All-trans retinoic acid (ATRA), a retinoid metabolite of vitamin A, regulates gene expression in a number of physiological processes, including embryogenesis, vision, growth, skin and lung maturation, cellular differentiation and homeostasis. In addition, nowadays it is a cardinal therapy against acute myeloid leukemia (AML), since it is able to induce terminal cell differentiation which is usually followed by malignant cell death. This compound was belieced last century to exert its effect genuinely through the nuclear retinoid receptor RAR-alpha. In the eary twentyonenth century, it became clear that it could bind to other cellular proteins. This way scientists discovered that rerinoic acid would prepare the cellular terrain for the subsequent genomic responses. One of these cellular protein is the protein kinase c-Raf1, which is pivotal for the activation of the mitogenic MAPK pathway. However, depending on the cellular context, MAPK may also drive cellular maturation and committment toward a differentiated status. Now Dr. Mengsu Yang from the Department of Biomedical Sciences, City University of Hong Kong, People’s Republic of China; and Dr. Andrew Yen from the Department of Biomedical Sciences, Cornell University, Ithaca, USA, have discovered a new step incell signaling.
They reported that using the HL-60 human non-APL AML model where ATRA causes nuclear enrichment of c-Raf that drives differentiation/G0-arrest, their research team now observe that a cell cycle inhibitor, roscovitine, enhanced nuclear translocation of certain traditionally cytoplasmic signaling molecules and differentiation after cycle arrest. The initial step invoves an ATRA-induced loss of pRB (retinoblastoma gene) phosphorylation, with cell cycle arrest associated with loss of RB-sequestered c-Raf, thereby coupling cell cycle arrest and increased availability of c-Raf to promote differentiation. Roscovitine is a cyclin-dependent kinase (CDKs) inhibitor that also enhanced the ATRA-induced nuclear enrichment of other signaling molecules traditionally perceived as cytoplasmic promoters of proliferation, but now known to promote differentiation. These included in particular: Src-like tyrosine kinases c-Lyn and c-Fgr; adaptor proteins like c-Cbl and SLP-76; a nucleotide exchange factor, Vav1; and the transcription factor IRF-1. ATRA induces these cells to undergo myeloid differentiation and G0 cell cycle arrest that depends on a sustained MAPK pathway signal with up-regulation and unanticipated translocation of c-Raf1 inside the cell nucleus.
In ATRA-induced differentiation of leukemia cells, Vav1 also interacts with PU.1, recruiting it to the promoter to transcriptionally activate expression of the CD11b marker. PU.1 is a transcription factor that coordinates the expression of the core network of T cell regulatory genes and human embryo develops also thanks to it. This enhances its interest in chemotherapy and, as the authors now report, which is an entirely novel mechanism for this drug that reveals novel therapeutic vulnerabilities as well as basic molecular mechanistic features of ATRA-induced differentiation of leukemic cells. When scientists genetically knocked down c-Lyn, it augmented certain roscovitine enhancements of ATRA effects on nuclear signaling and cell cycle regulatory molecules. This makes sense since this tyrosine kinase is involved in cell proliferation but not differentiation. Protein tyrosine phoshorilation is classicaly coupled with a mitogenic MAPK signaling in the absence of a pro-differentiating urge. Likely this scenario changes when differentiation is enhanced by molecules like ATRA. The novel activation of the signaling molecules and translocation to the nucleus during ATRA-induced differentiation is enhanced by roscovitine with concomitant enhancement of induced differentiation.
The research Team concluded in their paper, now published on Oncotarget: “The current study showed that roscovitine exhibits effects beyond its original presentation as a CDK inhibitor. Roscovitine may modulate nuclear molecules and enhance the therapeutic effects of ATRA in HL-60 cells. To the best of our knowledge, this study is the first to report that roscovitine potentiates ATRA in inducing myeloid leukemia cell differentiation, the mechanistic insights of which suggests new therapeutic targets to improve the clinical efficiency of ATRA to treat myeloid leukemia”.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
Rashid A et al., Yen A. Cell Signal. 2020 Feb 4:109555.
Wang S et al. Drug Des Devel Ther 2020 Jan; 14:297-308.
Shao X, Xiang S et al. Pharmacol Res. 2020; 151:104545.
Liang C et al. Food Chem Toxicol. 2019 Apr; 126:303-312.
Klobuch S et al. Front Pharmacol. 2018 Nov 27; 9:1380.