MDL-800, an allosteric activator of SIRT6, suppresses proliferation and enhances EGFR-TKIs therapy in non-small cell lung cancer.

Sirtuin 6 (SIRT6), a member of the sirtuin family, is a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase that is involved in various physiological and pathological processes. SIRT6 is generally downregulated and linked to tumorigenesis in non-small cell lung carcinoma (NSCLC), thus regarded as a promising therapeutic target of NSCLC. In this study, we investigated whether MDL-800, an allosteric activator of SIRT6, exerted antiproliferation effect against NSCLC cells in vitro and in vivo. We showed that MDL-800 increased SIRT6 deacetylase activity with an EC50 value of 11.0 ± 0.3 µM; MDL-800 (10-50 µM) induced dose-dependent deacetylation of histone H3 in 12 NSCLC cell lines. Treatment with MDL-800 dose dependently inhibited the proliferation of 12 NSCLC cell lines with IC50 values ranging from 21.5 to 34.5 µM. The antiproliferation effect of MDL-800 was significantly diminished by SIRT6 knockout. Treatment with MDL-800 induced remarkable cell cycle arrest at the G0/G1 phase in NSCLC HCC827 and PC9 cells. Furthermore, MDL-800 (25, 50 µM) enhanced the antiproliferation of epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) in osimertinib-resistant HCC827 and PC9 cells as well as in patient-derived primary tumor cells, and suppressed mitogen-activated protein kinase (MAPK) pathway. In HCC827 cell-derived xenograft nude mice, intraperitoneal administration of MDL-800 (80 mg · kg-1 · d-1, for 14 days) markedly suppressed the tumor growth, accompanied by enhanced SIRT6-dependent histone H3 deacetylation and decreased p-MEK and p-ERK in tumor tissues. Our results provide the pharmacological evidence for future clinical investigation of MDL-800 as a promising lead compound for NSCLC treatment alone or in combination with EGFR-TKIs.

cProx1 immunoreactivity distinguishes progenitor cells and predicts hair cell fate during avian hair cell regeneration.

In birds, mature sensory hair cells are regenerated continually in vestibular epithelia and after damage in the auditory basilar papilla. Molecular mechanisms governing the cellular processes associated with hair cell regeneration are poorly understood. Transcription factors are critical regulators of cell proliferation and differentiation in developing tissues. We examined immunoreactivity for cProx1 during both ongoing and damage-induced hair cell regeneration in chickens. Homologues of this divergent homeobox transcription factor are required for cell cycle withdrawal and differentiation in several vertebrate and invertebrate tissues. In the mitotically quiescent basilar papilla, a population of resting progenitor cells (supporting cells) shows faint nuclear immunoreactivity for cProx1. When auditory hair cell regeneration is triggered by experimental damage, nuclear cProx1 immunolabel is highly elevated in approximately 50% of dividing progenitor cells. Shortly after cytokinesis, all sibling pairs show symmetric patterns of nuclear cProx1 labeling, but pairs with asymmetric labeling emerge shortly thereafter. Strongly immunoreactive cells acquire the hair cell fate, whereas cells with low nuclear immunoreactivity differentiate as supporting cells. By contrast, cProx1 is not detected in any dividing progenitor cells during ongoing regeneration in the utricle. However, nuclear cProx1 immunoreactivity becomes asymmetric in postmitotic sibling cells, and as in the basilar papilla, cells with elevated cProx1 label differentiate as hair cells. In conclusion, cProx1 immunolabeling varies across sensory epithelial progenitors and distinguishes early differentiating hair cells from supporting cells. cProx1 may regulate the proliferative or differentiative capacities of progenitor cells and specify hair cell fate in postmitotic cells during avian hair cell regeneration.

Expression of Prox1 defines regions of the avian otocyst that give rise to sensory or neural cells.

The simple primordium of the inner ear (otocyst) differentiates into many cell types, including sensory neurons and hair cells. We examined expression of the divergent homeobox transcription factor, cProx1, during otocyst development in chickens. Nuclear cProx1 protein is not evident in the otic placode but emerges in the otic cup by stage 12. At stage 16, cProx1-positive nuclei are scattered continuously throughout the neuroepithelium, from anteroventral to posteromedial. These labeled cells are neural precursors; they express betaIII-tubulin and migrate to the cochleovestibular ganglion between stages 13 and 21. By stage 18, two areas develop a dense pattern of cProx1 expression in which every nucleus is labeled. These areas emerge at the anterior and posterior extremes of the band of scattered cProx1 expression and express the sensory markers cSerrate1 and Cath1 by stage 23. Four discrete patches of dense cProx1 expression appear by stage 23 that correspond to the future superior crista, lateral crista, saccular macula, and posterior crista, as confirmed by immunolabeling for hair cell antigen (HCA) by stage 29. The remaining sensory epithelia display a dense pattern of cProx1 expression and label for HCA by stage 29. In the basilar papilla, nuclear cProx1 expression is down-regulated in most hair cells by stage 37 and in many supporting cells by stage 40. Our findings show that regions of the otocyst that give rise to neurons or hair cells are distinguished by their relative density of cProx1-positive nuclei, and suggest a role for cProx1 in the genesis of these cell types.