PMA activation of cells

Hi all,

I am interested in activation U251 cells with PMA and then check the
expression levels of a particular protein using RT-PCR. I went through a
few papers to get a protocol and found that cells were grown in complete
media till 70% confluence and then cells were grown in a serm free media
for 16-18 hrs prior to PMA activation. However I also found a few papers
in which they have not used any serum free media and directly activated
the cells using PMA.

Now can anyone tell me what can be real use of growing cells in serum
free media and also if this step is necessary or not?

Regards,
Anshul