Abstract: | Kinetic resistance plays a major role in the failure of chemotherapy towards many solid tumors. Kinetic resistance to cytotoxic
drugs can be reproduced in vitro by growing the cells as multicellular spheroids (Multicellular Resistance) or as hyperconfluent
cultures (Confluence-Dependent Resistance). Recent findings on the cell cycle regulation have permitted a better understanding
why cancer cells which arrest in long quiescent phases are poorly sensitive to cell-cycle specific anticancer drugs. Two cyclin-dependent
kinase inhibitors (CDKI) seem particularly involved in the cell cycle arrest at the G1 to S transition checkpoint: the p53-dependent
p21cip1 protein which is activated by DNA damage and the p27kip1 which is a mediator of the contact inhibition signal. Cell quiescence could alter drug-induced apoptosis which is partly
dependent on an active progression in the cell cycle and which is facilitated by overexpression of oncogenes such as c-Myc
or cyclins. Investigations are yet necessary to determine the influence of the cell cycle on the balance between antagonizing
(bcl-2, bcl-XL...) or stimulating (Bax, Bcl-XS, Fas...) factors in chemotherapy-induced apoptosis. Quiescent cells could also be protected from toxic agents by an enhanced
expression of stress proteins, such as HSP27 which is induced by confluence. New strategies are required to circumvent kinetic
resistance of solid tumors: adequate choice of anticancer agents whose activity is not altered by quiescence (radiation, cisplatin),
recruitment from G1 to S/G2 phases by cell pretreatment with alkylating drugs or attenuation of CDKI activity by specific
inhibitors.
This revised version was published online in August 2006 with corrections to the Cover Date. |