Pim1 maintains telomere length in mouse cardiomyocytes by inhibiting TGFβ signalling

AIMS

Telomere attrition in cardiomyocytes is associated with decreased contractility, cellular senescence, and up-regulation of proapoptotic transcription factors. Pim1 is a cardioprotective kinase that antagonizes the aging phenotype of cardiomyocytes and delays cellular senescence by maintaining telomere length, but the mechanism remains unknown. Another pathway responsible for regulating telomere length is the transforming growth factor beta (TGFβ) signalling pathway where inhibiting TGFβ signalling maintains telomere length. The relationship between Pim1 and TGFβ has not been explored. This study delineates the mechanism of telomere length regulation by the interplay between Pim1 and components of TGFβ signalling pathways in proliferating A549 cells and post-mitotic cardiomyocytes.

METHODS AND RESULTS

Telomere length was maintained by lentiviral-mediated overexpression of PIM1 and inhibition of TGFβ signalling in A549 cells. Telomere length maintenance was further demonstrated in isolated cardiomyocytes from mice with cardiac-specific overexpression of PIM1 and by pharmacological inhibition of TGFβ signalling. Mechanistically, Pim1 inhibited phosphorylation of Smad2, preventing its translocation into the nucleus and repressing expression of TGFβ pathway genes.

CONCLUSION

Pim1 maintains telomere lengths in cardiomyocytes by inhibiting phosphorylation of the TGFβ pathway downstream effectors Smad2 and Smad3, which prevents repression of telomerase reverse transcriptase. Findings from this study demonstrate a novel mechanism of telomere length maintenance and provide a potential target for preserving cardiac function.

Hypoxia Prevents Mitochondrial Dysfunction and Senescence in Human c-Kit+ Cardiac Progenitor Cells

Senescence-associated dysfunction deleteriously affects biological activities of human c-Kit⁺ cardiac progenitor cells (hCPCs), particularly under conditions of in vitro culture. In comparison, preservation of self-renewal and decreases in mitochondrial reactive oxygen species (ROS) are characteristics of murine CPCs in vivo that reside within hypoxic niches. Recapitulating hypoxic niche oxygen tension conditions of ∼1% O₂ in vitro for expansion of hCPCs rather than typical normoxic cell culture conditions (21% O₂ ) could provide significant improvement of functional and biological activities of hCPCs. hCPCs were isolated and expanded under permanent hypoxic (hCPC-1%) or normoxic (hCPC-21%) conditions from left ventricular tissue explants collected during left ventricular assist device implantation. hCPC-1% exhibit increased self-renewal and suppression of senescence characteristics relative to hCPC-21%. Oxidative stress contributed to higher susceptibility to apoptosis, as well as decreased mitochondrial function in hCPC-21%. Hypoxia prevented accumulation of dysfunctional mitochondria, supporting higher oxygen consumption rates and mitochondrial membrane potential. Mitochondrial ROS was an upstream mediator of senescence since treatment of hCPC-1% with mitochondrial inhibitor antimycin A recapitulated mitochondrial dysfunction and senescence observed in hCPC-21%. NAD⁺ /NADH ratio and autophagic flux, which are key factors for mitochondrial function, were higher in hCPC-1%, but hCPC-21% were highly dependent on BNIP3/NIX-mediated mitophagy to maintain mitochondrial function. Overall, results demonstrate that supraphysiological oxygen tension during in vitro expansion initiates a downward spiral of oxidative stress, mitochondrial dysfunction, and cellular energy imbalance culminating in early proliferation arrest of hCPCs. Senescence is inhibited by preventing ROS through hypoxic culture of hCPCs. Stem Cells 2019;37:555-567.

Abstract 468: Mitochondrial Dysfunction and Senescence of Human Cardiac Progenitor Cells Are Prevented by Hypoxic Culture

Rationale: Therapeutic potential of c-Kit + cardiac progenitor cells (CPCs) is modest in autologous cell therapy clinical trials and could benefit from optimization to maximize functional activity. Early proliferation arrest limits expansion potential of human CPCs in vitro . In contrast, self-renewal is preserved and mitochondrial reactive oxidative species (mtROS) minimized within the CPC hypoxic niche in vivo . Expansion under oxygen tension comparable to the hypoxic niche may therefore improve CPC reparative function.

Hypothesis: Senescence of CPCs derived from heart failure patients is suppressed by dampening mtROS through long-term hypoxic culture.

Methods and Results: Environmental oxygen was maintained with constant hypoxia (1% O 2 ; H-CPCs) or atmospheric oxygen tension (20 - 21% O 2 ; N-CPCs) for isolation and expansion of CPCs from patients undergoing left ventricular assist device implantation. A 4-fold increase in clonogenesis and 73% fewer senescence-associated β-Gal-positive cells were found in H-CPCs. Oxidative stress and DNA damage were also reduced with 80% lower malondialdehyde content and significantly fewer nuclear γ-H2AX foci in H-CPCs. Accumulation of dysfunctional mitochondria was revealed in N-CPCs through functional metabolic analyses. Basal (-59%), maximal (-50%), and ATP production-coupled (-62%) oxygen consumption rates were significantly lower in N-CPCs, despite upregulation of electron transport chain subunits and 72% greater mitochondrial DNA content than H-CPCs. MtROS was elevated, and large mitochondria with swollen morphology were found by TEM in N-CPCs, confirming mitochondrial damage. Key indicators of mitochondrial function including NAD + /NADH ratio and autophagic flux were preserved in H-CPCs.

Conclusions: Development of mitochondrial dysfunction and senescence are consequences of normoxic CPC expansion. CPC senescence is delayed by preserving mitochondrial health and limiting oxidative damage through hypoxic culture.