is believed to be the crucial step. A novel PP2Ac subunit C-terminal Leucine modification by LCMT1-mediated carboxymethylation is also essential for the catalytic activity of PP2A. PME-1, a carboxymethylesterase specific for the removal of PP2Ac/L309Me could complex with PP2Ac and inhibits its activity. I have also uncovered that SIK2-PP2A complex preserves the phosphatase activity. PME-1 is excluded from that complex. I also showed that SIK2 knockdown resulted in the activation of CaMK1 and phosphorylation of PME-1/S15. It turned out that the activity of CaMK1 is negatively regulated by PP2A in a SIK2-dependent manner. I have demonstrated that PME-1/S15 is the target of CaMK1. Unexpectedly, kinase-dead mutant of SIK2 (i.e., SIK2/K49M) could also form SIK2/K49M-PP2A complex and preserve the PP2A activity. Together, these results suggest that one of the functions of SIK2 may be an adaptor. PP2A activity is protected in the SIK2-PP2A complex from the inhibition of PME-1. The Ca2+-dependent activation of CaMK1 during UPS- or autophagosome-induction and phosphorylation of PME-1/S15 may be regulated by PP2A-dependent feedback inhibition. Taken together, these findings suggest that SIK2 is an important regulator for ER homeostasis through maintaining PP2A activity and the level of antiapoptotic protein Bcl-2 in the ER membrane.
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