Two concepts which help explain this observation have gathered experimental support. The first concept suggests that HA size may influence affinity to receptors; also, receptor complexes may cluster differently depending on HA size. The second concept, less well understood, is that size may affect HA uptake by the cell, and HA intracellular signaling may also modulate biological responses. An additional impediment on the elucidation of size-dependent HA signaling and biological effects is the confusing language that is used in scientific publications.
For scientific papers it may be better to simply define the size or range of sizes the investigators are working with, until a clear mechanistic understanding of fragment size classification emerges. In the following we will provide brief overviews of the mechanisms of HA synthesis and degradation, which lead to the generation of different fragment sizes; the current state of knowledge on HA size-dependent signaling; and a conclusive discussion of implications and future directions.
HA is uniquely synthesized at the plasma membrane rather than in the Golgi apparatus as is typical of other glycosaminoglycans GAGs [ 3 ]. Synthesis of mammalian HA is accomplished by a family of membrane-bound glycosyltransferases composed of three isozymes, hyaluronan synthases HAS 1, 2, and 3. Although the three HAS isoforms are similar and synthesize an identical product, they exhibit differences in half-life and stability, the rate of HA synthesis, and affinity for HA substrates, all of which potentially affect the regulation of HA synthesis and biological function [ 8 ].
Of particular interest is the finding that the three HAS enzymes synthesized HA of varying molecular masses.
Because of the biological differences exhibited by HA of differing polymer lengths, the innate biochemical and synthetic capabilities of the HAS enzymes may serve an important regulatory role in development, injury, and disease. The HAS genes exhibit different temporal patterns of expression during morphogenesis [ 4 ]. HAS2 is expressed throughout all stages of embryogenesis [ 9 ] and is considered to be the major hyaluronan synthase during development.
HAS1 and HAS3 expressions, on the other hand, are restricted to early and late stages of development, respectively, although expression overlaps with HAS2 [ 9 ]. At the tissue level, HAS gene expression and subsequent HA synthesis is regulated by a wide range of cytokines and growth factors reviewed in [ 2 , 11 ]. Dysregulation of HAS gene expression plays important roles in disease and injury, consistent with the biological roles of HA in disease progression, wound healing, and tissue regeneration.
In cancer, overexpression of hyaluronan synthases influences tumor growth, metastatic potential, and progression in several malignancies, including prostate, colon, breast, and endometrial cancers reviewed in [ 13 ]. Ectopic expression of HAS genes may also functionally alter the biological responses of cells to injury in vivo [ 14 , 15 ] reviewed more extensively below.
Taken together, available studies suggest that HA synthases are critical mediators in development, injury, and disease.
HYALs are also found in lower organisms, like bacteria, which catabolize HA to generate primarily disaccharides and in part facilitate mobility within tissue [ 2 ], and in leeches and crustaceans, which produce predominately tetra- and hexasaccharide fragments [ 18 ]. HYAL3 is even more limited in its expression pattern, with low levels in brain, liver, testis, and bone marrow [ 22 ]. Experimental details and confirmation of this model are still outstanding.
The significance of HYAL-mediated degradation of HA is demonstrated in mucopolysaccharide hyaluronidase deficiency, first described by [ 25 ]. This lysosomal storage disorder is now termed mucopolysaccharidosis IX [ 22 ], and subsequent characterization revealed that hyaluronidase activity is specifically abrogated through mutations in HYAL1. The disease has a relatively mild phenotype, limited to specific cell types fibroblasts and histiocytes and characterized by accumulation of HA, short stature, and multiple soft tissue masses in the joints.
A further demonstration of how HYALs contribute to developmental processes was shown with a mouse model of HYAL2 deficiency, which, similar to mucopolysaccharidosis IX in humans, was characterized by increased plasma concentrations of HA, and a relatively mild phenotype, in this case with mild craniofacial and hematological defects [ 26 ].
Interactions with other genetic loci are suspected, as HYAL2 deficiency in an outbred mouse resulted in much more severe cardiopulmonary pathology and early mortality compared to HYAL2 deficiency in an inbred genetic background [ 27 ]. Increased HYAL levels have also been found in several carcinomas, including prostate and bladder, as well as breast and head and neck cancer, and tend to correlate with more invasive and metastatic phenotypes reviewed in [ 21 ].
Cumulatively, this suggests that HYALs have distinctive roles in developmental and disease processes through the regulation of HA metabolism. Although many diverse biological responses have been attributed to HA and its various size polymers, interpretation of experimental findings, both in vivo and in vitro , may be complicated by, for example, low levels of bacterial contamination, which may independently activate key HA receptors. Recently, Muto et al.
These results were recapitulated by injection of tetrasaccharide oHA into the skin of wild type mice, resulting in increased migration of DCs out of the skin and functionally, a diminished CHS response. HYAL1 is only active at a low pH, which is unlikely to have been present in the uninflamed skin of these mice.
Therefore, the exact mechanism of HYAL1 activity in this case and in inflammation generally is still far from completely understood. However, it should be noted that much is unknown about hyaluronidase activity and function in vivo , and it is possible that posttranslational processing or other factors association with proteins or salts substantially change activity and specificity from what is found in vitro [ 29 , 30 ]. Aside from the specific enzymatic degradation pathways described above, HMW-HA can be fragmented by nonspecific pathways as well.
Reactive oxygen species ROS , including superoxide, hydrogen peroxide, nitric oxide and peroxynitrite, and hypohalous acids reviewed in [ 31 ] , are generated during the inflammatory response in sepsis, tissue inflammation, and ischemia-reperfusion injury and can degrade HA [ 31 ]. The most direct evidence for this has been accumulated in the synovial fluid, where inflammatory oxidation leads to degradation of native HMW-HA with resulting decrease in synovial fluid viscosity and cartilage degeneration, and in the airways, where ROS can degrade luminal epithelial HA [ 32 ].
No matter the origin, it seems clear that excessive generation of ROS contributes to a proinflammatory status by the oxidative degradation of hyaluronan. The corollary to this is that neutralization of ROS, for example, through superoxide dismutase, decreases HMW-HA degradation and inflammation [ 33 — 35 ].
Finally, the possibility may be entertained that ROS scavenging is in fact one of the physiological functions of HMW-HA as was proposed recently [ 36 ]; however, no experimental support for this hypothesis exists at this point. Beyond this hypothesis, it should also be noted that degradation of HMW-HA by ROS may also have salutary effects, such as the promotion of ciliary beating in the airways [ 37 ].
Thus, ROS may engage hyaluronan in a fine-tuned interaction rather than a monolithic response, and in fact hyaluronan may be involved in the emerging signaling pathway for these molecules [ 32 , 38 ]. As part of the extracellular matrix ECM , HA plays an important role in the maintenance of appropriate cell-cell communication. When ECM homeostasis is disrupted during pathological conditions tumor invasion, inflammation, tissue remodeling, etc. For example the interaction of CD44 and HA is strongly influenced by cell-specific factors, cell type, state of activation, and HA size.
Different sizes of HA have distinct effects on CD44 clustering: Long chains of HA possess multivalent sites for CD44 binding while oHA have only 1 or 2 binding sites [ 1 , 52 ] suggesting that oHA binding can act as an antagonist by replacing these interactions with low affinity, low valency interactions [ 50 ]. HA of different sizes can also signal through toll-like receptors TLRs , either independently or in concert with other HA receptors. Interestingly, receptor binding and activation by oHA can even differ depending on the number of disaccharides present.
Additionally, 6- to mer oHA bind monovalently to CD44, whereas larger polymers bind multivalently [ 60 ], which can affect clustering and signaling of this receptor. Size-dependent HA signaling can also differ according to cell type.
In aggregate, available evidence suggests that HA size influences receptor complex formation in a size-specific manner and thus modifies downstream signaling cascades. Photoaffinity labeling of the multidrug-resistance-related P-glycoprotein with photoactive analogs of verapamil.
There are a number of things that could be clearer - I usually keep wikipedia in the background to quickly lookup key phrases. Cancer Therapy ; 3: An added plus are the clinical correlations several per chapter as well as online access to the book with a special "scratch-off" code inside the front cover. Besides the problematic formatting, the book doesn't explain what sympathetic and parasympathetic are, despite putting both phrases in bold font to indicate their apparent importance in understanding histology. All chapters I've read so far are clearly delimited. Molec Cell Biochem ;
Molecular basis of preferential resistance to colchicine in multidrug-resistant human cells conferred by GlyVal substitution in P-glycoprotein. Tamai I, Safa AR. Competitive interaction of cyclosporins with the Vinca alkaloid binding site of P-glycoprotein in multidrug resistant cells.
Azidopine noncompetitively interacts with vinblastine and cyclosporin A binding to P-glycoprotein in multidrug resistant cells. Modulation of P-glycoprotein-mediated drug transport by alteration in lipid fluidity of rat liver canalicular membrane vesicles. N- p -azido-3[ I]iodo-phenethyl Spiperone binds to specific regions of P-glycoprotein and another multidrug binding protein, Spiperophilin, in human neuroblastoma cells.
Ogretmen B, Safa AR. Proteinase-3, a serine protease which mediates doxorubicin-induced apoptosis in the HL leukemia cell line, and is downregulated in its doxorubicin resistant variant. Oncogene ; 21, Taxol induces caspasedependent apoptosis. Biochem Pharmacol ; Hum Gene Ther ; Human beta-galactoside alpha-2,3-sialyltransferase ST3Gal III attenuated Taxol-induced apoptosis in ovarian cancer cells by downregulating caspase-8 activity.
Mol Cell Biochem ; Dynamic assessment of mitoxantrone resistance and modulation of multidrug resistance by PSC in multidrug resistant human cancer cells.