Description:
<jats:p>
Our recent study demonstrated that the histone methyltransferase Smyd1 positively regulates cardiac metabolism through transcriptional control of PGC-1α, a key regulator of mitochondrial energetics. However, it is largely unknown whether Smyd1 plays a role in adaptive responses to metabolic stress in cardiomyocytes. Here, we hypothesized that Smyd1 is required for cell survival during metabolic stress through maintaining energetic balance. To address this hypothesis, neonatal rat ventricular myocytes (NRVMs) were cultured in glucose-free media for 24 hours and real-time quantitative PCR was performed. We found that Smyd1 and its downstream target PGC-1α were upregulated during glucose starvation, concurrent with the significant increase of mRNA levels of
<jats:italic>PPARα</jats:italic>
and
<jats:italic>CPT1b</jats:italic>
, the key regulators of fatty acid β-oxidation (all p<0.05), indicating enhanced energetics. Control NRVMs were able to withstand 24-hr glucose starvation, maintaining mitochondrial inner membrane potential (MIMP) and the cell membrane integrity. However, siRNA-mediated
<jats:italic>Smyd1</jats:italic>
knockdown (siSmyd1) prior to glucose starvation led to massive cell death, evidenced by the dissipation of MIMP and the uptake of the normally cell-impermeable indicator YO-PRO. To better understand the mechanism of siSmyd-NRVMs vulnerability to glucose starvation, we examined the expression of genes involved in metabolism and apoptosis at the early stage of glucose starvation (3 hr), when autophagy was activated as evidenced by upregulation of LC3 and FoxO1. We found that the upregulation of PGC-1α, PPARα, CPT1b, as well as LC3 and FoxO1, adaptively induced by glucose starvation in control NRVMs, were all abolished by
<jats:italic>Smyd1</jats:italic>
knockdown, concomitant with a significant increase in gene expression of tumor necrosis factor receptor type 1-associated DEATH domain protein (Tradd). Moreover, RNA-seq analysis revealed that
<jats:italic>Smyd1</jats:italic>
-knockdown
<jats:italic>per se</jats:italic>
led to upregulation of the genes involved in regulation of apoptosis and cell death (i.e. Tradd, Casp8, Ripk3, Smad1). These data suggest that upregulation of Smyd1 in response to metabolic stress is a critical adaptive mechanism for cell survival, operating via enhancement of fatty acid metabolism, activation of autophagy, and suppression of cell death signaling.
</jats:p>