As
multiple myeloma tumors universally dysregulate
cyclin D genes we conducted high-throughput chemical library screens for compounds that induce suppression of
cyclin D2 promoter transcription. The top-ranked compound was a natural
triterpenoid,
pristimerin. Strikingly, the early transcriptional response of cells treated with
pristimerin closely resembles cellular responses elicited by proteosome inhibitors, with rapid induction of
heat shock proteins,
activating transcription factor 3 (ATF3), and CHOP. Enzymatic assays and immunoblotting confirm that
pristimerin rapidly (< 90 minutes) and specifically inhibits
chymotrypsin-like proteosome activity at low concentrations (< 100 nM) and causes accumulation of cellular
ubiquitinated proteins. Notably, cytotoxic
triterpenoids including
pristimerin inhibit
NF-kappaB activation via inhibition of
IKK alpha or
IKK beta, whereas proteosome inhibitors instead suppress
NF-kappaB function by impairing degradation of ubiquitinated I kappaB. By inhibiting both IKK and the proteosome,
pristimerin causes overt suppression of constitutive
NF-kappaB activity in myeloma cells that may mediate its suppression of
cyclin D.
Multiple myeloma is exquisitely sensitive to proteosome or
NF-kappaB pathway inhibition. Consistent with this,
pristimerin is potently and selectively lethal to primary myeloma cells (IC(50) < 100 nM), inhibits xenografted
plasmacytoma tumors in mice, and is synergistically cytotoxic with
bortezomib--providing the rationale for pharmaceutical development of
triterpenoid dual-function proteosome/
NF-kappaB inhibitors as
therapeutics for human
multiple myeloma and related
malignancies.