7. DISCUSSION Over the last decades, many studies have shown numerous dietary constituents and nutraceuticals as cancer chemopreventive agents (Aggarwal et al, 2009); in fact, it has generally been accepted that they can suppress transformation, hyperproliferation, invasion, angiogenesis and metastasis of various tumors (Shanmugam et al, 2011). Because oxidative and inflammatory stress contributes to malignant transformation, dietary agents with antioxidative, anti-inflammatory and pro-apoptotic properties would be good candidates for preventing human malignancies (Gao et al, 2012; Huang et al, 2012). Taking into consideration that CF is a nutritional supplement whose antioxidant properties have been well documented in vitro (Benedetti et al, 2011), that the marine algae from which the organic and inorganic components of CF are extracted, has demonstrated a growth-inhibitory effect, both in vitro (Aslam et al, 2009) and in vivo (Aslam et al, 2012) and that CF is able to modulate O2 availability improving respiratory metabolism and mitochondrial activity (Ferrero et al, 2011), in the present thesis we wonder if CF could also affect cancer cell growth by inducing metabolism modifications and apoptosis. Initially, we performed assays to evaluate the effect of CF on cell growth and viability to determine the best experimental conditions without interferences caused by cytotoxicity. Hence, once identified the most efficient concentration (5 μl/ml) of CF in inhibiting cancer cell growth, we observed that CF treatment reduced cancer cell proliferation and viability in both leukemic cell lines and in all solid tumor cells tested (mesothelioma, melanoma, lung, breast and colon cancer). Cell growth reduction reached 50% in the U937 108
leukemic cell line as compared to untreated cells and percentages were even higher in some solid cancer cells, such as the mesothelioma MSTO (>80%) and the colon cancer HCT116 cell lines (about 90%), which were selected for further and detailed experiments, given their particularly high sensitivity to CF treatment. To investigate whether CF could affect healthy cell growth, human lymphocytes and fibroblasts were also seeded in the same experimental conditions. Data revealed no significant differences between untreated and treated cells, confirming that CF did not affect healthy cell viability and inhibited selectively only cancer cell growth. This inhibition was firstly revealed by the trypan blue exclusion method and then confirmed by cell viability assays, measuring mitochondrial dehydrogenase activity. Furthermore, the antiproliferative effect of CF was also examined in HFF (fibroblasts), MSTO (mesothelioma) and HCT116 (colon) cell lines by clonogenic assays, evaluating cell colony formation capacity. No visible change in healthy fibroblast growth was noted while no colony was observed in cancer cells, suggesting that CF significantly decreased cell growth only in the above mentioned cancer cell lines.
Moreover, to clarify whether CF was able to reduce cancer cell viability by promoting apoptotic cell death and cell cycle arrest, further experiments were conducted on leukemic cells and on two solid cancer cell lines, representing the most sensitive to CF treatment (the mesothelioma cell line MSTO and the colon carcinoma cell line HCT116). The capacity of CF to induce pro-apoptotic mechanisms was evaluated studying some typical apoptotic markers in the leukemic cells (cell morphological changes, caspase-3 activity and DNA fragmentation) and the expression of numerous proteins (caspase-3, PARP, p53, c-109
myc, p21 and p27, pAKT, AKT and Bcl-2) involved both in apoptotic and survival processes and in the cell cycle control, in the two solid tumor cell lines.
Caspase-3 is considered to be the most important effector of apoptosis and an early marker for both intrinsic and extrinsic pathways (Elmore, 2007) and our results, obtained using a colorimetric kit, show that CF treatment significantly stimulated caspase-3 activity in all leukemic cells at each experimental time point, as compared to untreated cells. In particular, caspases-3 activity was considerably and significantly increased in U937 cells after 72 hours of incubation with CF (2,6 fold higher than untreated cells). Moreover, cells undergoing apoptosis are characterized by the presence of morphological changes, such as contracted cell bodies, condensed chromatin and membrane bound apoptotic bodies (Elmore, 2007). To evaluate these morphological changes, the three leukemic cell lines were stained with MGG and observed using a light microscope at 400x magnification. Our results show that, upon CF treatment, the morphology observed was compatible with that of apoptotic cells. As a common hallmark of late-stage apoptosis, genomic DNA fragmentation was analyzed through gel electrophoresis. In this context, our data suggest that after 72 hours of incubation with CF an internucleosomal DNA cleavage (or DNA laddering) was observed in the leukemic cell lines, as compared to the respective untreated controls. The laddering is particularly evident in the CF-treated U937 cell line.
Taken together, these data lead us to suggest that the observed cancer cell growth inhibition was due to apoptosis induction in the three leukemic cell lines.