Breakdown of the crystalloid endoplasmic reticulum of UT-1 cells

Abstract

The studies described focus on the sterol-regulated mechanism by which UT-1 cells dispose of an elaborate intracellular membrane system of smooth endoplasmic reticulum (ER) termed crystalloid ER (CER). The CER is formed in response to lipoprotein deprivation and competitive inhibition o f 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). Under these conditions, UT-1 cells produce up to 500-fold more HMGR than parental CHO cells. HMGR has been shown to occupy the CER. Addition of sterols to the growth media of UT-1 cells has been shown to cause the degradation of HMGR and breakdown of the CER membrane system. It has also been shown that sterol-accelerated degradation of CER membranes occurs via a non-lysosomal pathway. Other possible mechanisms of CER elimination include intra- ER degradation and expulsion of CER membranes to the extracellular media. There were four specific aims included in these studies. First, indirect immunofluorescence microscopy and brefeldin A were used to determine the existence of an intra-ER degradative pathway for HMGR in UT-1 cells. Second, silver stain, 45ca2+, and 32p_(}TP overlay techniques were used on proteins separated by 2-D SDS-PAGE to determine changes in proteins during incubation of UT-1 cells in sterols. Third, immunoblotting of immunoprecipitated proteins was used to account for both intra- and extracellular disposition of CER membranes and proteins during CER breakdown. Lastly, a major constituent protein of the CER of UT-1 cells, CERp60, was purified and submitted for sequencing to determine if it was related to a multifiinctional ER protein, protein disulfide isomerase. The goals of these studies was to further define the location of HMGR degradation, to determine sterol-mediated changes in UT-1 proteins, in particular Ca^+- and GTP-binding proteins, and to determine if CERp60 was related to protein disulfide isomerase.