Role ofβ1 Integrin Distribution in Morphology and Function of Collagen-Sandwiched Hepatocytes

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Abstract
Culturing adult rat hepatocytes in a type I collagen gel sandwich has been documented to enhance differentiated function when compared with those cultured on a single collagen gel. The collagen gel sandwich maintains the polarity and cytoskeletal organization of hepatocytes, but the exact mechanism of its influence on liver-specific functions is unknown. In this report, we investigated the role of matrix adhesion receptors in determining hepatocyte morphology and function during culture in a collagen sandwich configuration. This was accomplished by examining the fate of hepatocyte β1 integrin membrane receptor during early and late phases of collagen sandwich cultures. A rapid redistribution of cell surface integrins following the addition of collagen overlay was observed, and densitometry of 3-D reconstructions of integrin-stained hepatocytes revealed that the ratio of integrin localization at the top membrane surface to that at the bottom surface increases steadily upon sandwiching of hepatocytes. Flow cytometric studies showed that the cumulative surface β1 integrin expression did not change, indicating that there is a distinct increase in expression of integrins bound to the collagen gel overlay and a concomitant decrease in the underlay-bound integrin pool. When integrin engagement with the collagen overlay was blocked by antibodies to β1 integrin, several processes typifying normal hepatocyte morphogenesis (cellular compaction, appearance of bright cell-cell borders, cytoskeletal and apical polarity) and function (albumin and urea secretion) were inhibited. These results suggest a primary role for integrin redistribution and differential surface binding to the extracellular matrix in the induction of function in sandwiched hepatocytes.