We searched for compounds that affect the cyclin D1/retinoblastoma protein pathway from the in-house natural product library using a recombinant adenovirus with the Cre/loxP-regulated cyclin D1 overexpression system, and we found that oligomycin inhibited cell growth more effectively in cyclin D1-overexpressing SW480 cells than in control SW480 cells. We also found that oligomycin reduced the expression levels of cyclin D1 protein and that this reduction is, at least in part, mediated by Thr-286 phosphorylation-dependent proteasomal degradation. The Journal of Antibiotics (2009) Keywords: cell growth inhibition; chemosensitivity; cyclin D1; pRB; E2F-1 INTRODUCTIONThe transition from G1 to S phase is regulated by cyclins, cyclindependent kinases (CDK) and their inhibitor proteins. The protooncogenic function of cyclin D1 has been attributed in part to its role in promoting cell cycle progression. Cyclin D1 is a key cell cycle regulator of the G1 to S phase progression. The binding of cyclin D1 to cyclin-dependent kinase (cdk4 or cdk6) leads to the phosphorylation of retinoblastoma protein (pRB) subsequently triggering the release of E2F transcription factors to allow the transcription of genes required for the G1 to S phase progression of the cell cycle. In several types of human cancer, there is abundant evidence that disturbances of specific cyclins, CDKs or CDK inhibitory proteins enhance tumor cell growth. [1][2][3] In particular, disruption of the regulation of the cyclin D1/pRB/E2F pathway, overexpression of cylin D1, loss of pRB function and amplification of E2F-1 are often observed in several types of cancer. 2,4,5 Although recent reports have shown that some antitumor drugs may affect the cyclin D1/pRB/E2F pathway, [6][7][8] it is unclear which types of antitumor drugs induced the disruption of this pathway. In this study, we searched for compounds that inhibit cell growth selectively in cyclin D1-overexpressing human colorectal carcinoma cell lines (SW480 and LoVo) using a recombinant adenovirus with the Cre/loxP-regulated expression system.