PI3K as Mediator of Apoptosis and Contractile Dysfunction in TGFβ(1)-Stimulated Cardiomyocytes

SIMPLE SUMMARY: TGFβ(1) is a growth factor that plays a major role in the remodeling process of the heart by inducing cardiomyocytes dysfunction and apoptosis, as well as fibrosis, thereby restricting heart function. TGFβ(1) mediates its effect via the TGFβ receptor I (ALK5) and the activation of SMAD transcription factors. But, TGFβ(1) is also known as activator of phosphoinositide-3-kinase (PI3K) via the non-SMAD signaling pathway. The aim of this study was to investigate whether PI3K is also involved in TGFβ(1)–induced cardiomyocytes apoptosis and contractile dysfunction. Pharmacological inhibition of PI3K with Ly294002 reduced TGFβ-induced apoptosis and reduced cell shortening. Inhibition of the PI3Kγ isoform also abolished the TGFβ effect on apoptosis and cell shortening. These data support a role for a PI3K and ALK5/SMAD pathway in TGFβ(1)-induced apoptosis and impaired cell shortening, which in part appears to be PI3Kγ-dependent. ABSTRACT: Background: TGFβ(1) is a growth factor that plays a major role in the remodeling process of the heart by inducing cardiomyocyte dysfunction and apoptosis, as well as fibrosis thereby restricting heart function. TGFβ(1) mediates its effect via the TGFβ receptor I (ALK5) and the activation of SMAD transcription factors, but TGFβ(1) is also known as activator of phosphoinositide-3-kinase (PI3K) via the non-SMAD signaling pathway. The aim of this study was to investigate whether PI3K is also involved in TGFβ(1)–induced cardiomyocytes apoptosis and contractile dysfunction. Methods and Results: Incubation of isolated ventricular cardiomyocytes with TGFβ(1) resulted in impaired contractile function. Pre-incubation of cells with the PI3K inhibitor Ly294002 or the ALK5 inhibitor SB431542 attenuated the decreased cell shortening in TGFβ(1)–stimulated cells. Additionally, TGFβ-induced apoptosis was significantly reduced by the PI3K inhibitor Ly294002. Administration of a PI3Kγ-specific inhibitor AS605240 abolished the TGFβ effect on apoptosis and cell shortening. This was also confirmed in cardiomyocytes from PI3Kγ KO mice. Induction of SMAD binding activity and the TGFβ target gene collagen 1 could be blocked by the PI3K inhibitor Ly294002, but not by the specific PI3Kγ inhibitor AS605240. Conclusions: TGFβ(1)-induced SMAD activation, cardiomyocyte apoptosis, and impaired cell shortening are mediated via both, the ALK5 receptor and PI3K, in adult cardiomyocytes. PI3Kγ specifically contributes to apoptosis induction and impairment of contractile function independent of SMAD signaling.