Tyrosine phosphorylation regulates maturation of receptor tyrosine kinases

Area of Science:

• Oncology
• Molecular Biology
• Cell Biology

Background:

• Constitutive activation of receptor tyrosine kinases (RTKs), such as Fms-like tyrosine kinase 3 (FLT-3), is common in human cancers.
• Internal tandem duplications (FLT-3 ITD) are a frequent mutation in FLT-3 linked to acute myeloid leukemia.

Purpose of the Study:

• To investigate the maturation and cellular localization of FLT-3 ITD compared to wild-type FLT-3.
• To explore the role of protein-tyrosine phosphatases (PTPs) in regulating FLT-3 maturation and cell surface expression.

Main Methods:

• Comparative analysis of FLT-3 ITD and wild-type FLT-3 expression, glycosylation, and localization in cancer cells.
• Assessment of FLT-3 maturation and surface expression upon inhibition of FLT-3 kinase activity or coexpression with various PTPs.
• Investigation of FLT-3 interactions with ER-resident chaperones like calnexin.

Main Results:

• FLT-3 ITD predominantly exists as an immature, underglycosylated 130-kDa form, retained in the endoplasmic reticulum (ER), unlike mature, 150-kDa wild-type FLT-3.
• Coexpression of specific PTPs (SHP-1, PTP1B, PTP-PEST) promotes FLT-3 ITD complex glycosylation and cell surface localization.
• FLT-3 ITD retention in the ER is linked to inefficient folding, evidenced by enhanced complex formation with calnexin.
• PTP inhibition or suppression of PTP1B impairs wild-type FLT-3 maturation.

Conclusions:

• FLT-3 ITD maturation and cell surface expression are regulated by tyrosine phosphorylation and PTP activity.
• ER-resident PTPs play a crucial role in RTK maturation, potentially influencing the altered signaling of oncogenic RTK mutants.
• Targeting PTPs could represent a therapeutic strategy for cancers driven by mutated RTKs like FLT-3 ITD.