CD44 is a marker of tumour-initiating cells and is upregulated in invasive breast carcinoma; however, its role in the cancer progression is unknown. Here, we show that antibody-mediated CD44-targeting in human breast cancer xenografts (HBCx) significantly reduces tumour growth and that this effect is associated to induction of growth-inhibiting factors. Moreover, treatment with this antibody prevents tumour relapse after chemotherapy-induced remission in a basal-like HBCx.

Acute myeloid leukemia (AML) is sustained by the extensive proliferation of leukemic stem and progenitor cells, which give rise to the population of leukemic blasts with defective differentiation and low proliferative capacity. We have recently shown that ligation of CD44, a cell surface molecule present on AML cells, with specific monoclonal antibodies (mAbs) inhibits their proliferation. However, its mechanism has not been investigated yet. Here, using the NB4 cell line as a model of proliferating human AML cells, and the A3D8 mAb to ligate CD44, we show for the first time that CD44 ligation stabilizes the cyclin-dependent kinase inhibitor p27 Kip1 (p27) protein, resulting in increased association with cyclin E/Cdk2 complexes and inhibition of their kinase activity. Moreover, using a p27 antisense vector, we provide direct evidence that p27 is the main mediator of cell growth arrest by CD44. CD44 ligation also leads to p27 accumulation in THP-1, KG1a, and HL60 cell lines and in primary leukemic cells, suggesting that this process is general in AML. Taken together, our present results suggest that CD44 is a new and efficient means to increase the expression of p27 in AML cells. Considering that elevated expression of p27 is a factor of good prognosis in AML, these results provide a new basis for developing CD44-targeted therapy in AML. IntroductionAcute myeloid leukemia (AML) is characterized by the excess production of leukemic blasts arrested at various stages of granulocytic and monocytic differentiation. These stages define distinct AML subtypes (AML0 to AML5). 1 Most leukemic blasts have low proliferative capacity, and the leukemic clone is maintained by the extensive proliferation of subpopulations of leukemic stem and progenitor cells. 2 Therefore, elimination of proliferating leukemic cells is central to effectively cure AML patients. Because chemotherapy rarely eradicates the leukemic clone, efforts are being made to find innovative therapies, including ways for inhibiting the proliferation of AML cells (which is compatible with associated chemotherapy). 3,4 CD44 is a transmembrane glycoprotein expressed on the surface of normal and leukemic myeloid cells. [5][6][7][8] It mediates the adhesion of normal progenitors and of leukemic cells to hyaluronan, 9-11 a glycosaminoglycan component of the extracellular matrix. 12 CD44 has an important role in normal myelopoiesis, because anti-CD44 antibodies profoundly alter in vitro myelopoiesis in long-term bone marrow cultures (some antibodies fully abrogate it [13][14][15] and others enhance it [15][16][17] ). In the context of AML, our own experiments have shown that it is possible to reverse differentiation blockage in AML cells through CD44 ligation with specific antibodies. 18,19 This reversion was obtained in primary blasts from distinct AML subtypes, indicating new possibilities for the development of CD44-targeted differentiation therapy in AML. 18 It was also obtained in cell lines that are models of AML3 (NB4, promyelocytic), AML5 (THP-1, monoblastic)...