Epithelial cells possess apical-basolateral polarity and form tight junctions (TJs) at the apical-lateral border, separating apical and basolateral membrane domains. The PAR3-aPKC-PAR6 complex plays a central role in TJ formation and apical domain development during tissue morphogenesis. Inactivation and overactivation of aPKC kinase activity disrupts membrane polarity. The mechanism that suppresses active aPKC is unknown. KIBRA, an upstream regulator of the Hippo pathway, regulates tissue size in Drosophila and can bind to aPKC. However, the relationship between KIBRA and the PAR3-aPKC-PAR6 complex remains unknown. We report that KIBRA binds to the PAR3-aPKC-PAR6 complex and localizes at TJs and apical domains in epithelial tissues and cells. The knockdown of KIBRA causes expansion of the apical domain in MDCK three-dimensional cysts and suppresses the formation of apical-containing vacuoles through enhanced de novo apical exocytosis. These phenotypes are restored by inhibition of aPKC. In addition, KIBRA directly inhibits the kinase activity of aPKC in vitro. These results strongly support the notion that KIBRA regulates epithelial cell polarity by suppressing apical exocytosis through direct inhibition of aPKC kinase activity in the PAR3-aPKC-PAR6 complex.

KIBRA is a WW domain-containing protein that can bind to protein kinase C (PKC ). The SNP of the ninth intron of the KIBRA gene is associated with human episodic memory performance. Protein kinase M (PKM ), a brain-specific variant of PKC , plays important roles in memory formation. Here we examined the interaction of KIBRA and PKM in the adult mouse brain. Immunoprecipitation using newly-raised anti-KIBRA antibody revealed the interaction between KIBRA and PKM in the brain. KIBRA was colocalized with PKM in a single cultured neuron. Distribution analysis by immunohistochemistry and in situ hybridization indicated that KIBRA was highly localized with PKM in the hippocampal CA1, CA3, and dentate gyrus. These results suggest that KIBRA functions in memory performance via interaction with PKM .

The kidney and brain protein (KIBRA) is a scaffold or an adaptor-like protein with WW, C2-like and atypical protein kinase C (aPKC)-binding domains. Genetic studies in Drosophila revealed that KIBRA is an upstream regulator of the conserved Hippo pathway, which is implicated in organ size determination. In addition, genome-wide studies revealed an association between the single nucleotide polymorphism in the KIBRA gene locus and human episodic memory performance. However, the mechanism of action through which KIBRA is linked to these functions remains poorly understood. Recent studies on the biochemical and cellular properties of KIBRA reveal the role of KIBRA as a regulator of membrane trafficking. Further, KIBRA directly inhibits the activity of the cell polarity regulator, aPKC, which is required for apical protein exocytosis. Here, we discuss how this KIBRA-aPKC connection, a potential regulator of membrane trafficking and cell polarity, can contribute to the recently discovered functions of KIBRA.

Overexpression of atypical protein kinase Ck ⁄ ι (aPKCk ⁄ ι), a regulator of cell polarity, is frequently associated with the poor prognoses of several cancers, including gastric cancer. Recent studies revealed a molecular link between aPKC and KIBRA, an upstream regulator of tumor suppressor Hippo pathway that regulates cell proliferation and apoptosis. Further, KIBRA directly inhibits the kinase activity of aPKC to regulate epithelial cell polarity. These observations suggest that the KIBRAaPKC connection plays a role in cancer progression; however, clinical significance of the correlation between these factors remains unclear. Here we examined the correlation between KIBRA ⁄ aPKCk ⁄ ι expression, as detected by immunohistochemistry, and clinicopathological outcomes in 164 gastric cancer patients using Fisher's exact test and Kaplan-Meier log-rank test. We found an intimate correlation between the expression level of KIBRA and aPKCk ⁄ ι (P = 0.012). Furthermore, high expression of KIBRA is correlated with lymphatic (P = 0.046) and venous invasion (P = 0.039). The expression level of KIBRA by itself did not correlate with the prognosis; however, high expression of KIBRA in low aPKCk ⁄ ι-expressing gastric cancer correlated with disease-specific (P = 0.037) and relapse-free survival (P = 0.041) by Kaplan-Meier with log-rank test and higher lymphatic invasion cases by Fisher's exact test (P = 0.042). Furthermore, overexpression of the aPKC-binding region of KIBRA disrupted tight junctions in epithelial cells. These results suggest that high expression of KIBRA in low aPKC-expressing cells causes massive loss of aPKC activity, leading to loss of polarity and invasiveness of gastric cancer cells. (Cancer Sci 2013; 104: 259-265) G astric cancer is the fourth most common cancer and the second highest cause of cancer-related death in the world.(1-4) Five-year survival rates for gastric cancer are approximately 20% in most areas of the world, and only 60% in Japan, where an extensive mass screening program has been implemented.(5) Although several combined chemotherapeutic regimens have been used for advanced and recurrent cases, their clinical outcomes are still unsatisfactory.(6,7) Therefore, new therapeutic targets, clinical markers and the establishment of more effective therapeutic strategies for gastric cancer are still urgently required.Loss of epithelial cell polarity is one of the hallmarks of cancer, and recent studies have shown loss of polarity itself can cause malignancy. (8)(9)(10)(11)(12) Among the regulators of epithelial cell polarity, a serine ⁄ threonine kinase known as atypical protein kinase C (aPKC) plays pivotal roles in the establishment of epithelial cell polarity binding with several partner proteins such as partitioning defective proteins (PARs). (13,14) Overexpression of aPKCk ⁄ ι is observed in several malignancies, including non-small cell lung, ovarian, colon, breast and prostate cancers and glioma. (15)(16)(17)(18)(19)(20) We previously reported that high expression of aPKCk ⁄ ι has b...