The Truncated Isoform of the Receptor Tyrosine Kinase ALK Generated by Alternative Transcription Initiation (ALK^ATI) Induces Chromatin Structural Changes in the Nucleus in a Kinase Activity-Dependent Manner

Abstract

Anaplastic lymphoma kinase (ALK) is a receptor-type tyrosine kinase that promotes cell growth upon stimulation with ligands such as midkine and pleiotrophin. Recently, a truncated isoform of ALK was identified in a variety of tumors. This isoform is expressed from a novel ALK transcript initiated from a de novo alternative transcription initiation (ATI) site in ALK intron 19 (referred to as ALK ATI ). ALK ATI , which consists of only the intracellular kinase domain, localizes to the nucleus as well as the cytoplasm. However, its nuclear role is unknown. In this study, we determined that ALK ATI promoted chromatin structural changes in the nucleus in a kinase activity-dependent manner. We found that expression of ALK ATI increased the level of the heterochromatin marker Lys9 tri-methylated histone H3. In addition, we demonstrated that ALK ATI phosphorylated the nuclear protein A-kinase anchoring protein 8 (AKAP8) and altered its subcellular localization from the insoluble fraction to the soluble fraction. These results suggest that ALK ATI induces chromatin structural changes and heterochromatinization through phosphorylation of AKAP8 in the nucleus.Key words anaplastic lymphoma kinase; nuclear tyrosine kinase; A-kinase anchoring protein 8; heterochromatinization; alternative transcription initiation; chromatin structural change Chromatin structure and organization are dynamically changed during cellular processes, such as proliferation, differentiation, DNA damage responses, and tumorigenesis.

The Transcriptional Roles of ALK Fusion Proteins in Tumorigenesis

“…EML4-ALK is a notable example, due to its relative predominance in NSCLC [119]. Recently, a novel isoform of ALK (ALK ATI ) (a truncated isoform, driven from an alternative transcription initiation (ATI) site in intron 19, encoding only the intracellular domain of ALK) has been described as being expressed in ~11% of melanomas and sporadically in other human cancers [120,121].…”

Targeting ALK in Cancer: Therapeutic Potential of Proapoptotic Peptides

“…Expression of ALK ATI allows growth factor-independent proliferation of murine BaF3 cells and promotes tumor formation from transfected NIH-3T3 cells in a mouse model. Similar to other ALK isoforms, ALK ATI proteins are activated by dimerization and autophosphorylation [40,41] (Figure 1A).…”

“…), and subcellular localization [35,40,42,43]. The nuclear ALK ATI isoforms are also phosphorylated, and have been shown to induce chromatin structural modifications, however, their substrates and function in the nucleus need to be further clarified [41]. The type of ALK-expressing tumor (ALCL, IMT, NSCLC, neuroblastoma, melanoma, etc.)…”