Credit: GETTY

The survival of chronic lymphocytic leukaemia (CLL) cells partly depends on pro-survival signals that come from the stromal cells. Research published in Cancer Cell has uncovered a signalling pathway that controls this cancer cell–stromal cell crosstalk.

Using co-culture of primary human CLL cells and a mouse stromal cell line (EL08-1D2), Ingo Ringshausen and colleagues showed that the presence of stromal cells protects the CLL cells from apoptosis. These stromal cells share genetic and morphological properties with so-called cancer-associated fibroblasts. Protein kinase C βII (PKCβII) in CLL cells has been shown to be required for apoptosis resistance, and the PKCβ inhibitor enzastaurin blocked the protective effect of EL08-1D2 cells, indicating the importance of this kinase for stroma–CLL interactions. Expression of PKCβII was increased in EL08-1D2 cells following contact with CLL cells, and CLL cells were no longer protected from apoptosis when co-cultured with EL08-1D2 cells in which PKCβII was suppressed by small interfering RNA (siRNA) or when co-cultured with bone marrow stromal cells (BMSCs) from Prkcb−/− (which encodes PKCβII) mice. Furthermore, ectopic expression of PKCβ in Prkcb−/− BMSCs rescued CLL cell survival. Together, these data indicate that PKCβII is important in the stromal cells.

lack of PKCβ in host mice prevented the formation of CLL-like malignancies

PKCβII is involved in the activation of nuclear factor-κB (NF-κB) signalling; indeed, NF-κB was activated in wild-type, but not in Prkcb−/−, BMSCs following co-culture with CLL cells, and the survival of CLL cellsin co-culture with EL08-1D2 cells wasblocked by pharmacological inhibition of NF-κB using an inhibitor of IKKβ (inhibitor of NF-κB (IκB) kinase-β). To confirm that this effect was specific to the stromal cells, the authors used BMSCs from mice deficient in the IκB complex member NEMO (also known as IKKγ). BMSCs with conditional knockout of Nemo did not protect CLL cells from apoptosis, and although NF-κB activation was blocked PKCβII was still induced, suggesting that PKCβII acts upstream in this pathway. Transcriptome analysis of BMSCs with and without Nemo knockout following co-culture with CLL cells pointed to decreased expression of cytokines in Nemo-null cells, and the authors showed that interleukin-1α (IL-1α) and IL-15 were induced in stromal cells in a NEMO-dependent manner. Exogenous administration of these cytokines to CLL cells co-cultured with NEMO-deficient BMSCs could also protect the CLL cells from apoptosis.

Is this pathway relevant in vivo? The authors transplanted malignant cells from mice bearing a CLL-like disease into Prkcb−/− or wild-type mice and found that lack of PKCβ in host mice prevented the formation of CLL-like malignancies. In patients with CLL, PKCβII was expressed in endothelial cells (these stromal cells were analysed owing to technical difficulties with analysing other subsets) in areas of bone marrow with CLL infiltration but not in areas lacking CLL cells.

Interestingly, the authors also observed a similar requirement for stromal PKCβII–NF-κB signalling in permitting the survival of primary human acute lymphoblastic leukaemia (ALL) and mantle cell lymphoma (MCL) cells, suggesting that this pathway could be crucial for several haematological malignancies.