Yen A, Roberson M S, Varvayanis S, Lee A T
Department of Pathology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, USA.
Cancer Res. 1998 Jul 15;58(14):3163-72.
Retinoic acid (RA) activated the extracellular signal-regulated kinase (ERK) 2 mitogen-activated protein kinase (MAPK) of HL-60 human myeloblastic leukemia cells before causing myeloid differentiation and cell cycle arrest associated with hypophosphorylation of the retinoblastoma (RB) tumor suppressor protein. ERK2 activation by mitogen-activated protein/ERK kinase (MEK) was necessary for RA-induced differentiation in studies using PD98059 to block MEK phosphorylation. G0 growth arrest and RB tumor suppressor protein hypophosphorylation (which is typically associated with induced differentiation and G0 arrest), two putatively RB-regulated processes, also depended on ERK2 activation by MEK. Activation of ERK2 by RA occurred within hours and persisted until the onset of RB hypophosphorylation, differentiation, and arrest. ERK2 activation was probably needed early, because delaying the addition of PD98059 relative to that of RA restored most of the RA-induced cellular response. In contrast to RA (which activates RA receptors (RARs) and retinoid X receptors in HL-60 cells with its metabolite retinoids), a retinoid that selectively binds RAR-gamma, which is not expressed in HL-60 cells, was relatively ineffective in causing ERK2 activation. This is consistent with the need for a nuclear retinoid receptor function in RA-induced ERK2 activation. RA reduced the amount of unphosphorylated RAR-alpha, whose activation is necessary for RA-induced differentiation and arrest. This shifted the ratio of phosphorylated:unphosphorylated RAR-alpha to predominantly the phosphorylated form. Unlike other steroid thyroid hormone receptors susceptible to phosphorylation and activation by MAPKs, RAR-alpha was not phosphorylated by the activated ERK2 MAPK. The results thus show that RA augments MEK-dependent ERK2 activation that is needed for subsequent RB hypophosphorylation, cell differentiation, and G0 arrest. The process seems to be nuclear receptor dependent and an early seminal component of RA signaling causing differentiation and growth arrest.
维甲酸（RA）在导致人早幼粒细胞白血病HL - 60细胞的髓系分化和细胞周期停滞（与视网膜母细胞瘤（RB）肿瘤抑制蛋白的低磷酸化相关）之前，激活了细胞外信号调节激酶（ERK）2丝裂原活化蛋白激酶（MAPK）。在使用PD98059阻断MEK磷酸化的研究中，丝裂原活化蛋白/ERK激酶（MEK）对ERK2的激活是RA诱导分化所必需的。G0期生长停滞和RB肿瘤抑制蛋白低磷酸化（通常与诱导分化和G0期停滞相关）这两个推测受RB调节的过程，也依赖于MEK对ERK2的激活。RA对ERK2的激活在数小时内发生，并持续到RB低磷酸化、分化和停滞开始。ERK2的激活可能在早期是必需的，因为相对于RA延迟添加PD98059可恢复大部分RA诱导的细胞反应。与RA（其代谢产物类视黄醇在HL - 60细胞中激活RA受体（RARs）和类视黄醇X受体）相反，一种选择性结合HL - 60细胞中未表达的RAR - γ的类视黄醇，在引起ERK2激活方面相对无效。这与RA诱导ERK2激活中需要核类视黄醇受体功能一致。RA减少了未磷酸化RAR - α的量，其激活对于RA诱导的分化和停滞是必需的。这使磷酸化：未磷酸化RAR - α的比例主要转变为磷酸化形式。与其他易受MAPKs磷酸化和激活的类固醇甲状腺激素受体不同，RAR - α不会被激活的ERK2 MAPK磷酸化。因此，结果表明RA增强了MEK依赖的ERK2激活，这是随后RB低磷酸化、细胞分化和G0期停滞所必需的。该过程似乎依赖于核受体，并且是RA信号传导中导致分化和生长停滞的早期关键组成部分。
