Background：

Tumor suppressor serine/threonine-protein kinase that controls the activity of AMP-activated protein kinase (AMPK) family members， thereby playing a role in various processes such as cell metabolism， cell polarity， apoptosis and DNA damage response. Acts by phosphorylating the T-loop of AMPK family proteins， thus promoting their activity: phosphorylates PRKAA1， PRKAA2， BRSK1， BRSK2， MARK1， MARK2， MARK3， MARK4， NUAK1， NUAK2， SIK1， SIK2， SIK3 and SNRK but not MELK. Also phosphorylates non-AMPK family proteins such as STRADA， PTEN and possibly p53/TP53. Acts as a key upstream regulator of AMPK by mediating phosphorylation and activation of AMPK catalytic subunits PRKAA1 and PRKAA2 and thereby regulates processes including: inhibition of signaling pathways that promote cell growth and proliferation when energy levels are low， glucose homeostasis in liver， activation of autophagy when cells undergo nutrient deprivation， and B-cell differentiation in the germinal center in re