Germline mutations in the gene encoding LKB1, a serine/threonine kinase, results in Peutz-Jeghers Syndrome (PJS), characterized by intestinal hamartomas and increased incidence of epithelial cancers . Inactivating mutations in LKB1 have also been found in sporadic human cancers, for example 34% of lung adenocarcinomas and 19% of squamous cell carcinomas . Many cervical cancer cell lines harbor LKB1 deletions and expression of LKB1 in the cervical cell line HeLa-S3 (which lack LKB1) is reported to induce a G1 cell-cycle arrest, in agreement with it playing a role as a tumor suppressor .
LKB1 regulates several important biochemical pathways, including cell metabolism, cell cycle and cell polarity, but it is not clear which of these are responsible for its tumor suppressor activity. Its ability to regulate metabolic pathways, such as enhanced uptake of glucose and fatty acid oxidation in response to a decrease in cellular ATP levels, is probably the best understood pathway in mammalian cells . In lower organisms, however, its ortholog PAR-4 is best characterized as a polarity determinant. PAR-4 was first identified in C. elegans as required for establishing the anterior-posterior axis during cell division of the zygote, while in D. melanogaster it regulates polarity establishment in the embryonic epithelium [5–9].
The contribution of LKB1 to cell polarity in mammalian cells has not been extensively explored. The activation of LKB1 in an intestinal epithelial cell line, through over-expression of its adaptor protein STRAD, was reported to induce a polarized morphology in single cells, as visualized by the assembly of an actin-rich brush border on one side of the cell to form an apical-like surface . Further analysis has revealed that the Mst4 kinase and the actin filament binding protein ezrin act downstream of LKB1 in the pathway leading to brush border formation . Since the polarized assembly of actin filaments is a key feature of all epithelial cells, the mechanism by which LKB1 interacts with the actin cytoskeleton is therefore of great interest. Whether the loss of LKB1 seen in epithelial cancers contributes to the tumorigenic process through effects on the actin cytoskeleton remains an interesting possibility.
Members of the Rho GTPase family are important regulators of the actin cytoskeleton and of cell polarity and dysregulated Rho pathways have been linked to the process of tumor progression . Rho and Rac are required for the assembly of cell-cell junctions in a wide variety of epithelial cells, while Cdc42, through its interaction with the Par6/atypical PKC polarity complex, is required for the establishment of apical-basal polarity . So far there have been few reports linking LKB1 to Rho family GTPases and actin. Depletion of LKB1 in migrating non-small cell lung cancer cells, for example, affects Cdc42 activity at the leading edge, though it is not clear if this directly influences actin filament assembly pathways . Intriguingly, Tuberous Sclerosis Protein 1 (TSC1), a protein that acts downstream of LKB1 to control mTORC1 activity, interacts with ezrin and can promote Rho-dependent assembly of actin filaments when expressed in cells . To explore the connection between LKB1 and actin filament assembly, we have introduced an LKB1 expression construct into HeLa-S3 cells, a cervical cancer cell line that lacks endogenous LKB1. We have found this promotes actin fiber formation, through activation Rho via the exchange factor Dbl.