Vl. Rudick et al., ROUTING OF A SECRETORY PROTEIN TO THE ENDOCYTIC COMPARTMENT IN TRANSFECTED MADIN-DARBY CANINE KIDNEY-CELLS, Cellular & molecular biology research, 39(8), 1993, pp. 773-788
Transfected Madin Darby canine kidney (MDCK) cells (3A) expressing hum
an growth hormone (hGH) contain twice as many Golgi stacks as untransf
ected cells. How MDCK cells, lacking a regulated pathway, deal with (o
ver)expression of a protein hormone, or any exogenous protein, has not
been examined in detail. Since hGH constituted 10% of total secreted
proteins, it was not apparent why Golgi amplification was needed, unle
ss some enters a nonsecretory compartment. Studies were undertaken to
determine hGH fate. By using an inhibitor of protein synthesis, or by
analyzing pulse labeled immunoprecipitated hGH, 20-30% of hGH was show
n to remain intracellular even after 4 h. That portion might be locali
zed in the endosome/lysosome compartment, because it is post-Golgi. Im
munoelectron microscopy with antibodies against hGH, clathrin, and cat
hepsin D demonstrated clathrin and hGH colocalized, as did hGH and cat
hepsin D. The latter were found in large vesicles, but no hGH appeared
in lysosomes, due to its degradation. Analysis of isolated lysosome/e
ndosomes revealed vesicles containing both hGH and cathepsin D, but mo
re containing only cathepsin D. Endocytosis studies suggested the 3A b
asolateral endosomal compartment may be more capacious than normal. Th
us, 3A Golgi amplification resulted in an expanded endosome compartmen
t to accommodate secretory protein (over)expression.