Our immunohistochemical studies have demonstrated that degranulating mast cells are located primarily in the peri-tumoral fibrous tissue in a wide variety of human cancers. Moreover, we have shown that heparin (a proteoglycan that is produced exclusively by mast cells) inhibits the clonogenic growth of human breast cancer cells co-cultured with normal fibroblasts but not tumor cells cultured alone. Significantly, the lysate of a human mast cell line, HMC-1, did not have any detectable effect on the clonogenic growth of the co-cultured tumor cells. We believe that this is probably due to the low levels of heparin in these cells, but we cannot exclude the possibility that some other component of mast cells may have neutralized the inhibitory effect of heparin. Similarly, various doses of tryptase had no effect on clonogenic growth. Taken together, these results provide strong evidence that mast cells can suppress the growth of tumor cells through an indirect mechanism that involves heparin and fibroblasts adjacent to the tumor cells.
This conclusion is consistent with our previous reports that depletion of endogenous heparin results in accelerated tumor growth in mice [5, 14]. In specific, we previously showed that syngeneic tumor cells implanted into NDST-2 knockout mice grew faster than tumor cells implanted into wild-type mice that synthesized normal heparin [5, 14]. NDST-2 knockout mice are unable to synthesize mast cell heparin and express abnormal mast cells with severely reduced amounts of histamine and mast cell proteases , probably because highly anionic heparin is required to stabilize the cationic compounds histamine, tryptase and chymase. Moreover, enzymatic depletion of mast cell heparin by injection of heparinase enzyme into tumor implants also accelerated tumor growth  and increased blood clotting within the tumors. Based on the results of the current study, we now propose that the accelerated tumor growth that we observed in heparin-depleted mice could be attributable, at least in part, to the absence of heparin-mediated inhibition of the growth-promoting interaction between fibroblasts and adjacent tumor cells.
Our findings are important because they may help to explain the well known ability of heparin and its derivatives to inhibit the growth of primary and metastatic tumors in various animal models and in humans with cancer [22–29]. Although the exact mechanism of this anti-tumor effect mediated by heparin remains uncertain, a number of possible explanations  have been proposed, including inhibition of thrombin and fibrin formation, immune system modulation, blockade of tumor cell adhesion to platelets, inhibition of angiogenesis, and functional inhibition of selectin-mediated cell-cell interactions leading to metastasis . Heparins can also directly inhibit proliferation of various normal cell types, including smooth muscle cells and epithelial cells [reviewed in ]. Under some conditions, however, heparin actually stimulated the growth of epithelial cells . On the other hand, only a few studies have directly evaluated the effects of heparin on the proliferation of cancer cells [also reviewed in ], and the results of these studies are generally inconclusive. Therefore, we propose that another possible mechanism for the anti-tumor effect of heparin observed in vivo may be the disruption of the critical, growth- promoting interactions that are known to occur between tumor cells and adjacent fibroblasts [7–9].
Although this study was not designed to explore the molecular mechanism of how heparin disrupts the interaction between fibroblasts and tumor cells, one possible explanation comes immediately to mind. Based on our findings with FGF-7, we speculate that heparin may disrupt the interaction of heparin-binding growth factors such as FGF-7 with heparan sulfate proteoglycans that are produced by fibroblasts and that act as essential co-factors for the growth factors. This explanation is consistent with our recent observation that optimal clonogenic growth of breast cancer cells in vitro requires direct physical contact between fibroblasts and breast cancer cells . Moreover, previous reports from other laboratories have confirmed that human breast cancer cells are generally in close contact with fibroblasts that express abundant FGF-7 [18, 19]. Finally, various heparan sulfate proteoglycans such as syndecan-1 and glypican are known to be expressed by stromal cells in cancer and to modulate the mitogenic effects of multiple heparin-binding growth factors [33, 34]. In this regard, it is especially notable that heparin had no effect on FGF-7-mediated stimulation of mammary epithelial cells grown in a collagen gel matrix in the absence of fibroblasts . Hence, it is reasonable to propose that heparin might interfere with the natural binding of heparin-binding growth factors to heparan sulfate proteoglycans produced by fibroblasts. Clearly, additional studies will be needed to test this hypothesis.
The possible biological and clinical significance of our in vitro experimental findings with regard to the naturally occurring mast cells that we observed around tumors remain speculative. Nevertheless, our observation that mast cells are abundantly present within the fibrous regions of tumors raises the intriguing possibility that a growth inhibitory mechanism similar to the one that we observed in our in vitro studies could also be naturally operative within tumors in vivo. The possible connection between our in vitro experiments and naturally occurring tumors is further strengthened by the reports from other laboratories that peri-tumoral fibroblasts express abundant FGF-7 [18, 19] and that mast cell tryptase is intimately associated with the concurrent presence of heparin [11, 12].
We acknowledge, however, that this possibility seems to be at odds with the accumulating evidence that mast cells promote rather than inhibit tumor growth [1, 2]. In this regard, it should be emphasized that mast cell granules contain numerous biologically active compounds in addition to heparin, such as histamine, tryptase, and chymase. Some of these mast cell compounds and metabolites are likely to have significant effects on fibroblasts that remain to be defined. In addition, a number of other mediators from fibroblasts and mast cells could potentially influence tumor growth through a variety of mechanisms such as cyclooxygenase metabolites, heparanases, etc. The net effect on tumor growth, therefore, is likely to be the result of multiple complex interactions between the various components of mast cell granules and adjacent stromal cells such as vascular endothelium and fibroblasts. Indeed, our immunohistochemical studies also demonstrated close proximity of mast cells to peri-tumoral blood vessels as well as to fibroblasts. Consequently, it is entirely conceivable that the stimulatory effects of mast cells on angiogenesis or fibroblasts or other functions within tumors might outweigh the inhibitory effects mediated by mast cell heparin and fibroblasts.