At P ¼ 5 of hMSCs, 1 106 of hMSCs were mixed with 1000, 100,
or 10 of HeLa cells and seeded into T175 flasks (Corning). The cells
were maintained in 40 ml of Dulbecco's Modified Eagle's medium
(DMEM; Gibco) supplemented with 10% FBS, 50 U/ml penicillin,
and 50 mg/ml streptomycin. Upon reaching approximately 90%
confluence, the cells were washed with phosphate buffered saline
(PBS) and treated with 0.05% trypsin-EDTA solution (Gibco) for
detachment from the flasks. The cells were centrifuged at 450 g
for 5 min and suspended with the fresh culture medium. Aliquots of
the suspended cells were stained with Trypan Blue solution and
counted by Countess Automated Cell Counter (Invitrogen) according
to the manufacture's protocol. One million cells in the suspension were re-seeded into T175 flasks and cultured until the
next passage. This process was repeated by P ¼ 10. The growth rate
(Rn) at P ¼ n was calculated by the following equation