SMER28 Attenuates PI3K/mTOR Signaling by Direct Inhibition of PI3K p110 Delta

Abstract

SMER28 (Small molecule enhancer of Rapamycin 28) is an autophagy-inducing compound functioning by a hitherto unknown mechanism. Here, we confirm its autophagy-inducing effect by assessing classical autophagy-related parameters. Interestingly, we also discovered several additional effects of SMER28, including growth retardation and reduced G1 to S phase progression. Most strikingly, SMER28 treatment led to a complete arrest of receptor tyrosine kinase signaling, and, consequently, growth factor-induced cell scattering and dorsal ruffle formation. This coincided with a dramatic reduction in phosphorylation patterns of PI3K downstream effectors. Consistently, SMER28 directly inhibited PI3Kδ and to a lesser extent p110γ. The biological relevance of our observations was underscored by SMER28 interfering with InlB-mediated host cell entry of Listeria monocytogenes, which requires signaling through the prominent receptor tyrosine kinase c-Met. This effect was signaling-specific, since entry of unrelated, gram-negative Salmonella Typhimurium was not inhibited. Lastly, in B cell lymphoma cells, which predominantly depend on tonic signaling through PI3Kδ, apoptosis upon SMER28 treatment is profound in comparison to non-hematopoietic cells. This indicates SMER28 as a possible drug candidate for the treatment of diseases that derive from aberrant PI3Kδ activity.

The autophagy inducer SMER28 attenuates microtubule dynamics mediating neuroprotection

“…We could readily show an increase of LC3-II in treated cells indicative of increased autophagy (Supplementary Fig. S3 ) as expected 31 and autophagic flux as recently reported 45 . More importantly, we noted a significant alteration in the appearance of the microtubule cytoskeleton.…”

“…3 d,e) or autophagy-inducing conditions (Fig. 3 f,g) at concentrations with comparable cytotoxicity 31 , 45 , 52 , 53 (Supplementary Fig. S2 c) implied two aspects: Firstly, our observations indicated that the effects of SMER28-induced MT straightening are distinct from physical MT stabilization as induced e.g.…”

“…Earlier reports have shown that the speed of MT assembly and of CLIP-170 at MT plus ends is regulated by the phosphorylation of CLIP-170 10 – 12 . A recent report from our laboratory suggests that the molecular target of SMER28 might be class I PI3 kinases 45 . It is tempting to speculate, therefore, that SMER28 might specifically interfere with a cellular kinase involved in CLIP-170 phosphorylation 12 , 13 , 82 , and induces autophagy at the level of or downstream of PI3Ks 83 .…”