Manumycin inhibits farnesyltransferase and induces apoptosis of drug‐resistant interleukin 6‐producing myeloma cells

Summary.  Interleukin 6 (IL‐6) is an important survival and growth factor for myeloma cells and exerts its effects by activating several transduction pathways, including the Ras cascade. As farnesylation of the activated Ras oncogene product by protein farnesyltransferase (FTase) is a critical step for Ras functional activity, FTase has emerged as a potential target for the development of new anti‐cancer agents. Based on our previous demonstration that IL‐6‐producing myeloma cells are refractory to drug‐induced apoptosis, we have analysed the effect of manumycin, a natural FTase inhibitor, on IL‐6‐producing myeloma cells resistant to Fas‐, dexamethasone‐ and doxorubicin‐induced apoptosis. Treatment of myeloma cells with manumycin prevented cell proliferation and induced apoptosis. Western blotting experiments demonstrated that this effect was related to inhibition of the post‐translational Ras processing.Further analysis showed that manumycin‐induced apoptosis involved caspase‐3. Activation of caspase‐3, in fact, was observed in 6 h‐treated myeloma cells expressing Apo 2·7 antigen, the marker of early apoptosis, whereas their treatment with cell‐permeable DEVD‐fmk, that irreversibly inhibits caspase‐3 activity, prevented their apoptosis. Over‐expression of caspase‐3 was also demonstrated by reverse transcription–polymerase chain reaction. Finally, over‐expression of Bcl‐2 and its homologue Bcl‐xL was observed in manumycin‐treated cells as well as in control myeloma cells, implying that the Bcl‐2 family is not involved. FTase inhibitors may thus be proposed as a potential pharmacological weapon, as they block the Ras pathway and induce the apoptosis of drug‐resistant IL‐6‐producing myeloma cells.