SUMMARY Accumulating evidence implicates heterogeneity within cancer cell populations in the response to stressful exposures, including drug treatments. While modeling the acute response to various anti-cancer agents in drug-sensitive human tumor cell lines, we consistently detected a small subpopulation of reversibly “drug-tolerant” cells. These cells demonstrate >100-fold reduced drug sensitivity, and maintain viability via engagement of IGF-1 receptor signaling and an altered chromatin state that requires the histone demethylase RBP2/KDM5A/Jarid1A. This drug-tolerant phenotype is transiently acquired and relinquished at low frequency by individual cells within the population, implicating the dynamic regulation of phenotypic heterogeneity in drug tolerance. The drug-tolerant subpopulation can be selectively ablated by treatment with IGF-1 receptor inhibitors or chromatin-modifying agents, potentially yielding a therapeutic opportunity. Together, these findings suggest that cancer cell populations employ a dynamic survival strategy in which individual cells transiently assume a reversibly drug-tolerant state to protect the population from eradication by potentially lethal exposures.

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We examined the expression and localization of herpesvirus proteins in monkey cells transfected with recombinant plasmids containing herpes simplex virus (HSV) DNA sequences. Low levels of expression of the major HSV DNA-binding protein ICP8 were observed when ICP8-encoding plasmids were introduced into cells alone. ICP8 expression was greatly increased when a recombinant plasmid encoding the HSV alpha (immediate-early) ICP4 and ICPO genes was transfected with the ICP8 gene. Deletion and subcloning analysis indicated that two separate functions capable of stimulating ICP8 expression were encoded on the alpha gene plasmid. One mapped in or near the ICP4 gene, and one mapped in or near the ICPO gene. Their stimulatory effects were synergistic when introduced on two separate plasmids. Thus, two separate viral functions can activate herpesvirus early gene expression in transfected cells.Gene products encoded by animal viruses have provided some of the few examples of well-characterized eucaryotic functions that stimulate the expression of other viral and cellular genes. These provide excellent systems in which to study the mechanisms by which eucaryotic gene expression can be regulated. The adenovirus ElA function (3,14), the herpes simplex virus (HSV) ICP4 (5,16,23,32), and the pseudorabies virus immediate-early gene product (2) have been shown to activate expression of their respective early and late genes during the viral lytic cycles. The pseudorabies virus immediate-early gene product can complement the defective function of an ElA mutant virus (6). Therefore, it was hypothesized that these two functions operate through a common mechanism (6). The action of the simian virus 40 (SV40) large T antigen is somewhat more complicated in that it stimulates the expression of late SV40 genes while repressing early transcription (4, 15).The ElA or pseudorabies functions can stimulate the expression of viral and even cellular genes when these genes are introduced into cells with the marker gene by transfection (9, 13). It has been suggested that these functions stimulate transcription nonspecifically by inactivating an inhibitory cellular function (22) or promoting assembly of stable transcription complexes (7). Although genes introduced into cells by transfection are activated by ElA, similar genes in the cellular genome are not activated (9). Therefore, several levels of transcriptional regulation exist.Adenovirus VAl RNA has been shown to stimulate the translation of late adenoviral mRNAs during viral infection (31). Transfection of this gene into cells with an adenoviral gene also leads to an increase in translation of the mRNA (30). Therefore, viral gene products can affect the transcription of viral genes or the translation of viral mRNA. Many of these effects can be observed in cells transfected with the cloned viral regulatory genes.The HSV ICP8 DNA-binding protein is a beta (delayedearly) gene product whose expression depends on the synthesis of alpha (immediate-early) HSV proteins (12. 25 Plasmids. Escherichia c...

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