F-box protein FBXO41 suppresses breast cancer growth by inducing autophagic cell death through facilitating proteasomal degradation of oncogene SKP2.

F-box proteins form SCF (Cullin1, SKP1 and F-box-protein) ubiquitin ligase complexes to ubiquitinate cellular proteins. They play key role in several biological processes, including cell cycle progression, cellular signaling, stress response and cell death pathways. Therefore, deregulation of F-box proteins is closely associated with cancer progression. However, the role of most of the F-box proteins, including FBXO41, in cancer progression remains elusive. Here, we unravel the role of FBXO41 in cancer progression. We show that FBXO41 suppresses cancer cell proliferation and tumor growth by inducing autophagic cell death through an alternative pathway. Results revealed that FBXO41-mediated autophagic cell death induction is dependent on accumulation of cell cycle checkpoint protein p21. We found that FBXO41 increases the expression levels of p21 at the post-translational level by promoting the proteasomal degradation of SKP2, an oncogenic F-box protein. Mechanistically, FBXO41 along with p21 disrupts the inhibitory BCL2 (anti-apoptotic protein)-Beclin1 (autophagy initiating factor) complex of autophagy induction to release Beclin1, thereby inducing autophagy. Overall, the present study establishes a new FBXO41-SKP2-p21 axis for induction of autophagic cell death to prevent cancer growth, which could be explored to develop promising cancer therapeutics.

F-box protein FBXO41 plays vital role in arsenic trioxide-mediated autophagic death of cancer cells.

Arsenic trioxide (ATO), a potent anti-neoplastic drug, is known to prevent cancer cell growth through induction of autophagic cell death. However, importance of cellular factors in ATO-mediated autophagic cell death is poorly understood. In this study, using biochemical and immunofluorescence techniques, we show that F-box protein FBXO41 plays a critical role in anti-proliferative activity of ATO. Our study reveals the importance of FBXO41 in induction of autophagic death of cancer cells by ATO. Further, we show that the autophagic cell death induced by FBXO41 is distinct and independent of apoptosis and necrosis, showing that FBXO41 may play vital role in inducing autophagic death of apoptosis resistant cancer cells. Overall, our study elucidates the importance of FBXO41 in ATO induced autophagic cell death to prevent cancer progression, which could be explored to develop promising cancer therapeutic strategy.

FBXL20 promotes breast cancer malignancy by inhibiting apoptosis through degradation of PUMA and BAX.

Apoptosis is a programmed cell death that efficiently removes damaged cells to maintain tissue homeostasis. Defect in apoptotic machinery can lead to tumor development, progression, and resistance to chemotherapy. PUMA (p53 upregulated modulator of apoptosis) and BAX (BCL2-associated X protein) are among the most well-known inducers of apoptosis. It has been reported that expression levels of BAX and PUMA are controlled at the posttranslational level by phosphorylation. However, the posttranslational regulation of these proapoptotic proteins remains largely unexplored. In this study, using biochemical, molecular biology, flow cytometric, and immunohistochemistry techniques, we show that PUMA and BAX are the direct target of the F-box protein FBXL20, which restricts their cellular levels. FBXL20 directs the proteasomal degradation of PUMA and BAX in a protein kinase AKT1-dependent manner to promote cancer cell proliferation and tumor growth. Interestingly, inactivation of AKT1 results in activation of another protein kinase GSK3α/ß, which facilitates the proteasomal degradation of FBXL20 by another F-box protein, FBXO31. Thus, a switch between two signaling kinases AKT1 and GSK3α/ß modulates the functional activity of these proapoptotic regulators, thereby determining cell survival or death. RNAi-mediated ablation of FBXL20 results in increased levels of PUMA as well as BAX, which further enhances the sensitivity of cancer cells to chemotherapeutic drugs. We showed that high level expression of FBXL20 in cancer cells reduces therapeutic drug-induced apoptosis and promotes chemoresistance. Overall, this study highlights the importance of targeting FBXL20 in cancers in conjunction with chemotherapy and may represent a promising anticancer strategy to overcome chemoresistance.

Molecular insights of NADPH oxidases and its pathological consequences.

Reactive oxygen species (ROS), formed by the partial reduction of oxygen, were for a long time considered to be a byproduct of cellular metabolism. Since, increase in cellular levels of ROS results in oxidative stress leading to damage of nucleic acids, proteins, and lipids resulting in numerous pathological conditions; ROS was considered a bane for aerobic species. Hence, the discovery of NADPH oxidases (NOX), an enzyme family that specifically generates ROS as its prime product came as a surprise to redox biologists. NOX family proteins participate in various cellular functions including cell proliferation and differentiation, regulation of genes and protein expression, apoptosis, and host defence immunological response. Balanced expression and activation of NOX with subsequent production of ROS are critically important to regulate various genes and proteins to maintain homeostasis of the cell. However, dysregulation of NOX activation leading to enhanced ROS levels is associated with various pathophysiologies including diabetes, cardiovascular diseases, neurodegenerative diseases, ageing, atherosclerosis, and cancer. Although our current knowledge on NOX signifies its importance in the normal functioning of various cellular pathways; yet the choice of ROS producing enzymes which can tip the scale from homeostasis toward damage, as mediators of biological functions remain an oddity. Though the role of NOX in maintaining normal cellular functions is now deemed essential, yet its dysregulation leading to catastrophic events cannot be denied. Hence, this review focuses on the involvement of NOX enzymes in various pathological conditions imploring them as possible targets for therapies. SIGNIFICANCE OF THE STUDY: The NOXs are multi-subunit enzymes that generate ROS as a prime product. NOX generated ROS are usually regulated by various molecular factors and play a vital role in different physiological processes. The dysregulation of NOX activity is associated with pathological consequences. Recently, the dynamic proximity of NOX enzymes with different molecular signatures of pathologies has been studied extensively. It is essential to identify the precise role of NOX machinery in its niche during the progression of pathology. Although inhibition of NOX could be a promising approach for therapeutic interventions, it is critical to expand the current understanding of NOX's dynamicity and shed light on their molecular partners and regulators.

FBXO32 activates NF-κB through IκBα degradation in inflammatory and genotoxic stress.

In response to diverse stresses, the canonical NF-κB pathway gets activated primarily to protect the cells and maintain their genomic integrity. It activates the cell cycle checkpoints allowing the cells with limited damage to restore a normal life cycle. One of the key events in activation of the canonical NF-κB pathway is the selective proteasomal degradation of IκBα. It has been previously shown that F-box protein ßTRCP1 has limited role in directing the proteasomal degradation of IκBα during stress conditions. Here, we report another member of F-box family proteins, FBXO32, as a potential activator of NF-κB signaling during genotoxic stress and inflammatory response. Following genotoxic or inflammatory stress, FBXO32 is stabilized, which leads to polyubiquitination and proteasome mediated degradation of IκBα. We also found that FBXO32 is required for physiological regulation of IκBα levels in unstressed cells. Thus, we decipher the new role of FBXO32 in regulation of NF-κB signaling pathway.

A MicroRNA/Ubiquitin Ligase Feedback Loop Regulates Slug-Mediated Invasion in Breast Cancer.

The transformation of a normal cell to cancer requires the derail of multiple pathways. Normal signaling in a cell is regulated at multiple stages by the presence of feedback loops, calibration of levels of proteins by their regulated turnover, and posttranscriptional regulation, to name a few. The tumor suppressor protein FBXO31 is a component of the SCF E3 ubiquitin ligase and is required to arrest cells at G1 following genotoxic stresses. Due to its growth-suppression activity, it is underexpressed in many cancers. However, the molecular mechanism underlying the translational regulation of FBXO31 remains unclear. Here we show that the oncogenic microRNAs miR-93 and miR-106a repress FBXO31, resulting in the upregulation of Slug, which is involved in epithelial-mesenchymal transition and cell invasion. FBXO31 targets and ubiquitylates Slug for proteasomal degradation. However, this mechanism is repressed in breast tumors where miR-93 and miR-106a are overexpressed. Our study further unravels an interesting mechanism whereby Slug drives the expression of miR-93 and miR-106a, thus establishing a positive feedback loop to maintain an invasive phenotype. Together, these results establish the presence of interplay between microRNAs and the ubiquitination machinery, which together regulate cancer cell invasion.