Quercetin Differentially Modulates MTT-Derived Cellular Metabolic Activity and Apoptosis-Related Gene Expression in A549 and BEAS-2B Cells

Background/Objectives: Quercetin is a dietary flavonoid with potential anticancer activity, but comparative effects in malignant and non-malignant lung-derived epithelial cells remain incompletely characterized. Methods: A549 and BEAS-2B cells were exposed to quercetin (1–250 µM) for 24, 48, and 72 h, and MTT-derived cellular metabolic activity was measured. Mean normalized concentration–response curves from three independent biological experiments (n = 3) were fitted with four-parameter logistic models, and Greenhouse–Geisser-corrected cell line × concentration × time interactions were tested. Apoptosis-related gene expression was assessed by qRT-PCR after exposure to 45 µM quercetin for 48 h; 45 µM was not a discrete concentration in the MTT series and was close to the directly tested 40 µM condition. Results: Apparent MTT-derived IC50 values were 58.28 µM (95% CI 44.18–74.81) for A549 and 121.22 µM (89.50–164.66) for BEAS-2B at 48 h, and 2.73 µM (2.31–3.67) and 1.49 µM (1.27–1.77), respectively, at 72 h. Cell line × concentration interactions were significant at all exposure times, as was the cell line × concentration × time interaction; transcriptional profiles differed descriptively between cell lines. Conclusions: The fitted IC50 ordering reversed between 48 and 72 h, so a stable cancer-selective response is not supported. MTT reflects metabolic activity rather than cell death, and transcript changes do not establish functional apoptosis.