Page 1036 - Williams Hematology ( PDFDrive )

1010 Part VII: Neutrophils, Eosinophils, Basophils, and Mast Cells Chapter 66: Disorders of Neutrophil Function 1011

responsible for diverting newly synthesized proteins to granules as transition, but in contrast to C/EBPε, continue to increase as the cells
opposed to immediate (constitutive) secretion. Not all granule proteins mature to neutrophils. ELF-1 reappears at the metamyelocyte stage fol-
are equally efficiently directed to granules. Lysozyme is poorly retained lowed by C/EBPξ, c-Jun, and c-fos that are expressed at the band cell
during biosynthesis. This explains the high concentration of lysozyme stage and increase in content as the cells mature. 99
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in plasma. MPO is efficiently retained and the plasma level of MPO is
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consequently very low. A particularly interesting observation pertains MicroRNA
to α defensins. These are localized exclusively to azurophil granules, but MicroRNAs (miRNAs) are important regulators of protein synthesis.
their window of biosynthesis is very similar to that of lactoferrin, 107,100 In general, they bind to the 3′-end of mRNA and inhibit translation.
and defensins and lactoferrin are both controlled by the transcription Just like genes for granule proteins, mRNAs are expressed during mat-
factor C/EBPε (CCAAT/enhancer binding protein ε), which is abso- uration of neutrophils in the marrow depending on the stage of neu-
lutely required for biosynthesis of specific granule proteins. 109,110 The trophil maturation and can be classified into six groups, each with its
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absence of defensins from specific granules, despite an active biosyn- characteristic expression profile. So far, miRNAs have been shown to
thesis when other specific granule proteins are formed, is explained by regulate proteins of importance for proliferation but not (yet) expres-
a complete lack of sorting of defensins to granules in myelocytes. 99,100,107 sion of individual granule proteins. Expression of the myeloid-specific
Only defensins synthesized at the late promyelocytic stage are routed miRNA-223 increases during maturation of neutrophils in the marrow,
to granules, whereas defensins synthesized at the myelocyte stage are and after their release into the circulating. One of the targets of miRNA-
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secreted from cells after biosynthesis and is present in relatively high 223 is the Mef2c transcription factor. Mice that lack miRNA-223 expand
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concentrations in plasma. The defensins that are targeted to gran- granulopoiesis and the mature neutrophils mount an enhanced respi-
ules are processed to mature defensins, whereas the defensins that are ratory burst in response to phorbol myristate acetate (PMA), indicat-
secreted remain unprocessed. Processing of defensins removes a charge ing that miRNA-223 acts as a negative regulator of granulopoiesis and
neutralizing propiece, sorting of defensins and other granule proteins to neutrophil activation. miRNA-130a is highly expressed in myeloblasts
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granules may depend on their ability to interact with negatively charged and promyelocytes and targets SMAD4, which, despite a high level of
proteoglycans that are present in the matrix of granules. 112–114 Serglycin, mRNA in promyelocytes, is not expressed at the protein level, and the
an intracellular proteoglycan is present in Golgi and immature granules cells are consequently insensitive to growth inhibition by transforming
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of promyelocytes and disappears as the cells mature. Serglycin is abso- growth factor β (TGF-β)–induced growth inhibition. miRNA-130a also
lutely critical for confining a variety of mast cell proteins to the mast inhibits C/EBPε which induces exit from cell cycle and growth arrest
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cell granules. Granulocytes from mice with a targeted disruption of at the myelocyte stage. Hence, miRNA-130a seems important for the
the serglycin gene are morphologically normal and contain normal lev- expansion of myeloblasts, promyelocytes and early myelocytes. 131,132
els of granule proteins except elastase. CD63 was demonstrated to be
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involved in sorting of elastase to azurophil granules, but this may be
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indirectly via serglycin. An N-terminal sorting domain has been iden- FUNCTION OF INDIVIDUAL GRANULE
tified in serglycin and was shown to be essential for routing of serglycin PROTEINS AND THEIR ROLE IN OXIDATIVE AND
to mast cell granules. No common denominator has been identified
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that can fully explain why neutrophil proteins are sorted to granules. NONOXIDATIVE MICROBIAL KILLING
Perhaps the lack of efficient sorting to granules may not solely be taken Proteins of Azurophil Granules
as inefficiency, but may be a way to secure a desirable level of antibiotic Table 66–1 lists the physical-chemical and functional properties of neu-
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protein such as lysozyme and hCAP-18 in plasma which renders trophil granules.
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the myeloid cells of the marrow a major secretory organ. The protein MPO is a marker of azurophil granules. It is formed as
a 90-kDa precursor with an internal disulphide bridge that forms a link
Control of Neutrophil Granule Protein Expression between the 57- and 13.5-kDa subunits that are generated by the prote-
The biosynthesis of neutrophil granule proteins is controlled at the olytic processing that takes place during routing to granules. The heme
transcriptional and not the translational level (see Fig. 66–3). 98–100 Not group, which is necessary for the reduction-oxidation (redox) functions
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all transcription factors that are responsible for biosynthesis of granule of MPO, associates with the 90-kDa subunit. This seems to be a nec-
protein have been identified, and the role of an individual transcription essary prerequisite for subsequent processing. MPO reacts with H O ,
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2
2
factor may be difficult to identify from gene knockout studies as tran- formed by the NADPH oxidase, and increases the toxic potential of this
scription factors may work at multiple stages during myelopoiesis. The oxidant. Through oxidation of chloride, tyrosine, and nitrite, the hydro-
transcription factor PU.1 is essential for myelopoiesis because knock- gen peroxide (H O )-MPO system induces formation of hypochlor-
2
2
out mice do not form myeloid progenitors beyond myeloblasts 121,122 ; ous acid (HOCl), other chlorination products, tyrosine radicals, and
but this does not preclude PU.1 from regulating transcription of reactive nitrogen intermediates, each of which can attack the surface
individual granule proteins at a later stage of development. 123–126 membrane of microorganisms. 135,136 MPO may be found on endothelial
Figure 66–3 shows the profile of important myeloid transcription fac- cells during inflammation and can inactivate nitric oxide (NO). In
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tors during maturation of normal myeloid cells in the marrow in vivo. addition to the activities of MPO itself, MPO is known for the anti-
RUNX1 (AML-1), c-MYB, CASP, C/EBPα, C/EBPγ, GATA-1, and ELF- MPO autoantibodies that are characteristic of the pANCAs (perinuclear
1 gene products are all strongly expressed in the myeloblast and pro- antineutrophil cytoplasmic antibodies) that are found in vasculitides, in
myelocyte, and some of these are required for azurophil granule protein particular those that primarily affect kidneys. 138,139
expression. Then c-MYB, AML-1, GATA-1, and ELF-1 gene products BPI is a 55-kDa protein with high homology to the LPS-binding
are downregulated as the cells enter the myelocyte stage, heralded by a protein of plasma. It is organized into two largely symmetrical sub-
brisk and transient upregulation of C/EBPε to initiate expression of per- domains, one of which is responsible for the binding of LPS and for
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oxidase-negative granule proteins in agreement with the lack of spe- the antimicrobial activity against Gram-negative microorganisms. In
cific granules in C/EBPε –/– mice and with the observation of a C/EBPε contrast to LPS-binding protein, which presents endotoxin to CD14
mutation in patients with a rare specific granule deficiency. 109,110,127,128 and elicits a proinflammatory response, BPI binds LPS independent
PU.1, C/EBPβ, and C/EBPδ also appear at the promyelocyte myelocyte of CD14 and neutralizes the effects of LPS. A transgene expressing
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Kaushansky_chapter 66_p1005-1042.indd 1011 9/21/15 10:48 AM