Tumors are relatively more sensitive to methionine restriction than corresponding normal tissues, a phenomenon known as methionine auxotrophy. The current studies were undertaken to elucidate the molecular mechanisms for methionine auxotrophy of prostate cancer cells. We found that the activity of c-Jun N-terminal kinase 1 (JNK1) increased dramatically in response to methionine restriction. Over expression of wild type JNK1 by transient transfection enhanced apoptosis in response to methionine restriction, whereas over expression of a kinase inactive mutant of JNK1 protected PC-3 human prostate cancer cells from apoptosis. We conclude that JNK1 plays a critical role in signaling cancer cells to undergo apoptosis in response to methionine restriction.