Gelatinase B (MMP-9), but not its inhibitor (TIMP-1), dictates the growth rate of experimental thymic lymphoma

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Abstract
Dysregulation of metalloproteinase production at tumor sites contributes to the modification of local stromal tissue necessary for tumor development. Gelatinase B (matrix metalloproteinase‐9, MMP‐9) is one of the key enzymes that have been associated with the progression of several tumors. Paradoxically, MMP‐9 expression by tumor cells, most notably by lymphoma cells, is concomitant with the expression of its physiological inhibitor, TIMP‐1. Not only are both genes often co‐expressed in the most aggressive forms of lymphomas but also both are up‐regulated upon contact with stromal cells. Since TIMP‐1 is known to regulate growth in several cell types and some aggressive lymphoma cells express TIMP‐1 constitutively without MMP‐9, it is unclear whether the over‐expression of MMP‐9 is counterbalanced by TIMP‐1 and whether TIMP‐1 expression alone could favor the development of lymphoma. To gain further insight into the respective roles of MMP‐9 and TIMP‐1 in lymphoma, we generated lymphoma cell lines expressing constitutively high levels of MMP‐9 or TIMP‐1 and compared these cells for the ability to form thymic lymphoma in vivo. Moreover, we generated lymphoma cell lines expressing constitutively high levels of both MMP‐9 and TIMP‐1 to reproduce the net physiological balance resulting from the expression of both genes simultaneously and to determine which gene overrides the other. Our results show that mice injected with lymphoma cells expressing MMP‐9 constitutively developed thymic lymphoma more rapidly than those injected with control lymphoma cells. Over‐expression of TIMP‐1 alone did not significantly influence tumor progression of lymphoma nor did it delay the capacity of MMP‐9 to accelerate the development of thymic lymphoma. Int. J. Cancer 82:743–747, 1999.