Germline mutations of the LKB1 gene are responsible for the cancer-prone Peutz-Jeghers syndrome (PJS). LKB1 encodes a serine-threonine kinase that acts as a regulator of cell cycle, metabolism and cell polarity. The majority of PJS missense mutations abolish LKB1 enzymatic activity and thereby impair all functions assigned to LKB1. Here, we have investigated the functional consequences of recurrent missense mutations identified in PJS and in sporadic tumors which map in the LKB1 C-terminal non-catalytic region. We report that these C-terminal mutations neither disrupt LKB1 kinase activity nor interfere with LKB1-induced growth arrest. However, these naturally occuring mutations lessened LKB1-mediated activation of the AMP-activated protein kinase (AMPK) and impaired downstream signaling. Furthermore, C-terminal mutations compromise LKB1 ability to establish and maintain polarity of both intestinal epithelial cells and migrating astrocytes. Consistent with these findings, mutational analysis reveals that the LKB1 tail exerts an essential function in the control of cell polarity. Overall, our results ascribe a crucial regulatory role to the LKB1 C-terminal region. Our findings further indicate that LKB1 tumor suppressor activity is likely to depend on the regulation of AMPK signaling and cell polarization.

Peutz-Jeghers syndrome (PJS) is an autosomal dominant disorder characterized by the presence of multiple gastrointestinal polyps and an increased risk for various types of cancers. Inactivating germline mutations of the LKB1 gene, which encodes a serine/threonine kinase, are responsible for the majority of PJS cases. Here, we show that the heteromeric complex containing the molecular chaperones Hsp90 and Cdc37/p50 interacts with the kinase domain of LKB1. Treatment of cells with either geldanamycin or novobiocin, two pharmacological inhibitors of Hsp90 causes the destabilization of LKB1. Furthermore, geldanamycin treatment leads to the ubiquitination and the rapid degradation of LKB1 by the proteasome-dependent pathway. In addition, we found that a LKB1 point mutation identified in a sporadic testicular cancer, weakens the interaction of LKB1 with both Hsp90 and Cdc37/p50 and enhances its sensitivity to the destabilizing effect of geldanamycin. Collectively, our results demonstrate that the Hsp90/Cdc37 complex is a major regulator of the stability of the LKB1 tumor suppressor. Furthermore, these data draw attention to the possible adverse consequences of antitumor drugs that target Hsp90, such as antibiotics related to geldanamycin, which could disrupt LKB1 function and promote the development of polyps and carcinomatous lesions. Oncogene (2003) 22, 9165-9175.

LKB1 is a tumor suppressor that is constitutionally mutated in a cancer-prone condition, called Peutz-Jeghers syndrome, as well as somatically inactivated in a sizeable fraction of lung and cervical neoplasms. The LKB1 gene encodes a serine/threonine kinase that associates with the pseudokinase STRAD (STE-20-related pseudokinase) and the scaffolding protein MO25, the formation of this heterotrimeric complex promotes allosteric activation of LKB1. We have previously reported that the molecular chaperone heat shock protein 90 (Hsp90) binds to and stabilizes LKB1. Combining pharmacological studies and RNA interference approaches, we now provide evidence that the co-chaperone Cdc37 participates to the regulation of LKB1 stability. It is known that the Hsp90-Cdc37 complex recognizes a surface within the N-terminal catalytic lobe of client protein kinases. In agreement with this finding, we found that the chaperones Hsp90 and Cdc37 interact with an LKB1 isoform that differs in the C-terminal region, but not with a novel LKB1 variant that lacks a portion of the kinase N-terminal lobe domain. Reconstitution of the two complexes LKB1-STRAD and LKB1-Hsp90-Cdc37 with recombinant proteins revealed that the former is catalytically active whereas the latter is inactive. Furthermore, consistent with a documented repressor function of Hsp90, LKB1 kinase activity was transiently stimulated upon dissociation of Hsp90. Finally, disruption of the LKB1-Hsp90 complex favors the recruitment of both Hsp/Hsc70 and the U-box dependent E3 ubiquitin ligase CHIP (carboxyl terminus of Hsc70-interacting protein) that triggers LKB1 degradation. Taken together, our results establish that the Hsp90-Cdc37 complex controls both the stability and activity of the LKB1 kinase. This study further shows that two chaperone complexes with antagonizing activities, Hsp90-Cdc37 and Hsp/Hsc70-CHIP, finely control the cellular level of LKB1 protein.