Unfinished business: Incomplete laminin processing exposes a tumor target

The G45 domain of laminin-332 is cleaved in normal tissues but remains in squamous cell carcinomas. Targeting this domain using a G45 antibody reduced in vivo tumor growth and invasion and suggests a role for G45 as a new therapeutic target.Key words: laminin, extracellular matrix, cancer, squamous cell carcinoma, invasionLaminin-332 (previously known as laminin-5) is a trimeric extracellular glycoprotein important for the integrity of the basement membrane zone.¹ It is found at high levels in squamous cell carcinomas (SCC) and its expression correlates with increased tumor invasiveness and poor prognosis.² In a recent article in Cancer Research, Marinkovich and colleagues³ show that the G45 domain of laminin-332, normally removed by proteolytic processing, remains present in SCC tissue.To determine the function of the G45 domain, the authors expressed the wild-type α3 chain of laminin-332 or the α3 chain lacking G45 (ΔG45) in normal and transformed laminin-332-null keratinocytes. They then examined whether the phenotype of the G45 deletion mutant could be reverted by re-expressing the G45 domain peptide.When expressed in laminin-332-null keratinocytes, the wild-type form of the protein was found deposited in the extracellular matrix. Reduced matrix deposition was observed with the ΔG45 mutant. In addition, the ΔG45 mutant was associated with abnormal peripheral cell adhesions, in contrast to central α6 integrin-associated adhesions in the wild-type. ΔG45 cells detached more rapidly in trypsin and showed increased cell migration in vitro. Expression of the G45 domain peptide restored laminin-332 deposition and reversed the changes in cell adhesion and migration.After transformation with Ras and IκBα, laminin-null keratinocytes are tumorigenic and invade Matrigel. Expression of wild-type laminin-332 increases invasiveness in these cells. Invasion was also increased, but to a lesser degree, with the ΔG45 mutant expressing cells. ΔG45 cells had reduced MMP levels, and both invasion and MMP production were restored by expression of the G45 domain. Similar results were seen in an in vivo SCC model where reduced tumor growth and muscle invasion correlated with expression of the ΔG45 mutant relative to wild-type protein. Interestingly, activation of the PI3K pathway almost completely restored tumor growth and laminin-332 deposition in the ΔG45 mutant expressing cells. In vitro, ΔG45 cells show decreased Akt phosphorylation and phospho- ERK nuclear translocation. Both could be corrected by activating PI3K. Together, these results suggest that G45 may promote laminin- 332 deposition via a signaling mechanism rather than acting simply as a matrix anchor.As G45 promotes tumorigenesis in SCC and is absent in normal tissues, could it be a target for anti cancer therapy in vivo? The authors show that an antibody targeting G45 dramatically inhibited subcutaneous SCC tumor growth. Intriguingly, despite recognizing both G45 and native laminin-332 by western blot, the antibody caused no disruption of epithelial-mesenchymal integrity in normal tissues.This study has shown an important role for the G45 domain of laminin-332 in SCC tumorigenesis. Considering its absence in normal tissues, G45 represents a potential target for anti-cancer therapy in human SCC.