Involvement of Sirt1-FoxO3a-Bnip3 axis and autophagy mediated mitochondrial turnover in according protection to hyperglycemic NRK-52E cells by Berberine.

Aberrant accumulation of dysfunctional mitochondria in renal cells during hyperglycemia signifies perturbed autophagy and mitochondrial turnover. This study aims to focus on the underlying mechanism involved in autophagy and mitophagy inducing efficacy of Berberine (isoquinoline alkaloid) in hyperglycemic NRK-52E cells. Berberine mediated protection to hyperglycemic cells prevented alteration in mitochondrial structure and function. Treatment with SRT-1720 (Sirt1 activator) enhanced autophagy, decreased apoptosis, upregulated expression of downstream moieties (FoxO3a and Bnip3) and ameliorated mitochondria related anomalies while nicotinamide (Sirt1 inhibitor) treatment exhibited reversal of the same. GFP reporter assay ascertained enhanced transcriptional activity of FoxO in Berberine-treated hyperglycemic cells, which was found to be correlated to increased expression of downstream protein Bnip3. Knocking down FoxO3a disrupted autophagy and stimulated apoptosis. N-acetyl-L-cysteine pre-treatment confirmed that generation of ROS intervened high glucose induced toxicity in NRK-52E cells. Berberine co-treatment resulted in differential expressions of key proteins involved in autophagy and mitophagy like LC3B, ATGs, Beclin1, Sirt1, Bnip3, FoxO3a and Parkin. Further, enhanced mitophagy in Berberine-treated cells was confirmed by transmission electron microscopy. Thus, our findings give evidence that the protection accorded by Berberine against hyperglycemia in renal proximal tubular cells (NRK-52E) involves instigation of Sirt1-FoxO3a-Bnip3 axis and autophagy mediated mitophagy induction.

Critical role of mitochondrial dysfunction and impaired mitophagy in diabetic nephropathy.

Mitochondrial dynamics play a critical role in deciding the fate of a cell under normal and diseased condition. Recent surge of studies indicate their regulatory role in meeting energy demands in renal cells making them critical entities in the progression of diabetic nephropathy. Diabetes is remarkably associated with abnormal fuel metabolism, a basis for free radical generation, which if left unchecked may devastate the mitochondria structurally and functionally. Impaired mitochondrial function and their aberrant accumulation have been known to be involved in the manifestation of diabetic nephropathy, indicating perturbed balance of mitochondrial dynamics, and mitochondrial turnover. Mitochondrial dynamics emphasize the critical role of mitochondrial fission proteins such as mitochondrial fission 1, dynamin-related protein 1 and mitochondrial fission factor and fusion proteins including mitofusin-1, mitofusin-2 and optic atrophy 1. Clearance of dysfunctional mitochondria is aided by translocation of autophagy machinery to the impaired mitochondria and subsequent activation of mitophagy regulating proteins PTEN-induced putative kinase 1 and Parkin, for which mitochondrial fission is a prior event. In this review, we discuss recent progression in our understanding of the molecular mechanisms targeting reactive oxygen species mediated alterations in mitochondrial energetics, mitophagy related disorders, impaired glucose transport, tubular atrophy, and renal cell death. The molecular cross talks linking autophagy and renoprotection through an intervention of 5'-AMP-activated protein kinase, mammalian target of rapamycin, and SIRT1 factors are also highlighted here, as in-depth exploration of these pathways may help in deriving therapeutic strategies for managing diabetes provoked end-stage renal disease.

Berberine induced modulation of PHLPP2-Akt-MST1 kinase signaling is coupled with mitochondrial impairment and hepatoma cell death.

Pleckstrin homology domain leucine-rich repeat protein phosphatase 2 (PHLPP2) has been known to exert tumor suppressive activity for long without much knowledge about its regulation and implications. Protein kinase B (Akt), Protein kinase C (PKC) and Ribosomal protein S6 Kinase (S6K) are known downtargets of PHLPP2, regulating a plethora of life processes viz. cell growth, survival and evasion from apoptosis. Present study decoded the crucial role of PHLPP2 in inducing apoptosis by its interaction with the newly found binding partner Mammalian sterile 20-like kinase 1 (Mst1) in berberine (BBR)-treated human hepatoma cells. HepG2 cells were exposed to (50 µM, 100 µM) berberine for different time intervals (18 h, 24 h). The results showed enhanced expression of PHLPP2 at transcriptional (2.13 fold, P < 0.01) and translational level (4 fold, P < 0.001), but not of PHLPP1, in berberine-treated HepG2 cells. Elevated expression of PHLPP2 was reported to inactivate Akt by dephosphorylating it on Ser473 (P < 0.001). As Akt is known to inhibit apoptotic effect of Mst1, we found that PHLPP2 mediated inactivation of Akt releases its repression from Mst1 leading to heightened phosphorylation of Mst1 on its activating site Thr183 (1.5 fold, P < 0.001). Consequently, coordination between PHLPP2, Akt and Mst1 stimulated downstream targets c-jun N-terminal kinase (JNK), Bim and Bak which are direct activators of pro-apoptotic proteins leading to cell death. Further, PHLPP2/Mst1 knock-down efficiently curtailed anti-proliferative effect of berberine by restoring the basal level of downstream anti-apoptotic proteins. In addition, pre-treatment of NAC (5 mM) showed that ROS generation was a primitive event to initiate activation of stress kinases. Thus, our findings suggest that PHLPP2, Akt and Mst1 constitute an autoinhibitory triangle which may be partly responsible for antiproliferative effect of berberine.

BH3-only protein BIM: An emerging target in chemotherapy.

BH3-only proteins constitute major proportion of pro-apoptotic members of B-cell lymphoma 2 (Bcl-2) family of apoptotic regulatory proteins and participate in embryonic development, tissue homeostasis and immunity. Absence of BH3-only proteins contributes to autoimmune disorders and tumorigenesis. Bim (Bcl-2 Interacting Mediator of cell death), most important member of BH3-only proteins, shares a BH3-only domain (9-16 aa) among 4 domains (BH1-BH4) of Bcl-2 family proteins and highly pro-apoptotic in nature. Bim initiates the intrinsic apoptotic pathway under both physiological and patho-physiological conditions. Reduction in Bim expression was found to be associated with tumor promotion and autoimmunity, while overexpression inhibited tumor growth and drug resistance as cancer cells suppress Bim expression and stability. Apart from its role in normal homeostasis, Bim has emerged as a central player in regulation of tumorigenesis, therefore gaining attention as a plausible target for chemotherapy. Regulation of Bim expression and stability is complicated and regulated at multiple levels viz. transcriptional, post-transcriptional, post-translational (preferably by phosphorylation and ubiquitination), epigenetic (by promoter acetylation or methylation) including miRNAs. Furthermore, control over Bim expression and stability may be exploited to enhance chemotherapeutic efficacy, overcome drug resistance and select anticancer drug regimen as various chemotherapeutic agents exploit Bim as an executioner of cell death. Owing to its potent anti-tumorigenic activity many BH3 mimetics e.g. ABT-737, ABT-263, obatoclax, AT-101and A-1210477 have been developed and entered in clinical trials. It is more likely that in near future strategies commanding Bim expression and stability ultimately lead to Bim based therapeutic regimen for cancer treatment.