Cytochromes P450 2C1/2 and P450 2E1 are retained in the endoplasmic reticulum membrane by different mechanisms

Correlating structure and function of drug-metabolizing enzymes: progress and ongoing challenges

This report summarizes a symposium sponsored by the American Society for Pharmacology and Experimental Therapeutics at Experimental Biology held April 20-24 in Boston, MA. Presentations discussed the status of cytochrome P450 (P450) knowledge, emphasizing advances and challenges in relating structure with function and in applying this information to drug design. First, at least one structure of most major human drug-metabolizing P450 enzymes is known. However, the flexibility of these active sites can limit the predictive value of one structure for other ligands. A second limitation is our coarse-grain understanding of P450 interactions with membranes, other P450 enzymes, NADPH-cytochrome P450 reductase, and cytochrome b5. Recent work has examined differential P450 interactions with reductase in mixed P450 systems and P450:P450 complexes in reconstituted systems and cells, suggesting another level of functional control. In addition, protein nuclear magnetic resonance is a new approach to probe these protein/protein interactions, identifying interacting b5 and P450 surfaces, showing that b5 and reductase binding are mutually exclusive, and demonstrating ligand modulation of CYP17A1/b5 interactions. One desired outcome is the application of such information to control drug metabolism and/or design selective P450 inhibitors. A final presentation highlighted development of a CYP3A4 inhibitor that slows clearance of human immunodeficiency virus drugs otherwise rapidly metabolized by CYP3A4. Although understanding P450 structure/function relationships is an ongoing challenge, translational advances will benefit from continued integration of existing and new biophysical approaches.

The signal-anchor sequence of CYP2C1 inserts into the membrane as a hairpin structure

Microsomal cytochrome P450s (CYPs) are anchored to the endoplasmic reticulum membrane by the N-terminal signal-anchor sequence which is predicted to insert into the membrane as a type 1 transmembrane helix with a luminally located N-terminus. We have mapped amino acids of the CYP2C1 signal-anchor, fused to Cys-free glutathione S-transferase, within the membrane by Cys-specific labeling with membrane-impermeant maleimide polyethylene glycol. At the C-terminal end of the signal-anchor, Trp-20 was mapped to the membrane-cytosol interface and Leu-19 was within the membrane. Unexpectedly, at the N-terminal end, Glu-2 and Pro-3 were mapped to the cytoplasmic side of the membrane rather than the luminal side as expected of a type 1 transmembrane helix. Similar results were observed for the N-terminal amino acids of the signal-anchor sequences of CYP3A4 and CYP2E1. These observations indicate that contrary to the current model of the signal-anchor of CYPs as a type 1 transmembrane helix, CYP2C1, CYP2E1, and CYP3A4 are monotopic membrane proteins with N-terminal signal-anchors that have a hairpin or wedge orientation in the membrane.

Progesterone receptor membrane component 1 inhibits the activity of drug-metabolizing cytochromes P450 and binds to cytochrome P450 reductase

Progesterone receptor membrane component 1 (PGRMC1) has been shown to interact with several cytochromes P450 (P450s) and to activate enzymatic activity of P450s involved in sterol biosynthesis. We analyzed the interactions of PGRMC1 with the drug-metabolizing P450s, CYP2C2, CYP2C8, and CYP3A4, in transfected cells. Based on coimmunoprecipitation assays, PGRMC1 bound efficiently to all three P450s, and binding to the catalytic cytoplasmic domain of CYP2C2 was much more efficient than to a chimera containing only the N-terminal transmembrane domain. Down-regulation of PGRMC1 expression levels in human embryonic kidney 293 and HepG2 cell lines stably expressing PGRMC1-specific small interfering RNA had no effect on the endoplasmic reticulum localization and expression levels of P450s, whereas enzymatic activities of CYP2C2, CYP2C8, and CYP3A4 were slightly higher in PGRMC1-deficient cells. Cotransfection of cells with P450s and PGRMC1 resulted in PGRMC1 concentration-dependent inhibition of the P450 activities, and this inhibition was partially reversed by increased expression of the P450 reductase (CPR). In contrast, CYP51 activity was decreased by down-regulation of PGRMC1 and expression of PGRMC1 in the PGRMC1-deficient cells increased CYP51 activity. In cells cotransfected with CPR and PGRMC1, strong binding of CPR to PGRMC1 was observed; however, in the presence of CYP2C2, interaction of PGRMC1 with CPR was significantly reduced, suggesting that CYP2C2 competes with CPR for binding to PGRMC1. These data show that in contrast to sterol synthesizing P450, PGRMC1 is not required for the activities of several drug-metabolizing P450s, and its overexpression inhibits those P450 activities. Furthermore, PGRMC1 binds to CPR, which may influence P450 activity.

Proteasome inhibition compromises direct retention of cytochrome P450 2C2 in the endoplasmic reticulum

To determine whether protein degradation plays a role in the endoplasmic reticulum (ER) retention of cytochromes P450, the effects of proteasomal inhibitors on the expression and distribution of green fluorescent protein chimeras of CYP2C2 and related proteins was examined. In transfected cells, expression levels of chimeras of full-length CYP2C2 and its cytosolic domain, but not its N-terminal transmembrane sequence, were increased by proteasomal inhibition. Redistribution of all three chimeras from the reticular ER into a perinuclear compartment and, in a subset of cells, also to the cell surface was observed after proteasomal inhibition. Redistribution was blocked by the microtubular inhibitor, nocodazole, suggesting that redistribution to the cell surface followed the conventional vesicular transport pathway. Similar redistributions were detected for BAP31, a CYP2C2 binding chaperone; CYP2E1 and CYP3A4, which are also degraded by the proteasomal pathway; and for cytochrome P450 reductase, which does not undergo proteasomal degradation; but not for the ER membrane proteins, sec61 and calnexin. Redistribution does not result from saturation of an ER retention "receptor" since in some cases protein levels were unaffected. Proteasomal inhibition may, therefore, alter ER retention by affecting a protein critical for ER retention, either directly, or indirectly by affecting the composition of the ER membranes.

Influence of protein-protein interactions on the cellular localization of cytochrome P450

BACKGROUND

Microsomal CYPs are integral membrane proteins that are localized in the endoplasmic reticulum (ER), which is critical for their function. CYPs are co-translationally inserted into the rough ER membrane and are then either directly retained in the smooth ER or retained by a retrieval mechanism or targeted for ER-associated degradation. Protein-protein interactions are likely to be important for proper cellular targeting of CYPs.

OBJECTIVE

Progress in understanding the mechanisms of cellular targeting and ER retention of CYPs is reviewed with emphasis on the role of protein-protein interactions. Possible mechanisms of direct retention are the incorporation of CYPs into an immobile complex in the ER membrane, homooligomerization that prevents inclusion in transport vesicles, exclusion of CYP monomers from transport vesicles or targeting of CYPs to an ER subdomain away from sites of transport vesicle formation. Degradation of CYPs occurs either by lysosomal mechanisms or by the ubiquitin-proteasomal pathway.

METHODS

The scope of this review includes studies published in the research literature that have defined the targeting of CYPs to the ER, the retention of CYPs in the ER and the degradation of CYPs.

RESULTS/CONCLUSION

Targeting of CYPs to the ER is well understood and involves signal recognition particle-mediated delivery to the sec61 complex. The mechanism of ER retention of CYPs remains unclear, but self-oligomerization or binding to large immobile networks do not underlie ER retention of CYPs. An ER retention 'receptor' remains elusive, but BAP31 is important for the proper cellular localization of CYPs and Dap1p is a CYP-binding protein that is a candidate for such a receptor. Identification of protein binding partners of CYPs will be critical to understanding the mechanism of ER retention.

BIMOLECULAR FLUORESCENCE COMPLEMENTATION ANALYSIS OF CYTOCHROME P450 2C2, 2E1, AND NADPH-CYTOCHROME P450 REDUCTASE MOLECULAR INTERACTIONS IN LIVING CELLS

Interactions between cytochromes P450 (P450s) and P450 reductase are required for enzymatic activity, and homo- or heterooligomerization of P450s may also be functionally important. Bimolecular fluorescence complementation (BiFC) was used to examine P450 interactions in a natural membrane context within living cells. BiFC detects protein interactions in living cells by reconstitution of a fluorescent protein from two fragments that are fused to the two interacting proteins. Nonspecific protein-protein interactions were detected if proteins were expressed at high levels. At low protein expression levels, homo-oligomerization of P450 2C2, but not P450 2E1, and interactions of these P450s with P450 reductase were detected by BiFC, consistent with interactions detected previously by fluorescence resonance emission transfer. Weak interaction of P450 2C2 with P450 2E1 and homooligomerization of P450 reductase was also detected by BiFC. Homo-oligomerization of the N-terminal P450 2C1 signal anchor sequence and interactions between the signal anchor and full-length P450 2C2 were detected, suggesting that homo-oligomerization of P450 2C2 is mediated by the signal anchor. However, interactions between the signal anchor and either P450 2E1 or P450 reductase were not detected by BiFC. Although high concentrations of the substrate lauric acid increased BiFC for both P450 2E1 and P450 2C2 with P450 reductase, the concentration dependence did not correlate with reported K(m) values. These results demonstrate that BiFC is an effective method to study the complex protein interactions that occur within the microsomal P450 system in living cells.

Gene expression changes associated with the endoplasmic reticulum stress response induced by microsomal cytochrome p450 overproduction

Induction of drug-metabolizing microsomal cytochromes p450 (p450s) results in a striking proliferation of the smooth endoplasmic reticulum (ER). Overexpression of P450s in yeast and cultured cells produces a similar response. The signals mediating this process are not known but probably involve signal transduction pathways involved in the unfolded protein response (UPR) or the ER overload response (EOR). We have examined the temporal response of specific genes in these pathways and genes globally to overexpression of p450 in cultured cells. Activity of NFkappaB, an EOR component, was substantially increased by overexpression of full-length p450 2C2 or a chimera with the 28-amino acid signal anchor sequence of p450 2C2 in HepG2 cells, and the activation correlated temporally with the accumulation of p450 in the cells. In the UPR pathway, activation of the transcription factor XBP1 by IRE1 also correlated with the accumulation of p450 in the cells, and in contrast, maximum activation of the BiP/grp78 promoter preceded the accumulation. Differential effects of expression of p450 on apoptosis were observed in nonhepatic COS1 and hepatic HepG2 cells. In COS1 cells, apoptosis was induced, and this correlated with sustained activation of the pro-apoptotic JNK pathway, induction of CHOP, and an absence of the increased NFkappaB activity. In HepG2 cells, JNK was only transiently activated, and CHOP expression was not induced. As assessed by DNA microarray analysis, up-regulation of signaling genes was predominant including those involved in anti-apoptosis and ER stress. These results suggest that both the EOR and UPR pathways are involved in the cellular response to induction of p450 expression and that in hepatic cells genes are also induced to block apoptosis, which may be a physiologically relevant response to prevent cell death during xenobiotic induced expression of p450 in the liver.

Membrane-bound fatty acid desaturases are inserted co-translationally into the ER and contain different ER retrieval motifs at their carboxy termini

Fatty acid desaturases (FADs) play a prominent role in plant lipid metabolism and are located in various subcellular compartments, including the endoplasmic reticulum (ER). To investigate the biogenesis of ER-localized membrane-bound FADs, we characterized the mechanisms responsible for insertion of Arabidopsis FAD2 and Brassica FAD3 into ER membranes and determined the molecular signals that maintain their ER residency. Using in vitro transcription/translation reactions with ER-derived microsomes, we show that both FAD2 and FAD3 are efficiently integrated into membranes by a co-translational, translocon-mediated pathway. We also demonstrate that while the C-terminus of FAD3 (-KSKIN) contains a functional prototypic dilysine ER retrieval motif, FAD2 contains a novel C-terminal aromatic amino acid-containing sequence (-YNNKL) that is both necessary and sufficient for maintaining localization in the ER. Co-expression of a membrane-bound reporter protein containing the FAD2 C-terminus with a dominant-negative mutant of ADP-ribosylation factor (Arf)1 abolished transient localization of the reporter protein in the Golgi, indicating that the FAD2 peptide signal acts as an ER retrieval motif. Mutational analysis of the FAD2 ER retrieval signal revealed a sequence-specific motif consisting of Phi-X-X-K/R/D/E-Phi-COOH, where -Phi- are large hydrophobic amino acid residues. Interestingly, this aromatic motif was present in a variety of other known and putative ER membrane proteins, including cytochrome P450 and the peroxisomal biogenesis factor Pex10p. Taken together, these data describe the insertion and retrieval mechanisms of FADs and define a new ER localization signal in plants that is responsible for the retrieval of escaped membrane proteins back to the ER.

Fluorescence resonance energy transfer analysis of cytochromes P450 2C2 and 2E1 molecular interactions in living cells

The molecular organization of microsomal cytochromes P450 (P450s) and formation of complexes with P450 reductase have been studied previously with isolated proteins and in reconstituted systems. Although these studies demonstrated that some P450s oligomerize in vitro, neither oligomerization nor interactions of P450 with P450 reductase have been studied in living cells. Here we have used fluorescence resonance energy transfer (FRET) to study P450 oligomerization and binding to P450 reductase in live transfected cells. Cytochrome P450 2C2, but not P450 2E1, forms homo-oligomeric structures, and this self-association is mediated by the signal-anchor sequence. Because P450 2C2, in contrast to P450 2E1, is directly retained in the endoplasmic reticulum (ER), these results could suggest that oligomerization may prevent transport from the ER. However, P450 2C1 signal-anchor sequence chimera defective in ER retention also formed oligomers, and chimera containing the cytoplasmic domain of P450 2C2, which is directly retained in the ER, did not exhibit self-oligomerization, which indicates that oligomerization is not correlated with direct retention. By using FRET, we have also detected binding of P450 2C2 and P450 2E1 to P450 reductase. In contrast to self-oligomerization, the catalytic domain can mediate an interaction of P450 2C2 with P450 reductase. These results suggest that microsomal P450s may differ in their quaternary structure but that these differences do not detectably affect interaction with the reductase or transport from the ER.

Retrieval-independent localization of lysyl hydroxylase in the endoplasmic reticulum via a peptide fold in its iron-binding domain

Lysyl hydroxylase (LH) is a peripheral membrane protein in the lumen of the endoplasmic reticulum (ER) that catalyses hydroxylation of lysine residues in collagenous sequences. Previously, we have mapped its primary ER localization motif within a 40-amino acid segment at its C-terminus. Here, we have characterized this localization mechanism in more detail, and our results indicate that this segment confers ER residency in a KDEL-receptor-independent manner, and without any apparent recycling of the enzyme between the Golgi apparatus and the ER. In addition, we show that a rather long peptide region, rather than a specific peptide sequence per se, is required for efficient retention of a reporter protein in the ER. Accordingly, the minimal retention motif was found to require the last 32 C-terminal amino acids, and sequential substitution of all five charged residues within this critical segment interfered only marginally with the retention or association of the enzyme with the ER membranes. Moreover, our fold-recognition and structure-prediction analyses suggested that this critical peptide segment forms an extended loop within LH's iron-binding domain, and that this loop is exposed and readily accessible for binding. Collectively, our results define a novel retrieval-independent retention mechanism in the ER.

Bimodal targeting of microsomal CYP2E1 to mitochondria through activation of an N-terminal chimeric signal by cAMP-mediated phosphorylation

Cytochrome P450 2E1 (CYP2E1) plays an important role in alcohol-induced toxicity and oxidative stress. Recently, we showed that this predominantly microsomal protein is also localized in rat hepatic mitochondria. In this report, we show that the N-terminal 30 amino acids of CYP2E1 contain a chimeric signal for bimodal targeting of the apoprotein to endoplasmic reticulum (ER) and mitochondria. We demonstrate that the cryptic mitochondrial targeting signal at sequence 21-31 of the protein is activated by cAMP-dependent phosphorylation at Ser-129. S129A mutation resulted in lower affinity for binding to cytoplasmic Hsp70, mitochondrial translocases (TOM40 and TIM44) and reduced mitochondrial import. S129A mutation, however, did not affect the extent of binding to the signal recognition particle and association with ER membrane translocator protein Sec61. Addition of saturating levels of signal recognition particle caused only a partial inhibition of CYP2E1 translation under in vitro conditions, and saturating levels of ER resulted only in partial membrane integration. cAMP enhanced the mitochondrial CYP2E1 (referred to as P450MT5) level but did not affect its level in the ER. Our results provide new insights on the mechanism of cAMP-mediated activation of a cryptic mitochondrial targeting signal and regulation of P450MT5 targeting to mitochondria.

The juxtamembrane sequence of cytochrome P-450 2C1 contains an endoplasmic reticulum retention signal

The N-terminal signal anchor of cytochrome P-450 2C1 mediates retention in the endoplasmic reticulum (ER) membrane of several reporter proteins. The same sequence fused to the C terminus of the extracellular domain of the epidermal growth factor receptor permits transport of the chimeric protein to the plasma membrane. In the N-terminal position, the ER retention function of this signal depends on the polarity of the hydrophobic domain and the sequence KQS in the short hydrophilic linker immediately following the transmembrane domain. To determine what properties are required for the ER retention function of the signal anchor in a position other than the N terminus, the effect of mutations in the linker and hydrophobic domains on subcellular localization in COS1 cells of chimeric proteins with the P-450 signal anchor in an internal or C-terminal position was analyzed. For the C-terminal position, the signal anchor was fused to the end of the luminal domain of epidermal growth factor receptor, and green fluorescent protein was additionally fused at the C terminus of the signal anchor for the internal position. In these chimeras, the ER retention function of the signal anchor was rescued by deletion of three leucines at the C-terminal side of its hydrophobic domain; however, deletion of three valines from the N-terminal side did not affect transport to the cell surface. ER retention of the C-terminal deletion mutants was eliminated by substitution of alanines for glutamine and serine in the linker sequence. These data are consistent with a model in which the position of the linker sequence at the membrane surface, which is critical for ER retention, is dependent on the transmembrane domain.

Alternative splicing regulates the endoplasmic reticulum localization or secretion of soluble secreted endopeptidase

A subfamily of zinc metalloproteases, represented by Neutral endopeptidase (EC ) and endothelin-converting enzyme, is involved in the metabolism of a variety of biologically active peptides. Recently, we cloned and characterized a novel member of this metalloprotease family termed soluble secreted endopeptidase (SEP), which hydrolyzes many vasoactive peptides. Here we report that alternative splicing of the mouse SEP gene generates two polypeptides, SEP(Delta) and SEP. After synthesis, both isoforms are inserted into the endoplasmic reticulum (ER) as type II membrane proteins. SEP(Delta) then becomes an ER resident, whereas SEP, which differs by only the presence of 23 residues at the beginning of its luminal domain, is proteolytically cleaved by membrane secretase(s) in the ER and transported into the extracellular compartment. An analysis of the chimeric proteins between SEP(Delta) and bovine endothelin-converting enzyme-1b (bECE-1b) demonstrated that the retention of SEP(Delta) in the ER is mediated by the luminal domain. In addition, the dissection of the chimeric bECE-1b/SEP insertion showed that its insertion domain is obviously responsible for its secretion. A series of mutagenesis in this region revealed that the minimal requirement for cleavage was found to be a WDERTVV motif. Our results suggest that the unique subcellular localization and secretion of SEP proteins provide a novel model of protein trafficking within the secretory pathway.

Identification and characterization of a mitochondrial targeting signal in rat cytochrome P450 2E1 (CYP2E1)

Cytochrome P450 2E1 (CYP2E1) lacking the hydrophobic NH(2)-terminal hydrophobic transmembrane domain is specifically targeted to mitochondria, where it is processed to a soluble and catalytically active form (Delta2E1) with a mass of about 40 kDa. Small amounts of Delta2E1 were also observed in mitochondria isolated from rat liver, indicating that this form of CYP2E1 is also present in vivo. In the present study the mitochondrial targeting signal was identified and characterized by the use of several NH(2)-terminally truncated and mutated forms of CYP2E1 that were expressed in the mouse H2.35 hepatoma cell line. Two potential mitochondrial targeting sequences were identified in the NH(2) terminus of CYP2E1. Deletion of the first potential mitochondrial targeting sequence located between amino acids 50 and 65, as in Delta(2-64)2E1, still resulted in mitochondrial targeting and processing, but when, in addition to the first, the second potential mitochondrial targeting sequence located between amino acids 74 and 95 was also deleted, as in Delta(2-95)2E1, the mitochondrial targeting was abolished. Mutation of the four positively charged Arg and Lys residues present in this sequence to neutral Ala residues resulted in the abrogation of mitochondrial targeting. Deletion of a hydrophobic stretch of amino acids between residues 76 and 83 also abolished mitochondrial targeting and import. Once imported in the mitochondria, these constructs were further processed to the mature protein Delta2E1. It is concluded that mitochondrial targeting of CYP2E1 is mediated through a sequence located between residues 74 and 95 and that positively charged residues as well as a hydrophobic stretch present in the beginning of this sequence are essential for this process.

Endoplasmic reticulum retention determinants in the transmembrane and linker domains of cytochrome P450 2C1

The cytochrome P450 2C1 N-terminal signal anchor sequence mediates direct retention of the protein in the endoplasmic reticulum and consists of a hydrophobic transmembrane domain, residues 3-20, followed by a hydrophilic linker, residues 21-28. Fusions of the N-terminal 21 or 28 amino acids of P450 2C1 to green fluorescent protein resulted in endoplasmic reticulum localization of the chimera in transfected cells. Disruption of microtubules by nocodazole treatment resulted in redistribution into a punctate pattern for the 1-21, but not for the 1-28, chimera indicating that the linker was preventing transport from the endoplasmic reticulum but was not required for retrieval to the endoplasmic reticulum from the pre-Golgi compartment. In the 1-28 chimera, mutations of residues 21-23 (KQS) in the linker resulted in redistribution of the chimera after nocodazole treatment. Mutations in the transmembrane domain affected both direct retention in the endoplasmic reticulum and retrieval from the pre-Golgi compartment, and although structural requirements for each process are distinct, in both cases the arrangement of amino acids and distribution of hydrophobicity are critical. In contrast, the linker region exhibits a sequence-specific requirement for direct retention in the endoplasmic reticulum.