Abstract LB-360: MYBPH, a novel transcriptional target of TTF-1/NKX2–1, inhibits ROCK1 and actomyosin assembly, and reduces cell motility and tumor metastasis (2011)

Abstract Lung cancer is the leading cause of cancer death in most economically developed countries, with lung adenocarcinoma the most prevalent form. The vast majority of lung cancer-related deaths is caused by invasion and metastasis, thus it is crucially important to elucidate the underlying mechanisms. We previously reported that lung adenocarcinomas are addicted to sustained expression of TTF-1 (also know as NKX2–1 and TITF1), a lineage-specific transcription factor required for branching morphogenesis and physiological lung functions, while others subsequently reached similar conclusions in studies that used genome-wide searches for focal genomic aberrations in lung adenocarcinomas. However, TTF-1/NKX2–1 expression is paradoxically known to be associated with favourable prognosis in lung adenocarcinoma cases. Emerging evidence indicates that non-muscle myosin II (NM II) members, especially NM IIA, are crucially involved in cancer cell migration, invasion, and metastasis via bivalent binding to actin filaments. Rho kinase 1 (ROCK1), a downstream effecter of RhoA, has been shown to be a major positive regulator of that process, which is thought to be executed through phosphorylation of myosin regulatory light chain (RLC) and subsequent unfolding of NM IIA into an assembly competent form capable of NM IIA dimer formation. In addition, ROCK1 phosphorylates LIM domain kinase (LIMK) and stabilizes actin filaments through inactivation of the actin-depolymerising factor cofilin. However, how actomyosin organization in non-muscle cells is regulated to counter-balance the positive regulatory function of ROCK1 remains to be elucidated. Herein, we report identification of myosin binding protein H (MYBPH) as a transcriptional target of TTF-1/NKX2–1, a lineage-survival oncogene in lung adenocarcinoma. MYBPH inhibits assembly competence-conferring phosphorylation of RLC as well as activating phosphorylation of LIMK. These are unexpectedly implemented through direct physical interaction of MYBPH with ROCK1 rather than with RLC, leading to inhibition of the ROCK1 kinase activity. In addition, MYBPH is shown to directly bind with non-muscle myosin heavy chain IIA (NMHC IIA), resulting in inhibition of NMHC IIA assembly. Thus, the present findings demonstrate that MYBPH plays multi-facetted roles in negative regulation of actomyosin organization, which we find results in reduction of cell motility, invasion, and metastasis. Finally, we also show that MYBPH is epigenetically inactivated by promoter DNA methylation in a fraction of lung adenocarcinomas abundantly expressing TTF-1/NKX2–1, which appears to be in accordance with its deleterious function for lung adenocarcinoma invasion and metastasis, as well as with the paradoxical association of TTF-1/NKX2–1 expression with favourable prognosis in lung adenocarcinoma patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-360. doi:10.1158/1538-7445.AM2011-LB-360