Phorbol esters stimulate non-transferrin iron uptake by K562 cells

Area of Science:

• Cell Biology
• Biochemistry
• Hematology

Background:

• Non-transferrin (non-Tf) iron transport in K562 cells exhibits unique characteristics compared to other cell types.
• K562 cells undergo megakaryocytic differentiation upon treatment with phorbol esters, a process involving protein kinase C activation.

Purpose of the Study:

• To investigate the effect of protein kinase C activation and subsequent megakaryocytic differentiation on non-Tf iron uptake in K562 cells.
• To characterize the changes in iron transport mechanisms, including Vmax, Km, and ferrireductase activity, in response to differentiation.

Main Methods:

• K562 cells were treated with phorbol esters (phorbol 12,13-dibutyrate) and/or Bryostatin-1 to induce differentiation.
• Cellular uptake of 55Fe was measured to quantify non-Tf iron transport.
• Ferrireductase activity and iron-binding site characteristics (Kd) were assessed.
• The role of protein synthesis was evaluated using actinomycin D.

Main Results:

• Phorbol ester treatment increased 55Fe uptake 4-6 fold in a time- and dose-dependent manner.
• Enhanced iron transport was associated with increased Vmax and apparent Km, indicating a change in transporter kinetics.
• Ferrireductase activity increased 5-10 fold, and saturable iron-binding sites appeared on the cell surface.
• Bryostatin-1, which does not induce differentiation, did not enhance iron uptake, and actinomycin D blocked the up-regulation.

Conclusions:

• Megakaryocytic differentiation of K562 cells, induced by phorbol esters, significantly enhances non-transferrin iron transport.
• The increased iron uptake is mediated by an increase in transporter Vmax and Km, elevated ferrireductase activity, and the expression of new cell-surface iron-binding proteins.
• These findings suggest that environmental factors promoting differentiation can control non-Tf iron transport and cell-surface protein expression in K562 cells.