Connecting apical endocytosis to the intracellular traffic infrastructure in polarized hepatocytes

Oncostatin M-stimulated apical plasma membrane biogenesis requires p27(Kip1)-regulated cell cycle dynamics

Oncostatin M regulates membrane traffic and stimulates apicalization of the cell surface in hepatoma cells in a protein kinase A-dependent manner. Here, we show that oncostatin M enhances the expression of the cyclin-dependent kinase (cdk)2 inhibitor p27(Kip1), which inhibits G(1)-S phase progression. Forced G(1)-S-phase transition effectively renders presynchronized cells insensitive to the apicalization-stimulating effect of oncostatin M. G(1)-S-phase transition prevents oncostatin M-mediated recruitment of protein kinase A to the centrosomal region and precludes the oncostatin M-mediated activation of a protein kinase A-dependent transport route to the apical surface, which exits the subapical compartment (SAC). This transport route has previously been shown to be crucial for apical plasma membrane biogenesis. Together, our data indicate that oncostatin M-stimulated apicalization of the cell surface is critically dependent on the ability of oncostatin M to control p27(Kip1)/cdk2-mediated G(1)-S-phase progression and suggest that the regulation of apical plasma membrane-directed traffic from SAC is coupled to centrosome-associated signaling pathways.

Shared apical sorting of anion exchanger isoforms AE2a, AE2b1, and AE2b2 in primary hepatocytes

AE2 (SLC4A2) is the member of the Na(+)-independent anion exchanger (AE) family putatively involved in the secretion of bicarbonate to bile. In humans, three variants of AE2 mRNA have been described: the full-length transcript AE2a (expressed from the upstream promoter in most tissues), and alternative transcripts AE2b(1) and AE2b(2) (driven from alternate promoter sequences in a tissue-restricted manner, mainly in liver and kidney). These transcripts would result in AE protein isoforms with short N-terminal differences. To ascertain their translation, functionality, and membrane sorting, we constructed expression vectors encoding each AE2 isoform fused to GFP at the C-terminus. Transfected HEK293 cells showed expression of functional GFP-tagged AE2 proteins, all three isoforms displaying comparable AE activities. Primary rat hepatocytes transfected with expression vectors and repolarized in a collagen-sandwich configuration showed a microtubule-dependent apical sorting of each AE2 isoform. This shared apical sorting is liver-cell specific, as sorting of AE2 isoforms was basolateral in control experiments on polarized kidney MDCK cells. Hepatocytic apical targeting of AE2 isoforms suggests that they all may participate in the canalicular secretion of bicarbonate to bile.

Role of Rab Proteins in Epithelial Membrane Traffic

Small GTPase rab proteins play an important role in various aspects of membrane traffic, including cargo selection, vesicle budding, vesicle motility, tethering, docking, and fusion. Recent data suggest also that rabs, and their divalent effector proteins, organize organelle subdomains and as such may define functional organelle identity. Most rabs are ubiquitously expressed. However, some rabs are preferentially expressed in epithelial cells where they appear intimately associated with the epithelial-specific transcytotic pathway and/or tight junctions. This review discusses the role of rabs in epithelial membrane transport.