Phorbol esters rapidly stimulate amiloride-sensitive Na+/H+ exchange in a human leukemic cell line

Area of Science:

• Cell Biology
• Biochemistry
• Molecular Pharmacology

Background:

• Human leukemic HL-60 cells differentiate upon exposure to tumor-promoting phorbol esters.
• The sodium-hydrogen (Na+/H+) antiporter is potentially involved in cellular differentiation and proliferation.
• Understanding the initial molecular events triggered by phorbol esters is crucial.

Purpose of the Study:

• To investigate the effects of phorbol esters on Na+-dependent H+ efflux in HL-60 cells.
• To determine if Na+/H+ antiporter stimulation is an early event in phorbol ester-induced differentiation.

Main Methods:

• Utilized pH-stat titration to measure Na+-dependent H+ efflux from HL-60 cells.
• Tested the effects of various phorbol esters, their inactive alcohol form, and different extracellular ions (sodium, lithium, choline).
• Assessed the impact of extracellular proton activity and amiloride inhibition.

Main Results:

• Tumor-promoting phorbol diesters, but not the parent alcohol, rapidly (within 1 min) and reversibly stimulated Na+-dependent H+ efflux.
• Stimulation was dependent on extracellular sodium and proton activity, and was inhibited by amiloride.
• Observed half-maximal stimulation concentrations align with known phorbol ester binding and protein kinase C activation.

Conclusions:

• Stimulation of the Na+/H+ antiporter by phorbol esters is a rapid and primary event in HL-60 cells.
• This antiporter activation likely plays a key role in the initial cellular response to phorbol esters.
• Findings support the hypothesis that Na+/H+ antiporter activity is integral to phorbol ester-mediated cellular differentiation.