Regulation of glucose metabolism by LKB1-SIK signaling cascade

自磷酸化是蛋白激酶调节自身生物活性的重要机制。盐诱导蛋白1(SIK1)是环磷酸腺苷(CAMP)介导的基因表达反馈级联反应的调节子，其蛋白结构域也具有自身磷酸化活性。我们提供的证据表明，激活环中的Serl86是自动磷酸化的部位，对激酶活性至关重要。Serl86位于临界点的+4位置。Thr残基ThrI82，被上游激酶如LKB1磷酸化。COS-7细胞中Ser186位和Thr182位的磷酸化关系表明，前者是后者的先决条件。糖原合成酶激酶-3β(GSK-3β)磷酸化位于预磷酸化的丝氨酸/苏氨酸残基前面的第四位的丝氨酸/苏氨酸残基，而β的抑制剂降低了Thr182位的磷酸化。体外重组实验结果也表明，GSK-3β可能是SIK1激酶。然而，在缺失LKB1的HeLa细胞中，GSK-3β的过表达和敲除表明，单独的GSK-3β可能不能磷酸化或激活SIK1，这表明LKB1可能通过在第82位磷酸化SIK1而发挥关键作用，可能是作为自动磷酸化级联反应的启动子，而GSK-3β可能通过识别培养细胞中SIK1在第86位的启动-自磷酸化而在第82位磷酸化SIK1。另一种异构体SIK2也可能是这种情况，但SIK3不是。利用SIK家族酶基因敲除的小鼠，我们正在研究它们的生理作用的细节。

Autophosphorylation is an important mechanism by which protein kinases regulate their own biological activities. Salt inducible kinase 1(SIK1) is a regulator in the feedback cascades of cAMP-mediated gene expression, while its kinase domain also features autophosphorylation activity. We provide evidence that Serl86 in the activation loop is the site of autophosphorylation and essential for the kinase activity. Serl86 is located at the +4 position of the critical. Thr residue ThrI82, which is phosphorylated by upstream kinases such as LKB1. The relationship between phosphorylation at Serl86 and at Thrl82 in COS-7 cells indicates that the former is a prerequisite for the latter. Glycogen synthase kinase-3β(GSK-3β) phosphorylates Ser/Thr residues located at the fourth position ahead of the pre-phosphorylated Ser/Thr residues, and inhibitors of GSK-3β reduce the phosphorylation at Thrl82. The results of an in vitro reconstitution assay also indicate that GSK-3β could be the SIK1 kinase. However, overexpression and knockdown of GSK-3β in LKB1-defective HeLa cells suggests that GSK-3β alone may not be able to phosphorylate or activate SIK1, indicating that LKB1 may play a crucial role by phosphorylating SIK1 at Thrl82, possibly as an initiator of the autophosphorylation cascade, and GSK-3β may phosphorylate SIK1 at Thrl82 by recognizing the priming-autophosphorylation at Serl86 in cultured cells. This may also be the case for the other isoform SIK2, but not for SIK3. Using knockout mice of SIK family enzyme, we are investing the details of their physiological roles.