Question:

The tyrosine kinase JAK3 (Janus kinase 3) contributes to the signalling of hematopoietic cell cytokine receptors. In lymphocytes and tumor cells JAK3 fosters cell proliferation and counteracts apoptosis. Accordingly, JAK3 inhibitors stimulate apoptosis of tumor cells. Replacement of the ATP coordinating lysine by alanine in the catalytic subunit results in the inactive ^K851AJAK3 mutant. The gain of function mutation of ^A568VJAK3 has been found in acute megakaryoplastic leukemia. The excessive nutrient demand of tumor cells requires up-regulation of transport processes at the cell membrane including the peptide transporter PepT1. Little is known about the signaling stimulating PepT1 transport in tumor cells. The present study explored, whether PepT1 is upregulated by JAK3.

Methods:

PepT1 was expressed in Xenopus oocytes with or without additional expression of JAK3 and electrogenic peptide (gly-gly) transport determined by dual electrode voltage clamp.

Results:

In PepT1-expressing oocytes, but not in oocytes injected with water, the dipeptide gly-gly generated an inward current, which was significantly increased following coexpression of JAK3. The effect was mimicked by ^A568VJAK3, but not by ^K851AJAK3. According to kinetic analysis JAK3 increased the maximal peptide induced current without significantly modifying the peptide concentration required to trigger halfmaximal peptide induced current (K_M). In PepT1 expressing oocytes, the peptide induced current was blunted by JAK3 inhibitor WHI-P154 (4-[(3'-Bromo-4'-hydroxyphenyl)amino]-6,7-dimethoxyquinazoline (11.2 µM).

Conclusions:

JAK3 is a powerful regulator of the peptide transporter PepT1.