SCCOHT tumors acquire chemoresistance and protection by interacting mesenchymal stroma/stem cells within the tumor microenvironment

Chemotherapeutic drug testing of SCCOHT-1 and BIN-67 tumor cells revealed synergistic growth-inhibition of >95% in vitro with a combination of foretinib and FK228. Application of this drug combination in vivo in NODscid mice induced SCCOHT-1(GFP) tumors was associated with similar to 6-fold reduction in tumor mass within 10 days, whereby synergistic effects of the two compounds remained undetectable compared to previous results with foretinib treatment alone. Histopathologic evaluation revealed a reduced vascularization and a lower amount of proliferating cells in the treated tumors. Surprisingly, a simultaneous significant accumulation of extracellular matrix structures with positive elastin-van Gieson staining was observed following foretinib/FK228 exposure. Expression analysis of treated animal tumors exhibited various changes including increased mouse transcript levels of elastin, laminin, and fibronectin. In parallel, markers for mesenchymal stroma/stem cells (MSC) including CD73 and CD90 were detectable in all mouse tumors suggesting a possible involvement of these cells in extracellular matrix restructure. Indeed, incubation of MSC with FK228 or foretinib/FK228 demonstrated morphologic alterations and enhanced expression of laminin and fibronectin. Moreover, a co-culture of MSC with lentiviral-labeled SCCOHT-1(GFP) cells contributed to protection of the tumor cells against FK228-mediated cytotoxicity. Furthermore, explant cultures of SCCOHT-1(GFP)-induced tumors acquired an increased resistance to FK228 and a combination of foretinib/FK228 in contrast to foretinib alone. Together, these data suggested that FK228-mediated extracellular matrix protein expression by MSC contributes to increased protection and enhanced resistance of SCCOHT tumors which could represent a more general mechanism of MSC during drug induced alterations of a tumor microenvironment.