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Flavopiridol Potently Induces Small Cell Lung Cancer Apoptosis during S Phase in a Manner That Involves Early Mitochondrial Dysfunction

Department of Medicine, Medical College of Virginia/Virginia Commonwealth University [S. G., G. W. K.], and McGuire Veterans Affairs Medical Center [J. L., P. C., G. S. W-G., G. W. K.], Richmond, Virginia 23249

Purpose: Accumulating evidence indicates that small cell lung cancer (SCLC) is defective in many of the regulatory mechanisms that control cell cycle progression. The purpose of this study was to determine the effects of flavopiridol, a pan-cyclin-dependent kinase inhibitor, on growth and apoptosis of SCLC cell lines.

Experimental Design: Cell growth was monitored using 3-(4,5dimethylthiazol-2yl)-2,5-diphenyl-tetrazolium bromide (MTT) and clonogenic assays. Induction of apoptosis was assessed using multiple assays, including flow cytometric determination of DNA content and mitochondrial membrane potential, terminal deoxynucleotide transferase-mediated dUTP nick end labeling (TUNEL), and Western blot analysis of procaspase 3 and poly(ADP-ribose) polymerase cleavage.

Results: Flavopiridol induced growth inhibition and cytotoxicity in multiple SCLC cell lines, with an IC₅₀ of 50–100 nM and an LD₅₀ of 150–200 nM in 72-h MTT assays. The cytotoxicity seen in the MTT assay proved to be apoptosis by several criteria. Interestingly, inhibition of caspase activation with the caspase inhibitor Boc-Asp(OMe)-CH₂F reduced TUNEL labeling by 40% but did not have any effect on the loss of mitochondrial membrane potential (detected as early as 4 h after drug exposure) or cytotoxicity in MTT assays. These results suggest that the primary event in flavopiridol-induced apoptosis involves induction of mitochondrial dysfunction. Cells synchronized with aphidicolin at the G₁-S border and treated with flavopiridol during S phase showed a marked increase in apoptosis compared with an asynchronous population or a population treated during G₂-M. Despite the increased apoptosis, a significant proportion of synchronized cells proceeded through S, G₂-M, and into G₁ phase in the presence of flavopiridol, demonstrating that a high-grade cell cycle arrest is not required for apoptosis. Cells synchronized at the G₁-S border treated with a short exposure to flavopiridol also showed more than a 10-fold decrease in clonogenicity compared with asynchronous cells treated identically.

Conclusions: Taken together, these data demonstrate that flavopiridol potently and selectively induces SCLC apoptosis preferentially during S phase, in a manner that involves early mitochondrial dysfunction without a requirement for a high-grade block to cell cycle progression. Furthermore, clonogenicity data suggests that prior S phase synchronization could be a highly effective way of enhancing the efficacy of bolus or short infusions of flavopiridol in the clinical setting.

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