Lack of impact of the loss of constitutive folate receptor α expression, achieved by RNA interference, on the activity of the new generation antifolate pemetrexed in HeLa cells

Pemetrexed [PMX (Alimta)] is a new generation antifolate with activity in a variety of solid tumors. It is an excellent substrate for most folate transporters, notably the reduced folate carrier (RFC) and folate receptor (FR)-alpha. The role of FR-alpha in PMX pharmacological activity is uncertain. Whereas high-level expression may enhance the activity of this agent, it is not clear what role constitutive levels of this transporter contribute to PMX activity. In this study, constitutive levels of FR-alpha expression were abolished by small interfering RNA-induced silencing in HeLa cells and RFC-null HeLa R5 cells as confirmed by Northern blotting, immunohistochemistry, and cell surface binding. PMX growth inhibition was unchanged in HeLa and R5 cells in the absence of FR-alpha expression. Loss of FR-alpha expression did not decrease net accumulation of PMX in either wildtype or RFC-null HeLa cells. Likewise, folate pools in wildtype HeLa cells were not decreased by FR-alpha gene silencing and were negligibly affected in the RFC-null R5 subline grown with 5-formyltetrahydrofolate. FR-alpha surface binding in HeLa cells was shown to be greater than that in a variety of other human solid tumor cell lines. Hence, constitutively expressed FR-a in HeLa cells does not contribute to PMX activity in the presence or absence of RFC function. This is likely the case in many human solid tumor cell lines.