The tyrosine kinase sf-Stk and its downstream signals are required for maintenance of friend spleen focus-forming virus-induced fibroblast transformation.
Infection of erythroid progenitor cells by Friend spleen focus-forming virus (SFFV) leads to acute erythroid hyperplasia and eventually to erythroleukemia in susceptible strains of mice. The viral envelope protein, SFFV gp55, forms a complex with the erythropoietin receptor (EpoR) and a short form of the receptor tyrosine kinase Stk (sf-Stk), activating both and inducing Epo-independent proliferation. Recently, we discovered that coexpression of SFFV gp55 and sf-Stk is sufficient to transform NIH 3T3 and primary fibroblasts. In the current study, we demonstrate that sf-Stk and its downstream effectors are critical to this transformation. Unlike SFFV-derived erythroleukemia cells, which depend on PU.1 expression for maintenance of the transformed phenotype, SFFV gp55-sf-Stk-transformed fibroblasts are negative for PU.1. Underscoring the importance of sf-Stk to fibroblast transformation, knockdown of sf-Stk abolished the ability of these cells to form anchorage-independent colonies. Like SFFV-infected erythroid cells, SFFV gp55-sf-Stk-transformed fibroblasts express high levels of phosphorylated MEK, ERK, phosphatidylinositol 3-kinase (PI3K), Gab1/2, Akt, Jun kinase (JNK), and STAT3, but unlike virus-infected erythroid cells they fail to express phosphorylated STATs 1 and 5, which may require involvement of the EpoR. In addition, the p38 mitogen-activated protein kinase (MAPK) stress response is suppressed in the transformed fibroblasts. Inhibition of either JNK or the PI3K pathway decreases both monolayer proliferation and anchorage-independent growth of the transformed fibroblasts as does the putative kinase inhibitor luteolin, but inhibition of p38 MAPK has no effect. Our results indicate that sf-Stk is a molecular endpoint of transformation that could be targeted directly or with agents against its downstream effectors.
Jelacic
Tanya M
TM
Laboratory of Cancer Prevention, Building 469, Room 205, National Cancer Institute-Frederick, Frederick, MD 21702-1201, USA.
Thompson
Delores
D
Hanson
Charlotte
C
Cmarik
Joan L
JL
Nishigaki
Kazuo
K
Ruscetti
Sandra
S
D000818:Animals; D049109:Cell Proliferation; D002472:Cell Transformation, Viral / Q000502:physiology*; D005347:Fibroblasts / Q000821:virology*; D020868:Gene Silencing; D051379:Mice; D041681:NIH 3T3 Cells; D011494:Protein Kinases / Q000096:biosynthesis; D020794:Receptor Protein-Tyrosine Kinases / Q000502:physiology*; D013155:Spleen Focus-Forming Viruses / Q000502:physiology*; D014759:Viral Envelope Proteins / Q000502:physiology*
D014759:Viral Envelope Proteins; C078780:glycoprotein gp55, Friend spleen focus-forming virus; D011494:Protein Kinases; C080835:RON protein; D020794:Receptor Protein-Tyrosine Kinases