Differential effects of methionine enkephalin on the growth of brain tumor cells.

The effect of methionine enkephalin on the growth of human brain tumor cells was investigated. The results show that this endogenous opioid has dual effects on tumor cell growth. This peptide exerted an inhibitory effect in SK-N-MC human neuroblastoma cell line; in contrast, in U-373 MG human astrocytoma cell line, the peptide showed a stimulatory effect. Treatment with naltrexone, an opioid receptor antagonist, also resulted in a similar alteration of tumor cell growth implicating that its action may be unrelated to opioid receptor blockade. These results suggest that in these tumor cell lines endogenous opioid systems may be involved in cell proliferation. Furthermore, these tumor cell lines may be useful model systems for the study of the signal transduction mechanisms of endogenous opioids.