Role of Subcellular Organelles and Structures

The roles of various subcellular organelles and structures in the hepatic metabolism and secretion of RBP have been studied with both normal and retinol-deficient rats. These studies have employed assays for a variety of marker enzymes and other constituents. After differential centrifugation of liver homogenates, 79 1% of the RBP was found associated with the liver microsomes (Harrison et al., 1979). Similar proportions of total liver RBP were found in the microsomal fractions of livers from both normal and retinol-deficient rats (J. E. Smith et al., 1975). Further subfractionation of the microsomal fraction showed that RBP was particularly enriched in the rough microsomal fraction (3.8 0.5-fold over the homogenate), which contained 49 4% of the liver microsomal RBP (Smith and Goodman, 1979). RBP was also enriched in the smooth microsomal fraction (3.2 0.2-fold over the homogenate). 

The effects of colchicine on RBP secretion and metabolism by the liver were explored by Smith et al. (1980). Colchicine treatment of retinol-deficient rats markedly inhibited the retinol-stimulated secretion of RBP from the liver into the serum. The inhibition of RBP secretion was quantitatively quite similar to the inhibition of very low-density lipoprotein secretion by colchicine seen in parallel experiments. In contrast, colchicine did not affect the overall rate of protein synthesis within the liver. The inhibition of RBP secretion by colchicine suggests that the microtubules play a role in RBP secretion. 

When retinol-deficient rats were first treated with colchicine and then injected with retinol to stimulate RBP secretion, the RBP content of a Golgi-rich fraction from liver increased markedly, to a maximum of 34% of the total liver RBP. The level of TTR in the Golgi was not influenced by retinol injection. 

Taken together, these studies are consistent with the conclusion that the Golgi apparatus, Golgi-derived secretory vesicles, and microtubules are involved in the normal pathway of RBP secretion in the liver cell. Thus these studies suggest that the RBP secretory process involves the same subcellular organelles and pathways previously shown to be involved in the secretion of other serum proteins such as albumin (Redman et al., 1975). These data also suggest that the block in RBP secretion found in retinol deficiency occurs at a site before the RBP molecule reaches the major portion of the Golgi apparatus. 

Further support for this conclusion was reported very recently by Rask et al. (1983) and Ronne et al. (1983) from studies of the subcellular localization of RBP and other marker components in normal and vitamin A-deficient rat liver, and from studies of the effects of retinol on RBP synthesis and secretion by primary rat hepatocytes in culture. From these studies, it was concluded that the transport of RBP from the endoplasmic reticulum to the Golgi complex is regulated by retinol. It is also possible that retinol is necessary not for transport of RBP from the endoplasmic reticulum to the Golgi, but for the appropriate directed movement of RBP through the Golgi apparatus. Thus, it has been reported that formation of the retinol-RBP complex may be necessary for the normal 

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