TRANSFORMING GROWTH-FACTOR-BETA (TGF-BETA)-INDUCED LOCALIZATION OF APOLIPOPROTEIN J CLUSTERIN IN EPITHELIAL-CELLS/

Apolipoprotein J (apoJ)/clusterin was first identified as an 80 kDa se cretory glycoprotein present in most body fluids. It has been implicat ed in a variety of physiological processes including cellular differen tiation and apoptosis. We demonstrate here that in addition to the wel l characterized secreted form of the protein, there exists an intracel lular, nuclear form of apoJ. This intracellular form of the protein is induced to accumulate in the nucleus of two epithelial cell lines (He pG2 and CCL64) in response to treatment with transforming growth facto r beta (TGF beta). We demonstrate in vitro that apoJ protein can be tr anslated from two in-frame ATG sites. Initiation from the first ATG en codes for the secretory form of apoJ and initiation from the second AT G, located 33 amino acids downstream of the first and lacking the hydr ophobic signal sequence, encodes for a truncated apoJ protein. This sh orter form of apoJ is not recognized by microsomes and therefore not g lycosylated, and we postulate that it is retained intracellularly, and targeted to the nucleus due to the presence of an SV40-like nuclear l ocalization sequence (NLS). This mechanism of nuclear targeting of apo J occurs in cells since the protein isolated from nuclei of TGF beta-t reated cells and the in vitro-translated truncated form are identical by V8 protease analysis. These results suggest that the diverse physio logical responses attributed to apoJ may be elicited through a common molecular mechanism involving a previously uncharacterized intracellul ar form of the protein.