Involvement of caveolin-2 in caveolar biogenesis in MDCK cells (original) (raw)
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Caveolins, caveolae, and lipid rafts in cellular transport, signaling, and disease
Biochemistry and Cell Biology-biochimie Et Biologie Cellulaire, 2004
Caveolae were initially described some 50 years ago. For many decades, they remained predominantly of interest to structural biologists. The identification of a molecular marker for these domains, caveolin, combined with the possibility to isolate such cholesterol-and sphingolipid-rich regions as detergent-insoluble membrane complexes paved the way to more rigorous characterization of composition, regulation, and function. Experiments with knock-out mice for the caveolin genes clearly demonstrate the importance of caveolin-1 and -3 in formation of caveolae. Nonetheless, detergent-insoluble domains are also found in cells lacking caveolin expression and are referred to here as lipid rafts. Caveolae and lipid rafts were shown to represent membrane compartments enriched in a large number of signaling molecules whose structural integrity is essential for many signaling processes. Caveolin-1 is an essential structural component of cell surface caveolae, important for regulating trafficking and mobility of these vesicles. In addition, caveolin-1 is found at many other intracellular locations. Variations in subcellular localization are paralleled by a plethora of ascribed functions for this protein. Here, more recent data addressing the role of caveolin-1 in cellular signaling and the development of diseases like cancer will be preferentially discussed.
Expression of Caveolin-1 Is Required for the Transport of Caveolin-2 to the Plasma Membrane
Journal of Biological Chemistry, 1999
Caveolins-1 and-2 are normally co-expressed, and they form a hetero-oligomeric complex in many cell types. These caveolin hetero-oligomers are thought to represent the assembly units that drive caveolae formation in vivo. However, the functional significance of the interaction between caveolins-1 and-2 remains unknown. Here, we show that caveolin-1 co-expression is required for the transport of caveolin-2 from the Golgi complex to the plasma membrane. We identified a human erythroleukemic cell line, K562, that expresses caveolin-2 but fails to express detectable levels of caveolin-1. This allowed us to stringently assess the effects of recombinant caveolin-1 expression on the behavior of endogenous caveolin-2. We show that expression of caveolin-1 in K562 cells is sufficient to reconstitute the de novo formation of caveolae in these cells. In addition, recombinant expression of caveolin-1 allows caveolin-2 to form high molecular mass oligomers that are targeted to caveolae-enriched membrane fractions. In striking contrast, in the absence of caveolin-1 expression, caveolin-2 forms low molecular mass oligomers that are retained at the level of the Golgi complex. Interestingly, we also show that expression of caveolin-1 in K562 cells dramatically up-regulates the expression of endogenous caveolin-2. Northern blot analysis reveals that caveolin-2 mRNA levels remain constant under these conditions, suggesting that the expression of caveolin-1 stabilizes the caveolin-2 protein. Conversely, transient expression
Caveolins and Cellular Cholesterol Balance
Traffic, 2000
Caveolins are major integral membrane components of caveolae. Over the last few years, evidence has accumulated for a close link between caveolin, caveolae, and the regulation of cellular cholesterol levels. However, the exact role of caveolin in this process, the intracellular trafficking routes followed by caveolin/cholesterol complexes, and the relationship of caveolin-cholesterol to other caveolin-mediated processes such as signal transduction have remained unclear. Recent findings from a number of systems suggest that specific signaling pathways require precise regulation of cellular cholesterol. Here we review evidence for caveolin regulation of cholesterol transport and consider how this may relate to signal transduction.
Journal of Biological Chemistry, 1999
We have characterized comparatively the subcellular distributions of caveolins-1 and-2, their interactions and their roles in caveolar formation in polarized epithelial cells. In Fischer rat thyroid (FRT) cells, which express low levels of caveolin-2 and no caveolin-1, caveolin-2 localizes exclusively to the Golgi complex but is partially redistributed to the plasma membrane upon co-expression of caveolin-1 by transfection or by adenovirus-mediated transduction. In Madin-Darby canine kidney (MDCK) cells, which constitutively express both caveolin-1 and-2, caveolin-2 localized to both the Golgi complex and to the plasma membrane, where it co-distributed with caveolin-1 in flat patches and in caveolae. In FRT cells, endogenous or overexpressed caveolin-2 did not associate with low density Triton insoluble membranes that floated in sucrose density gradients but was recruited to these membranes when co-expressed together with caveolin-1. In MDCK cells, both caveolin-1 and caveolin-2 associated with low density Triton-insoluble membranes. In FRT cells, transfection of caveolin-1 promoted the assembly of plasma membrane caveolae that localized preferentially (over 99%) to the basolateral surface, like constitutive caveolae of MDCK cells. In contrast, as expected from its intracellular distribution, endogenous or overexpressed caveolin-2 did not promote the assembly of caveolae; rather, it appeared to promote the assembly of intracellular vesicles in the peri-Golgi area. The data reported here demonstrate that caveolin-1 and-2 have different and complementary subcellular localizations and functional properties in polarized epithelial cells and suggest that the two proteins cooperate to carry out specific as yet unknown tasks between the Golgi complex and the cell surface.
Caveolins in cholesterol trafficking and signal transduction: implications for human disease
Frontiers in Bioscience, 2000
Caveolins are a family of proteins that coat the cytoplasmic face of caveolae, vesicular invaginations of the plasma membrane. These proteins are central to the organization of the proteins and lipids that reside in caveolae. Caveolins transport cholesterol to and from caveolae, and they regulate the activity of signaling proteins that reside in caveolae. Through studying the genes encoding the caveolae coat proteins, we have learned much about how they perform these multiple functions.
Regulation of Caveolin and Caveolae by Cholesterol In MDCK Cells
Journal of lipid research, 1998
We have examined the expression of caveolin in MDCK cells under conditions that vary cellular cholesterol concentration. Caveolin mRNA levels dropped to one-sixth of control levels after treatment with simvastatin, an inhibitor of cholesterol synthesis, or  -trimethyl cyclodextrin (CD), a cholesterol sequestering drug. Both simvastatin and CD treatment decreased total cellular cholesterol levels to about 50% of control values. The potent activator of the sterol regulatory element, 25-hydroxycholesterol, showed no direct regulation of caveolin mRNA levels. Caveolin protein concentration was also decreased to 50% of control values in cholesteroldepleted cells, giving rise to a severe attenuation of caveolin expression detected by indirect immunofluorescence labeling. Quantitative electron microscopy showed a total loss of morphologically recognizable invaginated caveolae after these cholesterol depletion treatments. When the number of invaginated caveolae per cell was expressed as a function of the cellular cholesterol content, a threshold phenomenon was observed, suggesting that caveolae only form when the steady state cellular cholesterol is above 50% of control values. These findings indicate that caveolins, and caveolae, may play an important part in cellular cholesterol homeostasis.-Hailstones, D., L. S. Sleer, R. G. Parton, and K. K. Stanley. Regulation of caveolin and caveolae by cholesterol in MDCK cells. J. Lipid Res. 1998. 39: 369-379. Abbreviations: CD,  -trimethyl cyclodextrin; HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; MDCK, Madin-Darby canine kidney cells; 25-OHC, 25-hydroxycholesterol; SREBP, sterol regulatory element binding protein.