Fig. 10.

A new model of nucleus-mitochondria positive feedback loop formed by p90RSK-ERK5-NRF2 module-mediated PARP activation, and subsequent persistent SASP induction and coronary atherosclerosis formation after chemo-radiation.

(1) Chemo-radiation initiates mtROS production before p90RSK-ERK5 S496 phosphorylation, because initial phase of mtROS production induced by low dose of IR and DOX could not inhibit by p90RSK inhibitor and ERK5 S496A mutant (Fig. 5E and F).

(2) Next, chemo-radiation increased p90RSK-mediated ERK5 S496 phosphorylation, which subsequently inhibited ERK5 and NRF2 transcriptional activity, because mtROS specific inhibiotor (MitoTEMPOL) significantly inhibited low dose of both IR and DOX-induced p90RSK activity and ERK5 S496 phosphorylation. However, no change of ERK5 kinase activation detected by ERK5 TEY motif phosphorylation was detected (Supplementary Figs. 6H and I).

(3) The impairment of NRF2 transcriptional activity decreases anti-oxidant expressions including HO1 and Trx1 (Fig. 1H and N), which plays a crucial role of instigating persistent SASP status (i. senescence, ii. Inflammation, iii. mtROS production, iv. Attenuation of efferocytosis) (Fig. 1H-N), and primes MCs for enhancing the secondary insult of extracellular ROS-induced p90RSK-ERK5 S496 phosphorylation (Fig. 1H, I, and N).

(4) p90RSK-mediated ERK5 S496 phosphorylation and subsequent downregulation of NRF2 transcriptional activity and anti-oxidant expression is necessary for low dose of IR and DOX-induced telomere shortening (Fig. 1J and K, Fig. 3E, and Supplementary Figs. 5D, H, and K).

(5) Telomere DNA damage-induced PARP activation has been reported [37].

(6) PARP activation-mediated mitochondrial damage and cell death have been extensively studied [[34], [35], [36]]. For example, the involvement of the reduction of MKP1 induced by PARylated ATF-4 in ERK1/2/JNK/p38 activation-mediated mitochondrial dysfunction and cell death has been reported [55]. The implication of PARP activation on significant inhibition of both glycolysis and OXPHOS has been reported, and these conditions lead to severe and irreversible mitochondrial damage and cell death [35,56,59]. However, we found that low dose of IR and DOX did not induce any immediate cell death (Fig. 7B and C), and the depletion of ATP and NAD⁺ was recovered by PARP and p90RSK inhibitors (Fig. 7D–G), suggesting the reversibility of mitochondrial dysfunction. We referred this unique reversible form of mitochondrial dysfunction with severe ATP depletion as “mitochondrial (mt) stunning”.

(7) Another unique aspect of this mitochondrial stunning is its metabolically active status even under the depleted ATP condition. We found persistent mtROS production and the increase of succinate level after low dose of IR (Fig. 5A, E, F and G), and the induction of late phase of mtROS (but not early phase of mtROS production) (Fig. 5E and F) and succinate were p90RSK dependent (Fig. 6H). We also observed the necessity of complex II activity for mitochondrial stunning-induced mtROS production (Fig. 6F). Significant and persistent mtROS production without killing the cells would be important for chronic inflammation and unceasing SASP status, which are noted even long after the completion of cancer therapy as late effects.