Proteolysis of Normal and Mutated Steroidogenic Acute Regulatory Proteins in the Mitochondria: the Fate of Unwanted Proteins

Abstract

Steroidogenic acute regulatory protein (StAR) is a nuclear encoded mitochondrial protein that enhances steroid synthesis by facilitating the transfer of cholesterol to the inner membranes of mitochondria in hormonally regulated steroidogenic cells. It is currently assumed that StAR activity commences before or during StAR import into the mitochondrial matrix. The present study was designed to demonstrate that, once imported and becoming physiologically irrelevant, exhaustive accumulation of StAR must be limited by a rapid degradation of the protein to prevent potential damage to the organelles. The use of uncouplers and manipulation of the interior mitochondrial pH in hormone-induced ovarian granulosa cells and StAR-expressing COS cells suggests that StAR degradation is biphasic and involves two classes of proteases. During phase I, which normally lasts for the first approximately 2 h following import, StAR is rapidly degraded by a protease, or proteases, that can be arrested by a nonclassical action of proteasome inhibitors such as MG132. StAR molecules that evade phase I are subjected to a second class of protease(s), which is slower and MG132 resistant. A third proteolytic entity was revealed in studies with C-28 StAR, a loss-of-function mutant of StAR. Upon initiation of its import, C-28 StAR dissipates the inner membrane potential and causes swelling of the mitochondria. Degradation of C-28 StAR, probably by an intermembrane space protease, is extremely rapid and MG132 insensitive. Collectively, this study defines StAR as the first naturally occurring mitochondrial protein that can serve as a substrate to probe multiple proteolytic activities in mammalian mitochondria.

Steroidogenic acute regulatory protein gene expression, steroid-hormone secretion and proliferative activity of adrenocortical cells in the presence of proteasome inhibitors: In vivo studies on the regenerating rat adrenal cortex

“…The striking down-regulation of the expression of the rate-limiting step enzyme of steroidogenesis StAR during the early stages of adrenal regeneration fits well with the reduced gland steroidogenic capacity. It is reasonable to conceive that at day 5 post-surgery highly proliferating adrenocortical cells are unable to produce adequate amounts of StAR, whose expression rapidly decrease due to its rather short half-life (3)(4)(5)(6).…”

“…Moreover, MG115 has been reported to cause a complete blockade of G1/S and metaphase transitions, as well as a delayed progression through S phase in various cell types cultured in vitro (34). Several in vitro experiments showed that relatively high concentrations of proteasome inhibitors are needed to inhibit proteasome-dependent StAR protein degradation (5,6,8). Hence, our present negative findings may depend on the relatively low doses of inhibitors administered in our in vivo experiments, coupled to the rapid inactivation of the inhibitors due to their rather short half-life (7).…”

“…In steroidogenic cells, the 37 kDa StAR preprotein is rapidly cleaved to the mature 30 kDa protein, and the half-lives of StAR preprotein trapped in the cytosol and mitochondria have been estimated at 15 min and 4.8 h, respectively (3)(4)(5)(6). Due to its short half-life, StAR preprotein must be produced continuously if steroidogenesis is to be maintained.…”

“…Compelling evidence indicates that the bulk of short-lived regulatory proteins, including StAR, is degraded by the ubiquitin-proteasome system (5,8). Moreover, experimental data suggested stabilization of regulatory proteins upon treatment with proteasome inhibitors (7).…”

Control of Mitochondrial Outer Membrane Permeabilization and Bcl-xL Levels by Thioredoxin 2 in DT40 Cells

“…Protein stability is a key regulatory mechanism in the control of cell development, the cell cycle, cell growth, and apoptosis. Proteolysis of mitochondrial proteins is regulated by multiple mechanisms (35,36). The ubiquitin/proteasome system regulates molecules such as Mcl-1, a member of the Bcl-2 family in mitochondria (37)(38)(39)(40), indicating the importance of the protease system in the regulation of levels of Bcl-2 family proteins.…”