Stabilizing the Proteomes of Acute Myeloid Leukemia Cells: Implications for Cancer Proteomics.

Previous work has shown that inhibition of abundant myeloid azurophil granule-associated serine proteases (ELANE [neutrophil elastase], PRTN3 [protease 3], and CTSG [Cathepsin G]) is required to stabilize some proteins in myeloid cells. We therefore hypothesized that effective inhibition of these proteases may be necessary for quantitative proteomic analysis of samples containing myeloid cells. To test this hypothesis, we thawed viably preserved acute myeloid leukemia cells from cryovials in the presence or the absence of diisopropyl fluorophosphate (DFP), a cell-permeable and irreversible serine protease inhibitor. Global proteomic analysis was performed, using label-free and isobaric peptide-labeling quantitation. The presence of DFP resulted in an increase of tryptic peptides (14-57%) and proteins (9-31%). In the absence of DFP, 11 to 31% of peptide intensity came from nontryptic peptides; 52 to 75% had cleavage specificity consistent with activities of ELANE-PRTN3. Treatment with DFP reduced the intensity of nontryptic peptides to 4-8% of the total. ELANE inhibition was 95%, based on diisopropyl phosphate modification of active site serine residue. Overall, the relative abundance of 20% of proteins was significantly altered by DFP treatment. These results suggest that active myeloid serine proteases, released during sample processing, can skew quantitative proteomic measurements. Finally, significant ELANE activity was also detected in Clinical Proteomics Tumor Analysis Consortium datasets of solid tumors (many of which have known myeloid infiltration). In the pancreatic cancer dataset, the median percentage of nontryptic intensity detected across patient samples was 34%, with many patient samples having more than half of their detected peptide intensity from nontryptic cleavage events consistent with ELANE-PRTN3 cleavage specificity. Our study suggests that in vitro cleavage of proteins by myeloid serine proteases may be relevant for proteomic studies of any tumor that contains infiltrating myeloid cells.

Proteogenomic analysis reveals cytoplasmic sequestration of RUNX1 by the acute myeloid leukemia-initiating CBFB::MYH11 oncofusion protein.

Several canonical translocations produce oncofusion genes that can initiate acute myeloid leukemia (AML). Although each translocation is associated with unique features, the mechanisms responsible remain unclear. While proteins interacting with each oncofusion are known to be relevant for how they act, these interactions have not yet been systematically defined. To address this issue in an unbiased fashion, we fused a promiscuous biotin ligase (TurboID) in-frame with 3 favorable-risk AML oncofusion cDNAs (PML::RARA, RUNX1::RUNX1T1, and CBFB::MYH11) and identified their interacting proteins in primary murine hematopoietic cells. The PML::RARA- and RUNX1::RUNX1T1-TurboID fusion proteins labeled common and unique nuclear repressor complexes, implying their nuclear localization. However, CBFB::MYH11-TurboID-interacting proteins were largely cytoplasmic, probably because of an interaction of the MYH11 domain with several cytoplasmic myosin-related proteins. Using a variety of methods, we showed that the CBFB domain of CBFB::MYH11 sequesters RUNX1 in cytoplasmic aggregates; these findings were confirmed in primary human AML cells. Paradoxically, CBFB::MYH11 expression was associated with increased RUNX1/2 expression, suggesting the presence of a sensor for reduced functional RUNX1 protein, and a feedback loop that may attempt to compensate by increasing RUNX1/2 transcription. These findings may have broad implications for AML pathogenesis.

Missense mutations in Myc Box I influence MYC cellular localization, mRNA partitioning and turnover to promote leukemogenesis.

Somatic missense mutations in the phosphodegron domain of the MYC gene ( M YC Box I) are detected in the dominant clones of a subset of acute myeloid leukemia (AML) patients, but the mechanisms by which they contribute to AML are unknown. To unveil unique proprieties of MBI MYC mutant proteins, we systematically compared the cellular and molecular consequences of expressing similar oncogenic levels of wild type and MBI mutant MYC. We found that MBI MYC mutants can accelerate leukemia by driving unique transcriptional signatures in highly selected, myeloid progenitor subpopulations. Although these mutations increase MYC stability, they overall dampen MYC chromatin localization and lead to a cytoplasmic accumulation of the mutant proteins. This phenotype is coupled with increased translation of RNA binding proteins and nuclear export machinery, which results in altered RNA partitioning and accelerated decay of select transcripts encoding proapoptotic and proinflammatory genes. Heterozygous knockin mice harboring the germline MBI mutation Myc p.T73N exhibit cytoplasmic MYC localization, myeloid progenitors' expansion with similar transcriptional signatures to the overexpression model, and eventually develop hematological malignancies. This study uncovers that MBI MYC mutations alter MYC localization and disrupt mRNA subcellular distribution and turnover of select transcripts to accelerate tumor initiation and growth.

Proteomic and phosphoproteomic landscapes of acute myeloid leukemia.

We have developed a deep-scale proteome and phosphoproteome database from 44 representative acute myeloid leukemia (AML) patients from the LAML TCGA dataset and 6 healthy bone marrow-derived controls. After confirming data quality, we orthogonally validated several previously undescribed features of AML revealed by the proteomic data. We identified examples of posttranscriptionally regulated proteins both globally (ie, in all AML samples) and also in patients with recurrent AML driver mutations. For example, samples with IDH1/2 mutations displayed elevated levels of the 2-oxoglutarate-dependent histone demethylases KDM4A/B/C, despite no changes in messenger RNA levels for these genes; we confirmed this finding in vitro. In samples with NPMc mutations, we identified several nuclear importins with posttranscriptionally increased protein abundance and showed that they interact with NPMc but not wild-type NPM1. We identified 2 cell surface proteins (CD180 and MRC1/CD206) expressed on AML blasts of many patients (but not healthy CD34+ stem/progenitor cells) that could represent novel targets for immunologic therapies and confirmed these targets via flow cytometry. Finally, we detected nearly 30 000 phosphosites in these samples; globally, AML samples were associated with the abnormal phosphorylation of specific residues in PTPN11, STAT3, AKT1, and PRKCD. FLT3-TKD samples were associated with increased phosphorylation of activating tyrosines on the cytoplasmic Src-family tyrosine kinases FGR and HCK and related signaling proteins. PML-RARA-initiated AML samples displayed a unique phosphorylation signature, and TP53-mutant samples showed abundant phosphorylation of serine-183 on TP53 itself. This publicly available database will serve as a foundation for further investigations of protein dysregulation in AML pathogenesis.

The glycan-specific sulfotransferase (R77W)GalNAc-4-ST1 putatively responsible for peeling skin syndrome has normal properties consistent with a simple sequence polymorphisim.

Expanded access to DNA sequencing now fosters ready detection of site-specific human genome alterations whose actual significance requires in-depth functional study to rule in or out disease-causing mutations. This is a particular concern for genomic sequence differences in glycosyltransferases, whose implications are often difficult to assess. A recent whole-exome sequencing study identifies (c.229 C > T) in the GalNAc-4-ST1 glycosyltransferase (CHST8) as a disease-causing missense R77W mutation yielding the genodermatosis peeling skin syndrome (PSS) when homozygous. Cabral et al. (Genomics. 2012;99:202-208) cite this sequence change as reducing keratinocyte GalNAc-4-ST1 activity, thus decreasing glycosaminoglycan sulfation, as the mechanism for this blistering disorder. Such an identification could point toward potential clinical and/or prenatal diagnosis of a harmful medical condition. However, GalNAc-4-ST1 has minimal activity toward glycosaminoglycans, instead modifying terminal ß1,4-linked GalNAc on N- and O-linked oligosaccharides on specific glycoproteins. We find expression, processing and catalytic activity of GalNAc-4-ST1 completely equivalent between wild type and (R77W) sulfotransferases. Moreover, keratinocytes have little or no GalNAc-4-ST1 mRNA, indicating that they do not express GalNAc-4-ST1. In addition, loss-of-function of GalNAc-4-ST1 primarily presents as reproductive system aberrations rather than skin effects. These findings, an allele frequency of 0.004357, and a 10-fold difference in prevalence of CHST8 (c.299 C > T, R77W) across different ethnic groups, suggest that this sequence represents a "passenger" distributed polymorphism, a simple sequence variant form of the enzyme having normal activity, rather than a "driver" disease-causing mutation that accounts for PSS. This study presents an example for guiding biomedical research initiatives, as well as medical and personal/family perspectives, regarding newly-identified genomic sequence differences.

Functional Consequences of Mannose and Asialoglycoprotein Receptor Ablation.

The mannose receptor (ManR, Mrc1) and asialoglycoprotein receptor (ASGR, Asgr1 and Asgr2) are highly abundant endocytic receptors expressed by sinusoidal endothelial cells and parenchymal cells in the liver, respectively. We genetically manipulated either receptor individually or in combination, revealing phenotypic changes in female and male mice associated with changes in circulating levels of many glycoproteins. Both receptors rise and fall in response to progesterone during pregnancy. Thirty percent of Asgr2(-/-) and 65% of Mrc1(-/-)Asgr2(-/-) mice are unable to initiate parturition at the end of pregnancy, whereas Mrc1(-/-) mice initiate normally. Twenty five percent of Mrc1(-/-)Asgr2(-/-) male mice develop priapism when mating due to thrombosis of the penile vein, but neither Mrc1(-/-) nor Asgr2(-/-) mice do so. The half-life for luteinizing hormone (LH) clearance increases in Mrc1(-/-) and Mrc1(-/-)Asgr2(-/-) mice but not in Asgr2(-/-) mice; however, LH and testosterone are elevated in all three knockouts. The ManR clears LH thus regulating testosterone production, whereas the ASGR appears to mediate clearance of an unidentified glycoprotein that increases LH levels. More than 40 circulating glycoproteins are elevated >3.0-fold in pregnant Mrc1(-/-)Asgr2(-/-) mice. Pregnancy-specific glycoprotein 23, undetectable in WT mice (<50 ng/ml plasma), reaches levels of 1-10 mg/ml in the plasma of Mrc1(-/-)Asgr2(-/-) and Asgr2(-/-) mice, indicating it is cleared by the ASGR. Elevation of multiple coagulation factors in Mrc1(-/-)Asgr2(-/-) mice may account for priapism seen in males. These male and female phenotypic changes underscore the key roles of the ManR and ASGR in controlling circulating levels of numerous glycoproteins critical for regulating reproductive hormones and blood coagulation.

Modulation of mannose and asialoglycoprotein receptor expression determines glycoprotein hormone half-life at critical points in the reproductive cycle.

The rate at which glycoproteins are cleared from the circulation has a critical impact on their biologic activity in vivo. We have shown that clearance rates for glycoproteins such as luteinizing hormone (LH) that undergo regulated release into the circulation determine their potency. Two highly abundant, carbohydrate-specific, endocytic receptors, the asialoglycoprotein receptor (ASGR) and the mannose receptor (ManR) are expressed in the liver by parenchymal and sinusoidal endothelial cells, respectively. We demonstrate that the ManR mediates the clearance of glycoproteins such as LH that bear N-linked glycans terminating with ß1,4-linked GalNAc-4-SO4, as well as glycoproteins bearing glycans that terminate with Man. Steady state levels of mRNA encoding the ASGR and the ManR are regulated by progesterone in pregnant mice, reaching maximal levels on day 12.5 of pregnancy. Protein expression and glycan-specific binding activity also increase in the livers of pregnant mice. In contrast, ManR mRNA, but not ASGR mRNA, decreases in male mice at the time of sexual maturation. We show that levels of ManR and ASGR expression control the clearance rate for glycoproteins bearing recognized glycans. Thus, reduced expression of the ManR at the time of sexual maturation will increase the potency of LH in vivo, whereas increased expression during pregnancy will reduce LH potency until progesterone and receptor levels fall prior to parturition.

Ablation of GalNAc-4-sulfotransferase-1 enhances reproduction by altering the carbohydrate structures of luteinizing hormone in mice.

Luteinizing hormone (LH), produced in the anterior lobe of the pituitary, is a member of the hypothalamic-pituitary-gonad axis that is required for production of the sex hormones estradiol, progesterone, and testosterone. Perturbations in levels of hormones associated with this axis can result in defects in sexual development and maturity. LH bears unique N-linked carbohydrate units that terminate with a sulfated N-acetylgalactosamine structure (GalNAc-4-SO(4)) that mediates its clearance from the blood. To determine the significance of this terminal structure, we ablated the gene encoding the sulfotransferase responsible for sulfate addition to GalNAc on LH, GalNAc-4-sulfotransferase-1 (GalNAc-4-ST1) in mice. Mice lacking GalNAc-4-ST1 exhibited increased levels of circulating LH. In male mice, this resulted in elevated levels of testosterone and precocious maturation of testis and seminal vesicles. Female mice lacking GalNAc-4-ST1 demonstrated elevated estrogen levels and exhibited precocious sexual maturation and increased fecundity. Female mice remained in estrus for prolonged periods and produced almost 50% more litters per mouse than wild-type mice over the same period of time. Thus, sulfate modification of the terminal glycosylation of LH plays a central role in regulating the hypothalamic-pituitary-gonad axis in vivo.

A necessary and sufficient determinant for protein-selective glycosylation in vivo.

A limited number of glycoproteins including luteinizing hormone and carbonic anhydrase-VI (CA6) bear N-linked oligosaccharides that are modified with beta1,4-linked N-acetylgalactosamine (GalNAc). The selective addition of GalNAc to these glycoproteins requires that the beta1,4-N-acetylgalactosaminyltransferase (betaGT) recognize both the oligosaccharide acceptor and a peptide recognition determinant on the substrate glycoprotein. We report here that two recently cloned betaGTs, betaGT3 and betaGT4, that are able to transfer GalNAc to GlcNAc in beta1,4-linkage display the necessary glycoprotein specificity in vivo. Both betaGTs transfer GalNAc to N-linked oligosaccharides on the luteinizing hormone alpha subunit and CA6 but not to those on transferrin (Trf). A single peptide recognition determinant encoded in the carboxyl-terminal 19-amino acid sequence of bovine CA6 mediates transfer of GalNAc to each of its two N-linked oligosaccharides. The addition of this 19-amino acid sequence to the carboxyl terminus of Trf confers full acceptor activity onto Trf for both betaGT3 and betaGT4 in vivo. The complete 19-amino acid sequence is required for optimal GalNAc addition in vivo, indicating that the peptide sequence is both necessary and sufficient for recognition by betaGT3 and betaGT4.

N-linked oligosaccharides on the low density lipoprotein receptor homolog SorLA/LR11 are modified with terminal GalNAc-4-SO4 in kidney and brain.

Sorting protein-related receptor (SorLA/LR11) is a highly conserved mosaic receptor that is expressed by cells in a number of different tissues including principal cells of the collecting ducts in the kidney and neurons in the central and peripheral nervous systems. SorLA/LR11 has features that indicate it serves as a sorting receptor shuttling between the plasma membrane, endosomes, and the Golgi. We have found that a fraction of SorLA/LR11 that is synthesized in the kidney and the brain bears N-linked oligosaccharides that are modified with terminal beta1,4-linked GalNAc-4-SO(4). Oligosaccharides located in the vacuolar sorting (Vps) 10p domain (Vps10p domain) are modified with beta1,4-linked GalNAc when the Vps10p domain is expressed in cells along with either of two recently cloned protein-specific beta1,4GalNAc-transferases, GalNAcTIII and GalNAcTIV. Either of two sequences with basic amino acids located within the Vps10p domain is able to mediate recognition by these beta1,4GalNAc-transferases. The highly specific modification of oligosaccharides in the Vps10p domain of SorLA/LR11 with terminal GalNAc-4-SO(4) suggests that this unusual modification may modulate the interaction of SorLA/LR11 with proteins and influence their trafficking.

The asialoglycoprotein receptor clears glycoconjugates terminating with sialic acid alpha 2,6GalNAc.

Endogenous ligands have not, to date, been identified for the asialoglycoprotein receptor (ASGP-R), which is abundantly expressed by parenchymal cells in the liver of mammals. On the basis of the rapid clearance of BSA bearing multiple chemically coupled sialic acid (Sia)alpha2,6GalNAcbeta1,4GlcNAcbeta1,2Man tetrasaccharides (SiaGGnM-BSA) from the circulation, and the ability of the ASGP-R hepatic lectin-1 subunit to bind SiaGGnM-BSA, we previously proposed that glycoproteins modified with structures terminating with Siaalpha2,6GalNAc may represent previously unrecognized examples of endogenous ligands for this receptor. Here, we have taken a genetic approach using wild-type and ASGP-R-deficient mice to determine that the ASGP-R in vivo does indeed account for the rapid clearance of glycoconjugates terminating with Siaalpha2,6GalNAc. We have also determined that the ASGP-R is able to bind core-substituted oligosaccharides with the terminal sequence Siaalpha2,6Galbeta1,4GlcNAc but not those with the terminal Siaalpha2,3Galbeta1,4GlcNAc. We propose that glycoproteins bearing terminals Siaalpha2,6GalNAc and Siaalpha2,6Gal are endogenous ligands for the ASGP-R, and that the ASGP-R helps to regulate the relative concentration of serum glycoproteins bearing alpha2,6-linked Sia.

Differential expression and enzymatic properties of GalNAc-4-sulfotransferase-1 and GalNAc-4-sulfotransferase-2.

We have cloned two GalNAc-4-sulfotransferases, GalNAc-4-ST1 and GalNAc-4-ST2, that transfer sulfate to terminal beta1,4-linked GalNAc. In conjunction with the action of protein-specific beta1,4GalNAc-transferases, GalNAc-4-ST1 and GalNAc-4-ST2 account for the presence of terminal beta1,4-linked GalNAc-4-SO(4) on glycoproteins such as lutropin, thyrotropin (TSH), proopiomelanocortin (POMC), carbonic anhydratase-VI (CA-VI), and tenascin-R. GalNAc-4-ST1 and GalNAc-4-ST2 can be distinguished by their differing specificity for oligosaccharide acceptors and temperature lability. The differences in properties have been used to show that the levels of GalNAc-4-ST1 and GalNAc-4-ST2 activity are proportionate to the levels of their respective transcripts. Furthermore, we have found that both transcript and activity levels of GalNAc-4-ST1 and GalNAc-4-ST2 vary widely among different tissues indicating that the regulation of their expression differs. Differences in specificity and the regulation of expression may account for existence of two GalNAc-4-sulfotransferases in vivo. The highest levels of both GalNAc-4-ST1 and GalNAc-4-ST2 transcripts are present in the pituitary of the mouse with multiple cell types that produce glycoproteins terminating with GalNAc-4-SO(4). Genetic ablation of both GalNAc-4-ST1 and GalNAc-4-ST2 may be necessary to alter the pattern and/or extent of sulfate addition to terminal beta1,4GalNAc in tissues such as pituitary.

Regulation of lutropin circulatory half-life by the mannose/N-acetylgalactosamine-4-SO4 receptor is critical for implantation in vivo.

Lutropin (LH) directs ovulation and implantation by regulating the production of estrogen and progesterone. We have shown that the circulatory half-life of LH is controlled by the Man/GalNAc-4-SO4 receptor, which binds GalNAc-4-SO4 on LH oligosaccharides. The short half-life in conjunction with episodic release of LH from the pituitary accounts for the pulsatile rise and fall in circulating LH. Complete genetic ablation of the Man/GalNAc-4-SO4 receptor results in death in utero. Heterozygous female mice clear LH from the circulation more slowly and have smaller litters due to a reduction in the rate of implantation. This reduction is fully correctable by exogenous progesterone and estrogen, indicating that the rate of LH clearance is critical for the production of sufficient progesterone and estrogen for implantation. Thus, the Man/GalNAc-4-SO4 receptor regulates the endocrinological status of the female and is essential for an early event in embryonic development.