Glioma apoptosis induced by macrophages involves both death receptor-dependent and independent pathways.

Apoptosis of glioma may represent a promising intervention for tumor treatment. Macrophages are able to induce apoptosis in a number of tumor cells, including glioma. It is known that apoptosis of cells is executed on either a death receptor-dependent or independent pathway. Whether and how apoptosis of glioma cells induced by activated macrophages is involved in these two pathways simultaneously are not known. Using in vitro and in vivo experimental models, we investigated Bcl-2 system and Fas/FasL channel, representing the death receptor-dependent and independent pathways, respectively, in glioma cells treated with the supernatant from the activated macrophages, which was rich in tumor necrosis factor-alpha and interferon-gamma. We found that levels of Fas and FasL were up-regulated both in vitro and in vivo, accompanying an increase in the expression of caspase-8. The number of apoptotic cells was also increased significantly, although the percentage of death cells exceeded the number of tumor cells positive for Fas or FasL. It was also evident that the expression of Bax was increased, whereas the level of Bcl-2 was decreased, in glioma cells treated with the supernatant from the activated macrophages. The alteration of molecules related to both death pathways led to apoptosis of glioma and the inhibition of xenograft glioma growth in mice. Apoptosis of glioma induced by the activated macrophage is executed by way of both death receptor-dependent and independent pathways, and such an apoptosis-induced approach can effectively inhibit the growth of glioma in vivo.

Negative correlation between the ratio of Bax to Bcl-2 and the size of tumor treated by culture supernatants from Kupffer cells.

Kupffer cells play an important role in keeping liver from occurrence of tumors. Apoptosis is thought to be a major mechanism responsible for the anti-tumor function of Kupffer cells. Previous studies have mainly concentrated on the direct contact and interaction between Kupffer cells and tumor cells. The present experiment is to investigate the apoptotic pathway in tumor induced by culture supernatant from activated Kupffer cells. The levels of Bax, Bcl-2 and iNOS were analyzed in an in vivo mouse tumor model, which was treated with culture supernatant from activated Kupffer cells. The results showed that the expression of Bax significantly increased while the expression of Bcl-2 decreased when tumor cells were treated with culture supernatants from activated Kupffer cells. The alteration of Bax and Bcl-2 levels resulted in an increase in the ratio of Bax to Bcl-2, which had negative correlation with the size of tumor and positive correlation with the expression of iNOS. The expression of TNF alpha and the occurrence of apoptosis were also increased in tumor treated with culture supernatants from activated Kupffer cells, compared with those which received no treatment. In conclusion, culture supernatants from activated Kupffer cells were able to change the balance between Bax and Bcl-2 in favor of the former. The ratio of Bax to Bcl-2 is a useful index to evaluate tumor apoptosis induced by Kupffer cells. Our experiment also suggests that alteration of the ratio of Bax to Bcl-2 may result from increased levels of iNOS and TNF alpha.

Activation of Kupffer cells inhibits tumor growth in a murine model system.

Kupffer cells, a liver organ-specific macrophage, play an important role in preventing the development of malignant tumors. The mechanism responsible for their tumoricidal activities is not completely known. In our study, we established in vivo models involving a rat malignant cell line, rat Kupffer cells and tumor implantation in nude mice. A series of relevant in vitro experiments were also carried out to determine possible pathways. LPS-activated Kupffer cells produced significant amounts of NO, TNFalpha and IFNgamma. Malignant cells treated with either Kupffer cells or culture supernatant of the activated Kupffer cells had an increase in caspase-8 activity. Implanted tumors originated from malignant cells treated with either Kupffer cells or culture supernatant of the activated Kupffer cells grew much smaller than those from malignant cells without treatment or treated with control supernatants. The alteration of anti-apoptotic Bcl-2 was inversely associated with the change of pro-apoptotic caspase-8 and their levels in the tumor tissues matched the size of the tumors and treatments they received. It appeared that the above changes resulted in an increase in cellular DNA damage and apoptosis seen in malignant cells. Therefore, Kupffer cells execute their anti-tumor effect via increasing the production of NO, TNFalpha and IFNgamma and these cytotoxic molecules inhibit the growth of tumor by damaging cellular DNA and inducing apoptosis that was featured by downregulation of Bcl-2 but upregulation of caspase-8.