Regulatory Mechanisms Of LKB1-AMPK Axis In Chromosomal Stability

LKB1 is widely considered as a tumor suppressor, inactivated germline mutation of which can lead to the Peutz-Jeghers syndrome (PJS) featured by hamartomatous polyps and elevated risks for cancers, and the somatic mutations of LKB1 are also frequently found in many kinds of malignancies. However, the tumor suppression mechanisms of LKB1 remain elusive currently. The key tasks of this study are exploring the functional mechanisms of LKB1 in regulating chromosomal stability, aiming to clarify the mechanisms of LKB1 in preventing tumorigenesis. The tumor suppressor gene LKB1 is an evolutionarily conserved kinase that can regulate a variety of cellular activities, such as cell growth, cell polarity and energy metabolism. The combination of STRADα with LKB1 is responsible for the intracellular localization and activation of LKB1, thereafter phosphorylating and activating its substrates, such as AMPK, a core kinase regulating the energy metabolism. It has been reported that inhibition of LKB1 led to multi-spindle and chromosome misalignment in mitosis, but the molecular mechanisms remain elusive. Moreover, previous studies revealed that AMPK could localize to mitotic apparatus such as centrosome, central spindle and midbody independent of LKB1. However, the functional mechanisms of AMPK in mitosis are also unclear.Here, we found that STRADα-LKB1 tumor suppressor complex could localize to kinetochores, and STRADα possessed microtubule binding capacity and can recruit LKB1 to centrosomes and kinetochores. Suppression of STRADα-LKB1 led to multi-spindle and chromosome misalignment in mitosis, moreover, persistent expression of LKB1 on kinetochores caused abnormal connection between microtubules and kinetochores associating lagging chromosomes. Then biochemical analysis revealed that STRADα-LKB1 interacted with MCAK directly, and LKB1 could phosphorylate MCAK at Ser115 site, thereby inhibiting its microtubule depolymerization activity. Our experiments thereafter unraveled that the roles of STRADα-LKB1 in regulation of mitotic spindle dynamics were mediated by MCAK. In addition, our studies clarified that LKB1 and PLK1 could coordinately regulate the microtubule depolymerization activity of MCAK to guarantee the dedicated functions of spindle microtubules in mitosis. So our work unraveled a new mechanism that STRADα-LKB1-MCAK axis governs chromosome stability in mitosis.Moreover, we also discovered that AMPKα-pT172 localized to kinetochores, which was directed by PLK1. Interestingly, AMPKα-pT172 may be not a direct substrate of PLK1 as shown in our in vitro biochemical experiments. Our preliminary results revealed that the kinetochore localization of AMPKα-pT172 may be coordinately modulated by PLK1 together with LKB1 and CAMKKβ.In conclusion, our studies unraveled relative functional mechanisms of the tumor suppressor complex LKB1-AMPK in mitosis, possibly providing experimental and theoretical basis for tumorigenesis caused by LKB1-AMPK dysfunction.