Tumor microenvironment changes tumor cell sensitivity to action of energy metabolism modifiers

Taking into account differences in the bioenergetics between malignant and normal cells a search of antitumor drugs among the modifiers of tumor metabolism has a reasonable excuse. Earlier it was found that the cytotoxic/cytostatic action of sodium dichloroacetate (DCA) against Lewis lung carcinoma (LLC) cells in vitro was enhanced in the case of its combination with metformin (MTF).

To study the antitumor action of DCA in combination with MTF against LLC in vivo.

LLC/R9, a low metastatic variant of LLC cells, was used. LLC/R9 bearing mice were treated with MTF (at a total dose 0.15 g/kg b.w.) alone or in combination with DCA (at a total dose of 0.75 g/kg b.w.). LLC/R9 growth kinetics and the primary tumor growth and metastasis indices on the 23^rd day after tumor cell inoculation were evaluated by routine procedures. The state of the electron transport chain of mitochondria in tumor cells was studied using electron paramagnetic resonance. The content of lactate and glucose in blood plasma from mice was measured by enzymatic methods using biochemical analyzer. The number of tumor-associated macrophages (TAMs) and their distribution by M1/M2 phenotype were estimated by flow cytometry using antibodies against CD68 and CD206.

In LLC/R9-bearing mice treated with DCA in combination with MTF, tumor growth and metastasis indices, as well as circulating glucose and lactate levels were not significantly different from those in the control group. The level of nitrosylation of non-heme and heme proteins and the content of iron-sulfur centers in the mitochondria of tumor cells in LLC/R9-bearing mice administered with DCA in combination with MTF did not also differ from the corresponding indices in control. Instead, in tumors treated with MTF alone and in combination with DCA the total CD68⁺ TAMs count was almost 27% (p < 0.05) and 43% lower (p < 0.05) correspondingly than that in control, but this decrease was not accompanied by redistribution of CD68⁺/CD206+ and CD68⁺/D206- subsets.

DCA in combination with MTF, at least in doses applied, did not affect LLC/R9 growth and metastasis in vivo. The complete absence of an antitumor effect of DCA in combination with MTF was simultaneously associated with the absence of significant changes in the functional state of electron transport chain of mitochondria in tumor cells, circulating glucose and lactate levels, and the decrease of the TAMs amount in tumors. It suggests that the antitumor activity of DCA and MTF could be determined by both their local effects within a tumor and their multiple systemic impacts.