Star-PAP controls oncogene expression through primary miRNA 3'-end formation to regulate cellular proliferation and tumour formation

Star-PAP is a non-canonical poly(A) polymerase that is down regulated in breast cancer. While Star-PAP down regulation impairs target mRNA polyadenylation, paradoxically, we see up regulation of a large number of oncogenes on Star-PAP knockdown. Using two breast cancer cells (MCF7 with high Star-PAP, and MDA-MB-231 with negligible Star-PAP level), we discover that Star-PAP negatively regulates oncogene expression and subsequently cellular proliferation. This regulation is compromised with Star-PAP mutant of 3'-end processing function (serine 6 to alanine, S6A phospho-mutation). Concomitantly, xenograft mice model using MDA-MB-231 cells reveals a reduction in the tumour formation on ectopic Star-PAP expression that is ameliorated by S6A mutation. We find that Star-PAP control of target oncogene expression is independent of Star-PAP-mediated alternative polyadenylation or target mRNA 3'-end formation. We demonstrate that Star-PAP regulates target oncogenes through cellular miRNAs (miR-421, miR-335, miR-424, miR-543, miR-205, miR-34a, and miR-26a) that are down regulated in breast cancer. Analysis of various steps in miRNA biogenesis pathway reveals that Star-PAP regulates 3'-end formation and synthesis of primary miRNA (host) transcripts that is dependent on S6 phosphorylation thus controlling mature miRNA generation. Using mimics and inhibitors of two target miRNAs (miR-421 and miR-424) after Star-PAP depletion in MCF7 or ectopic expression in MDA-MB-231 cells, we demonstrate that Star-PAP controls oncogene expression and cellular proliferation through targeting miRNAs that regulates tumour formation. Our study establishes a novel mechanism of oncogene expression independent of alternative polyadenylation through Star-PAP-mediated miRNA host transcript polyadenylation that regulates breast cancer progression.

Proto-oncogene cSrc-mediated RBM10 phosphorylation arbitrates anti-hypertrophy gene program in the heart and controls cardiac hypertrophy

AIMS

RBM10 is a well-known RNA binding protein that regulates alternative splicing in various disease states. We have shown a splicing-independent function of RBM10 that regulates heart failure. This study aims to unravel a new biological function of RBM10 phosphorylation by proto-oncogene cSrc that enables anti-hypertrophy gene program and controls cardiac hypertrophy.

MATERIALS AND METHODS

We employ in vitro and in vivo approaches to characterise RBM10 phosphorylation at three-tyrosine residues (Y81, Y500, and Y971) by cSrc and target mRNA regulation. We also use isoproterenol induced rat heart and cellular hypertrophy model to determine role of cSrc-mediated RBM10 phosphorylation.

KEY FINDINGS

We show that RBM10 phosphorylation is induced in cellular and animal heart model of cardiac hypertrophy and regulates target mRNA expression and 3'-end formation. Inhibition of cSrc kinase or mutation of the three-tyrosine phosphorylation sites to phenylalanine accentuates myocyte hypertrophy, and results in advancement and an early attainment of hypertrophy in the heart. RBM10 is down regulated in the hypertrophic myocyte and that its re-expression reverses cellular and molecular changes in the myocyte. However, in the absence of phosphorylation (cSrc inhibition or phospho-deficient mutation), restoration of endogenous RBM10 level in the hypertrophic heart or ectopic re-expression in vitro failed to reverse cardiomyocyte hypertrophy. Mechanistically, loss of RBM10 phosphorylation inhibits nuclear localisation and interaction with Star-PAP compromising anti-hypertrophy gene expression.

SIGNIFICANCE

Our study establishes that cSrc-mediated RBM10 phosphorylation arbitrates anti-hypertrophy gene program. We also report a new functional regulation of RBM10 by phosphorylation that is poised to control heart failure.

Isolation and Culture of Primary Fibroblasts from Neonatal Murine Hearts to Study Cardiac Fibrosis

Cardiac fibroblasts are one of the major constituents of a healthy heart. Cultured cardiac fibroblasts are a crucial resource for conducting studies on cardiac fibrosis. The existing methods for culturing cardiac fibroblasts involve complicated steps and require special reagents and instruments. The major problems faced with primary cardiac fibroblast culture are the low yield and viability of the cultured cells and contamination with other heart cell types, including cardiomyocytes, endothelial cells, and immune cells. Numerous parameters, including the quality of the reagents used for the culture, conditions maintained during digestion of the cardiac tissue, composition of the digestion mixture used, and age of the pups used for culture determine the yield and purity of the cultured cardiac fibroblasts. The present study describes a detailed and simplified protocol to isolate and culture primary cardiac fibroblasts from neonatal murine pups. We demonstrate the transdifferentiation of fibroblasts into myofibroblasts through transforming growth factor (TGF)-β1 treatment, representing the changes in fibroblasts during cardiac fibrosis. These cells can be used to study the various aspects of cardiac fibrosis, inflammation, fibroblast proliferation, and growth.

Cultured Neonatal Murine Primary Myotubes as a Model to Study Muscle Atrophy

Besides genetic disorders, skeletal muscle atrophy mainly occurs as a consequence of underlying conditions such as prolonged inactivity, aging, and metabolic diseases, ultimately contributing to the risk of disability. Disturbances in cellular and molecular mechanisms involved in proteolysis and protein synthesis underpin muscle fiber shrinkage and decreased muscle fiber diameter. Stress-induced primary myotube culture is an established model for studying muscle atrophy. An in vitro model is an essential criterion in establishing preliminary data in a cell-autonomous manner that can later be validated using in vivo models. Here, we describe protocols for the isolation, culture, and differentiation of primary murine myotubes and the induction of myotube atrophy using dexamethasone, a synthetic corticosteroid. We further elaborate the procedure to validate degenerative parameters, such as assessing muscle fiber diameter, expression of muscle atrophy genes, and protein synthesis status under dexamethasone treatment. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Isolation and culture of primary myoblasts from rat or mouse pups Support Protocol 1: Preparation of coated tissue culture ware Support Protocol 2: Subculture of myoblasts Basic Protocol 2: Induction and assessment of myotube atrophy.

Sirtuin 6 Is a Critical Epigenetic Regulator of Cancer

Sirtuin 6 (SIRT6) is a member of the mammalian sirtuin family with deacetylase, deacylase, and mono-ADP-ribosyl-transferase activities. It is a multitasking chromatin-associated protein regulating different cellular and physiological functions in cells. Specifically, SIRT6 dysfunction is implicated in several aging-related human diseases, including cancer. Studies indicate that SIRT6 has a tumor-specific role, and it is considered a tumor suppressor as well as a tumor growth inducer, depending on the type of cancer. In this chapter, we review the role of SIRT6 in metabolism, genomic stability, and cancer. Further, we provide an insight into the interplay of the tumor-suppressing and oncogenic roles of SIRT6 in cancer. Additionally, we discuss the use of small-molecule SIRT6 modulators as potential therapeutics.