Bortezomib resistance in NSCLC

Molecular basis of intrinsic and acquired bortezomib resistance in non-small cell lung cancer

L.H.A.M. de Wilt (1), G. Jansen(2), Y.G. Assaraf (3), J. van Meerloo(4), J. Cloos(4), A.D. Schimmer(5), C.J. Kirk (6), G.J. Peters (1), F.A.E. Kruyt (7).

1Dept Med Oncol, 2Dept Rheumatol, VU University Medical Center, Amsterdam, 3 The Fred Wyszkowski Cancer Research Lab, Dept Biol, Technion-Israel Institute of Technology, Haifa, Israel, 4Dept Pediatr Oncol/Hematol, VU University Medical Center, Amsterdam, 5Princess Margeret Hospital, Ontario Cancer Institute, Toronto, Canada, 6Onyx Pharmaceuticals, San Franscisco, U.S.A., 7Dept Med Oncol, Univ Medical Center Groningen
Grant: KWF


The proteasome inhibitor bortezomib is currently used for the treatment of multiple myeloma, but has limited activity in solid tumors including non-small cell lung cancer (NSCLC). Bortezomib exerts its cytotoxic effect via inhibition of the catalytic activity of proteasome ?5 subunit. Herein we studied the molecular mechanisms underlying intrinsic and acquired bortezomib resistance in NSCLC cells. Various NSCLC cell lines displayed differential intrinsic sensitivities to bortezomib. High basal chymotrypsin- and caspase-like proteasome activities strongly correlated with intrinsic bortezomib resistance in these cells. Moreover, acquired bortezomib resistance via stepwise selection of these cells resulted in 12 to 119-fold bortezomib resistance. Cross-resistance to proteasome inhibitors that specifically target ?-subunits was found, but not to the novel non-competitive ?-subunit-specific proteasome inhibitor (5AHQ). Consistently, bortezomib-resistant cells required higher bortezomib concentrations to induce a G2/M arrest and apoptosis. Bortezomib-resistant cells exhibited increased levels of both constitutive ?-subunits and immuno-?-subunits. Moreover, sequence analysis of the bortezomib-binding pocket encoded by exon 2 of the PSMB5 gene revealed Ala49Thr, Met45Val and Cys52Phe mutations which have never been described for solid tumor cells. One cell line even revealed two PSMB5 mutations. These alterations did not affect basal catalytic proteasome activities, although bortezomib-resistant cells required higher bortezomib concentrations to achieve comparable inhibition of proteasome activity. Taken together, these findings establish that low basal levels of proteasome activity correlate with bortezomib sensitivity. Furthermore, acquired bortezomib resistance in NSCLC is associated with proteasome subunit overexpression and emergence of mutant ?5-subunits that likely compromise bortezomib binding. The use of ?-subunit-specific proteasome inhibitors can efficiently bypass this resistance.