Cell-specific aptamers for targeted therapies

Abstract

Many signalling proteins involved in diverse functions such as cell growth and differentiation can act as oncogenes and cause cellular transformation. These molecules represent attractive targets for cancer diagnosis or therapy and therefore are subject to intensive investigation. Aptamers are small, highly structured nucleic acid molecules, isolated from combinatorial libraries by a procedure termed SELEX. Aptamers bind to a target molecule by providing a limited number of specific contact points imbedded in a larger, defined three-dimensional structure. Recently, aptamers have been selected against whole living cells, opening a new path which presents three major advantages: (1) direct selection without prior purification of membrane-bound targets, (2) access to membrane proteins in their native conformation similar to the in vivo conditions and (3) identification of (new) targets related to a specific phenotype. The ability to raise aptamers against living cells opens some attractive possibilities for new therapeutic and delivery approaches. In this chapter, the most recent advances in the field will be reviewed together with detailed descriptions of the relevant experimental approaches.

Fig. 5.1

The whole-cell SELEX technology allows identifying unique molecular features of cancer cells by selecting aptamers in a physiological context, and, most importantly, it can be done without prior knowledge of the target molecules.

Fig. 5.2

Schematic of PSMA aptamer–siRNA chimera.

Fig. 5.3

Mechanism of aptamer–siRNA chimera-mediated targeting and silencing. Target specificity by the RNA chimera can be achieved both at level of the aptamer (PMSA-specific) as well as at the level of the siRNA (by silencing cancer cell-specific survival factors). This approach leads to selective killing of cancer cells that express both the cell surface receptor PSMA and prostate cancer-specific survival factors (e.g. Plk1 and Bcl2).

Fig. 5.4

Schematic protocol for the selection of PC12/MEN2A cell-specific aptamers.

Fig. 5.5

PC12/MEN2A cells were either left untreated or treated with the indicated RNA aptamer, or the starting RNA pool (pool). Cell lysates were immunoblotted with anti-pErk antibody, then stripped and reprobed with anti-ERK antibody to confirm equal loading. Values below the blots indicate signal levels relative to untreated controls.