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。盐诱导激酶1（SIK1）是camp介导的基因表达反馈级联中的调节因子，其激酶结构域也具有自磷酸化活性。我们提供的证据表明，活化环中的Serl86是自磷酸化的位点，对激酶活性至关重要。Serl86位于临界的+4位置。苏氨酸残基thr82，它被上游激酶如LKB1磷酸化。COS-7细胞中Serl86和Thrl82位点磷酸化的关系表明，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Ser/Thr残基前面的第4位的Ser/Thr残基，GSK-3β的抑制剂降低了Thrl82的磷酸化。体外重组实验的结果也表明GSK-3β可能是SIK1激酶。然而，在LKB1缺陷的HeLa细胞中，GSK-3β的过表达和敲低表明，GSK-3β可能无法单独磷酸化或激活SIK1，这表明LKB1可能通过磷酸化SIK1的Thrl82位点发挥关键作用，可能作为自磷酸化级联的启动物，GSK-3β可能通过识别培养细胞中Serl86位点的启动自磷酸化来磷酸化SIK1的Thrl82位点。其他同工异构体SIK2可能也是如此，但SIK3则不然。利用SIK家族酶敲除小鼠，我们正在研究其生理作用的细节。

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.