MLN64 contains a domain with homology to the steroidogenic acute regulatory protein (StAR) that stimulates steroidogenesis

Abstract

MLN64 is a protein that is highly expressed in certain breast carcinomas. The C terminus of MLN64 shares significant homology with the steroidogenic acute regulatory protein (StAR), which plays a key role in steroid hormone biosynthesis by enhancing the intramitochondrial translocation of cholesterol to the cholesterol side-chain cleavage enzyme. We tested the ability of MLN64 to stimulate steroidogenesis by using COS-1 cells cotransfected with plasmids expressing the human cholesterol side-chain cleavage enzyme system and wild-type and mutant MLN64 proteins. Wild-type MLN64 increased pregnenolone secretion in this system 2-fold. The steroidogenic activity of MLN64 was found to reside in the C terminus of the protein, because constructs from which the C-terminal StAR homology domain was deleted had no steroidogenic activity. In contrast, removal of N-terminal sequences increased MLN64's steroidogenesisenhancing activity. MLN64 mRNA was found in many human tissues, including the placenta and brain, which synthesize steroid hormones but do not express StAR. Western blot analysis revealed the presence of lower molecular weight immunoreactive MLN64 species that contain the C-terminal sequences in human tissues. Homologs of both MLN64 and StAR were identified in Caenorhabditis elegans, indicating that the two proteins are ancient. Mutations that inactivate StAR were correlated with amino acid residues that are identical or similar among StAR and MLN64, indicating that conserved motifs are important for steroidogenic activity. We conclude that MLN64 stimulates steroidogenesis by virtue of its homology to StAR.

Differential expression of HPA axis homolog in the skin

“…The transport of cholesterol substrate for P450scc into mitochondria is performed by specific cholesterol transporting proteins, StAR or MLN64 (Bose et al, 2000;Stocco, 2000;Watari et al, 1997). Using specific antibodies that recognize a common epitope for both MLN64 and StAR (Bose et al, 2000), we detected the expected protein with MW of 55 kDa (Slominski et al, 2004c) corresponding to MLN64 (Uribe et al, 2003) in human and rodent skin samples.…”

“…Using specific antibodies that recognize a common epitope for both MLN64 and StAR (Bose et al, 2000), we detected the expected protein with MW of 55 kDa (Slominski et al, 2004c) corresponding to MLN64 (Uribe et al, 2003) in human and rodent skin samples. Additional immunoreactive proteins of lower MW (37 kDa and 18 kDa) could represent products of MLN64 processing and/or the full-length StAR protein (Bose et al, 2000;Stocco, 2000;Watari et al, 1997). This suggests that mammalian skin cells are biochemically equipped for cholesterol transport as the basis for steroidogenesis.…”

Metastatic lymph node 64 (MLN64), a gene overexpressed in breast cancers, is regulated by Sp/KLF transcription factors

“…The functional relation between MLN64 and StAR was previously addressed. It was shown that, like StAR, MLN64 can enhance steroidogenesis in vitro (Watari et al, 1997). Moreover, the region conserved between StAR and MLN64, the StAR-related transfer (START) domain was shown to be a cholesterol-binding domain for both proteins (Tsujishita and Hurley, 2000).…”

Targeted Mutation of the MLN64 START Domain Causes Only Modest Alterations in Cellular Sterol Metabolism

“…MLN64 was originally discovered as a gene amplified in breast and ovarian cancers. It contains a C-terminal domain with 37% amino acid identity and 60% amino acid similarity to the C-terminal domain of StAR (13,14). The N terminus of MLN64 contains a leader sequence and four putative transmembrane domains, distinguishing it from StAR and suggesting that MLN64 functions in a different subcellular compartment.…”

“…The N terminus of MLN64 contains a leader sequence and four putative transmembrane domains, distinguishing it from StAR and suggesting that MLN64 functions in a different subcellular compartment. The MLN64 START domain was subsequently found to have StAR-like activity in that it could promote steroidogenesis in a model cell system (14). Like the StAR START domain, the MLN64 START domain binds cholesterol, stimulates the movement of free cholesterol from sterol-rich donor vesicles to acceptor membranes, and augments steroid synthesis when added to isolated mitochondria (10,15).…”