Cell crowding induces interferon regulatory factor 9, which confers resistance to chemotherapeutic drugs.

The mechanism of multicellular drug resistance, defined as the reduced efficacy of chemotherapeutic drugs in solid tumors is incompletely understood. Here we report that colon carcinoma cells cultured as 3D microtissues (spheroids) display dramatic increases in the expression of a subset of type I interferon-(IFN)-stimulated genes (ISGs). A similar gene signature was associated previously with resistance to radiation and chemotherapy, prompting us to examine the underlying biological mechanisms. Analysis of spheroids formed by different tumor cell lines and studies using knock-down of gene expression showed that cell crowding leads to the induction of IFN regulatory factor-9 (IRF9) which together with STAT2 and independently of IFNs, is necessary for ISG upregulation. Increased expression of IRF9 alone was sufficient to induce the ISG subset in monolayer cells and to confer increased resistance to clinically used cytotoxic drugs. Our data reveal a novel mechanism of regulation of a subset of ISGs, leading to drug resistance in solid tumors. Therapy resistance represents a major hurdle in the management of cancer patients. Several resistance mechanisms have been described (e.g., overexpression of efflux pumps, mutations in the DNA repair system, activation of the antiapoptotic machinery). Multicellular resistance, suggested to be important for the resistance of advanced solid tumors, 1 is due both to limited penetration of drugs into tumor parenchyma 2 and to the presence of quiescent cell populations in hypoxic regions. These populations are insensitive to clinically used drugs and are believed to repopulate tumors between therapy cycles ("regrowth resistance"). 3