Miura O, Miura Y, Nakamura N, Quelle F W, Witthuhn B A, Ihle J N, Aoki N
First Department of Internal Medicine, Tokyo Medical and Dental University, Japan.
Blood. 1994 Dec 15;84(12):4135-41.
The receptor for erythropoietin (Epo) belongs to the cytokine receptor family and lacks a tyrosine kinase domain. However, it has been hypothesized that a tyrosine kinase, Jak2, associates with the membrane proximal cytoplasmic region of Epo receptor (EpoR) and mediates the growth signaling from the receptor through tyrosine phosphorylation of cellular substrates. To explore the growth signaling pathways from the EpoR, we analyzed substrates of tyrosine phosphorylation induced by Epo stimulation in cells expressing various mutant EpoRs. The vav proto-oncogene product was found to be tyrosine phosphorylated after Epo stimulation in cells expressing the wild-type EpoR or a truncated receptor, H mutant, that retains the growth signaling function. In these cells, Epo also induced the expression of a serine/threonine kinase, Pim-1. However, Epo stimulation did not have any effect on Vav or Pim-1 in cells expressing a mutant EpoR, PM4 mutant, inactivated by a point mutation, Trp282 to Arg, in the membrane proximal region, which abrogates the interaction with Jak2. On the other hand, both tyrosine phosphorylation of Vav and expression of Pim-1 were observed constitutively in cells expressing a mutant EpoR that is constitutively activated by a point mutation, Arg 129 to Cys, in the extracellular domain. Jak2 was also constitutively tyrosine phosphorylated and activated in cells expressing this mutant, which confirms the crucial role of Jak2 in growth signaling from the EpoR. Taken together, these observations suggest that the tyrosine phosphorylation of Vav and the expression of Pim-1 may play important roles in growth signaling from the EpoR.
促红细胞生成素（Epo）受体属于细胞因子受体家族，缺乏酪氨酸激酶结构域。然而，据推测，一种酪氨酸激酶Jak2与Epo受体（EpoR）膜近端的胞质区域相关联，并通过细胞底物的酪氨酸磷酸化介导来自该受体的生长信号。为了探究来自EpoR的生长信号通路，我们分析了在表达各种突变型EpoR的细胞中，Epo刺激诱导的酪氨酸磷酸化底物。在表达野生型EpoR或保留生长信号功能的截短受体H突变体的细胞中，发现vav原癌基因产物在Epo刺激后发生酪氨酸磷酸化。在这些细胞中，Epo还诱导了丝氨酸/苏氨酸激酶Pim-1的表达。然而，在表达因膜近端区域的点突变（Trp282突变为Arg）而失活的突变型EpoR（PM4突变体）的细胞中，Epo刺激对Vav或Pim-1没有任何影响，该点突变消除了与Jak2的相互作用。另一方面，在表达因胞外结构域的点突变（Arg 129突变为Cys）而组成性激活的突变型EpoR的细胞中，Vav的酪氨酸磷酸化和Pim-1的表达均持续存在。在表达这种突变体的细胞中，Jak2也持续发生酪氨酸磷酸化并被激活，这证实了Jak2在来自EpoR的生长信号中的关键作用。综上所述，这些观察结果表明，Vav的酪氨酸磷酸化和Pim-1的表达可能在来自EpoR的生长信号中发挥重要作用。
