Lysyl oxidase in human pathology



LOX physiological role is well described since 1968 (27) and know in its details (28, 29). Lysyl Oxidase (protein-6-oxidase; EC 1.4.3.13) is the key-enzyme in elastin and collagen maturation. In fact it catalyzes the oxidative deamination of peptidyl-lysine and hydroxylysine to peptidyl -alpha- aminoadipic-delta-semialdehyde into elastin and collagen chains. The reaction follows this basic stechiometric scheme:

RCH2-CHNH2 = RCH2-CHO + NH3 + H2O2

The consequent aldheydes lead to a spontaneous condensation forming inter- and intra-chain cross-links into these extracellular matrix components. This process is also known as insolubilization of the collagen. This post-translational modification of the ECM molecules seems to have a very important role either for its structural aspects and also for a possible triggering of still unknown signal transduction pathways.
Rat, chicken and human Lysyl Oxidase cDNAs have been recently cloned (29-31). The nucleotidic sequences are extremely well conserved, mainly between human and rat. Even higher are the homologies among the resultant aminoacidic sequences. Minor divergencies are confined at the -NH2 termini. Consensus sequence for an highly hydrophobic signal peptide has been identified as well as putative peptidase sites were the pro-lysyl oxidase is cleaved in order to produce the 32 kDa active protein(31). It has been identified a metallo-binding consensus sequence. In fact biochemical studies on the enzyme showed its requirement for Cu++, which explains some of the pathologies where LOX is directly or indirectly involved. Most of the copper dis-metabolism symptoms as in Menkes and Wilson's diseases can be ascribed to an impaired LOX function. Furthermore LOX has to be considered as pathogenetic factor or co-factor in some polygenic and monogenic disorders like atherosclerosis, type IX Ehlers-Danlos syndrome, pseudoxanthoma elasticum(32-36)
Those listed above are syndromes mostly compatible with a partial or total deficiency of LOX activity. There are though pathologies suggesting instead an enhanced or inappropriate activity of LOX. Several reports have recently suggested a clear association between fibrosis and increased LOX activity. This has been described in several human chronic liver diseases (37) in rat experimental hepatic fibrosis (CCl4) (38), in hydiopatic and experimental lung fibrosis (bleomycin) (39) and adriamycin kidney fibrosis (9) These observations have also an important clinical relevance, since LOX has been proposed as marker at least for liver fibrosis, where it turned out being more specific and reliable than the traditional ones.
An other important cofactor for LOX enzymic activity is the pyrroquinoline quinone (PQQ), that is covalently bound to the LOX catalytic site(40). The PQQ is probably the mediator of the inhibiting activity by several mono- and di-amines on LOX. An important inhibitor of the enzyme is the ß-aminopropionitrile (BAPN) that is one of the agents that cause lathyrism in rodent. This syndrome, due to the blocking of the lysyl oxidase activity, summarizes the complex and diversified actions of this enzyme, resulting in laxity of joints and skin, reduced resistency to traction of bones and connectives in general and also serious neurological damage, referred to a degeneration of Purkinje cells (39, 41-43).
The localization of the enzyme is obviously extracellular where its substrates are. Nevertheless this does not implies that regulation of ECM cross-linking is LOX only biochemical role. It is not known if LOX has substrates other than collagen and elastin, although it has been proved that "in vitro" LOX is able to catalyze the oxidative deamination in different peptides and complex proteins (29, 44, 45). As far as it is concerned, LOX might have an intracellular substrate(s), which would mediate its ability to control the cell phenotype (anti-oncogenic role). In support to this view there is a report by Di Donato et al. (46) in which it is proved a direct anti-p21-Ha-Ras activity of a recombinant LOX protein in isolated Xenopus Laevis oocyte system. In this same direction go recent findings showing that lysyl oxidase seems to have also an important nuclear localization, beside the classical extracellular translocation and processing(47).

Lysyl oxidase and cell transformation


Evidence suggests an important role of lysyl oxidase (LOX) in cell phenotype control and/or tumor suppressor activity. In oncogene transformed cellular models it has been described that LOX is one of the main genes induced in concomitance with the reversion process (1-4). In particular it seems that LOX is downregulated in ras and ras-dependent transformed cells, so that it was first identified as a "ras recision gene" (rrg) (1-6)
LOX regulation in these models seems to be more than a general consequence of the phenotype reversion, since anti-sense LOX oligonucleotides where able to "retransform" the revertant cells(3). Moreover LOX expression and activity is proved to be regulated by several growth factors (7-11), like TGFß-1, IL-1, PG, IGF1, that put this protein in a critical pathway for cell growth and phenotype control. A stronger support to LOX putative tumor suppressor activity was provided by the finding that the interleukin induced tumor suppressor IRF1 is able to induce LOX expression directly acting on its promoter as well as the fact that IRF1 deficiency can be overcome by inducing LOX expression (12). Moreover, the chromosomic region where LOX is mapped to, 5q23.3-q31.2, is included in a deletion appearing in an high percentage of tumors(13-16) and mielodysplasic/leukemic syndromes (17-19), supporting the idea that the region contains a tumor suppressor gene. A similar deletion has been described also in some ovarian cancers (20, 21). Also it seems that activated ras plays an important pathogenetic role in these tumors(22-26). Backed up by these experimental data, we decided to study the role of LOX in ovarian neoplasia, its relationship with ras oncogenes and eventually the mechanism by which LOX might achieve its function.
Despite all the above intriguing findings, there are no hypothesis about the mechanism through which LOX might actually work as tumor suppressor.


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