**2.3 HA fragmentation and its role in tumourigenesis**

In addition to HAS1-3 expression, the amount and polymer size of HA are also affected by reactive oxygen species (ROS) and secreted hyaluronidases (HYALs), which fragment HA to various sizes. Significant levels of ROS can be generated during times of oxidative stress and these are considered critical in cancer initiation, promotion and progression (Karihtala *et al*., 2007). ROS are produced in response to extracellular stimuli such as bacterial infections and environmental toxins, but can also be produced by cellular metabolism (Yu *et al*., 2011). Five HYALs fragment HA: HYAL-1-3, PH-20 and HYAL-5. The HYALs differ in their cellular location and enzymatic properties. HYAL-1 and 2 are the major HYALs produced by somatic tissues whereas HYAL-3 is expressed mostly in bone marrow and testes. Both PH-20 and HYAL-5 expression are normally restricted to testes but PH20 is aberrantly expressed in BCA (Stern, 2008). HYAL-1 and 2 cooperate to degrade HMW HA in a coordinated fashion. HYAL-2, which is GPI anchored to the cell surface, degrades extracellular HA to fragments of 20 kDa, which are then taken up into endocytic vesicles. HYAL-1 present in the lysosome further degrades intracellular HA into tetrasaccharides (Tammi *et al*., 2001, Stern, 2008, Simpson and Lokeshwar, 2008). Coordinated breakdown of HA by HYALs increases the rate of HA metabolism and this appears to be an important factor in tumourigenesis (Veiseh and Turley, 2011). For example, co-expression of HAS3 and HYAL-1 increases the aggressiveness and spread of prostate cancer cells compared to expression of either alone (Bharadwaj *et al*., 2009). In BCA, HYAL-1 and HYAL-2 are often coordinately overexpressed compared to non-malignant breast tissue. Knockdown of HYAL-1, which is overexpressed in MDA-MB-231 and MCF-7 BCA lines, reduces tumour xenograft size (Tan *et al*., 2010).
