leaf extract protects MIN6 pancreas-derived beta cells against the diabetic toxin streptozotocin: role of the NF-κB pathway.

BACKGROUND

. [; also known as ()] may have health-promoting activities, including resolution of diabetes mellitus (DM). This study evaluated whether and how a leaf extract of could directly protect pancreas-derived MIN6 cells against the diabetogenic toxin streptozotocin (STZ).

METHODS

Composition of the extract (by 100% methanol) was investigated using high resolution mass spectrometry, which revealed several compounds with beneficial bioactive efficacy. MIN6 cells were exposed to 50% lethal dose of STZ, with or without extract. Cell viability was assessed using the MTT method. Inflammatory activity of extract was assessed in MIN6 cells and macrophage-like RAW cells, and addition of TNF-α to combinations of and STZ were tested on MIN6 cell viability. The role of the NF-κB pathway in effects of STZ and were investigated using the proteosome inhibitor MG132.

RESULTS

Exposure of MIN6 cells to the extract (in concentrations that did not notably affect MIN6 cells: 5-15 mg/mL) was indeed able to minimize STZ-induced toxicity in MIN6 cells. Exposing macrophage-like RAW cells to extract (at 10-15 mg/mL) increased TNF-α gene expression, and this response was highly augmented by co-exposure to lipopolysaccharide (LPS, 1.0 mg/mL), indicating that extract contained inflammatory compounds too. The implication of this finding was investigated by exposing MIN6 cells to a subthreshold dose (100 ng/mL) of TNF-α, which 1) prevented the protective effect of the extract (10-15 mg/mL) against STZ toxicity, and 2) caused to become toxic to MIN6 cells even without the presence of STZ. TNF-α is known to activate the NF-κB pathway leading to cell death, however, NF-κB is also known to stimulate cell proliferation and survival. To investigate the relevance of the NF-κB pathway in our findings, we treated MIN6 cells with the proteasome inhibitor MG132 (at doses ≥0.2 μM), and observed that extract was no longer able to block STZ toxicity in MIN6 cells (p < 0.05), and to block CIAP2 expression, an anti-apoptotic target downstream from NF-κB.

CONCLUSION

These data suggest that a leaf extract of has anti-diabetogenic efficacy, which may depend on the integrity of the NF-κB pathway. This protective effect appears to be impeded in a pro-inflammatory environment.