Fer kinase sustains the activation level of ERK1/2 and increases the production of VEGF in hypoxic cells.

Fer is a nuclear and cytoplasmic tyrosine kinase that is ubiquitously expressed in mammalian cells. Herein we show that Fer sustains a key signaling step in hypoxic cells. Knock-down of the Fer protein using a specific siRNA decreased the production of VEGF by the hypoxic cells. Conversely, ectopic expression of this kinase led to an elevated production of VEGF under hypoxia. At the molecular level, Fer was found to associate with ERK1/2 and this interaction was intensified under hypoxia. Moreover, Fer increased the activation levels of ERK1/2, and reducing the level of Fer, impaired the activation of ERK1/2 in hypoxic cells. Blocking the MEK-ERK1/2 signaling pathway with the MEK inhibitors U0126, or PD98059 led to the abrogation of ERK1/2 activity in hypoxic cells, an effect that was counteracted by Fer. Hence, Fer sustains the activation of ERK1/2 and increases the production of VEGF in hypoxic cells, without affecting the MEK-ERK signaling pathway.

Fer is a downstream effector of insulin and mediates the activation of signal transducer and activator of transcription 3 in myogenic cells.

Fer is an intracellular tyrosine kinase that associates with signal transducer and activator of transcription 3 (Stat3) in mammalian cells. However, the signaling pathways in which this interaction plays a functional role have not been revealed. Herein, we show that insulin up-regulates the levels of the fer mRNA and Fer protein in myoblasts that undergo insulin-induced myogenic differentiation. Moreover, insulin directs the interaction of Fer with members of the Janus family of tyrosine kinases (Jak)-Stat3 signaling pathway. Although in untreated cells Fer binds Jak1 and its tyrosine phosphorylation level is low, insulin treatment induced the phosphorylation of Fer and its dissociation from Jak1. The up-regulation of Fer and its dissociation from Jak1 were accompanied by an augmented association of activated Fer with Stat3 and by a concomitant increase in the tyrosine phosphorylation of Stat3. Dissociation of Fer and Jak1, as well as elevation in the level of Fer and in the tyrosine phosphorylation of Stat3, depended on the activity of phosphatidylinositol 3-kinase (PI3K) and was abolished by a PI3K inhibitor. However, Fer and Stat3 were only mildly affected by low concentrations of IGF-I, another activator of the PI3K pathway that can also induce myogenic differentiation. RNA interference directed toward the fer mRNA did not affect the cellular levels of Stat3 but led to a dramatic reduction in the tyrosine phosphorylation level of this transcription factor. Thus, Fer is a downstream effector of insulin and mediates the activation of Stat3 in myogenic cells.