Suppression of apoptosis induced by growth factor withdrawal by an oncogenic form of c-Cbl

The v-Cbl oncogene induces myeloid and B-cell leukemia; however, the mechanism by which transformation occurs is not understood. An oncogenic form of c-Cbl (Cbl-Delta Y371) was expressed in the interleukin-3 (IL-3) dependent cell line 32Dc13 to determine whether it was able to induce growth factor-independent proliferation. We mere unable to isolate clones of transfected 32Dc13 cells expressing Cbl-Delta Y371 that proliferated in the absence of IL-3. in contrast, 32Dcl3/Cbl-Delta Y371 cells did not undergo apoptosis like parental 32Dc13 cells when cultured in the absence of IL-3. Both 32Dc13 and 32D/Cbl Delta Y371 cells arrested in G(1) when cultured in the absence of IL-3, Approximately 18% of the 32Dcl3 cells cultured in the absence of IL-3 for 24 h were present in a sub-G(1) fraction, while only 4% of the 32D/Cbl-Delta Y371 and 2% of the 32D/Bcl-2 cells were found in a sub-G(1) fraction. There was no difference in the pattern of tyrosine-phosphorylated proteins observed following stimulation of either cell type with IL-3. The phosphorylation of JAK2, STAT5, and endogenous c-Cbl was identical in both cell types. No differences were detected in the activation of Akt, ERK1, or ERK2 in unstimulated or IL-3-stimulated 32D/Cbl-Delta Y371 cells compared with parental 32Dc13 cells. Likewise, there was no difference in the pattern of phosphorylation of JAK2, STATE, ERK1, ERK2, or Akt when 32Dcl3 and 32D/CblDY371 cells were withdrawn from medium containing IL-3. The protein levels of various Bcl-2 family members were examined in cells grown in the absence or presence of IL-3. We observed a consistent increased amount of Bcl-2 protein in five different clones of 32D/Cbl-Delta Y317 cells. These data suggest that the Cbl-Delta Y371 mutant may suppress apoptosis by a mechanism that involves the overexpression of Bcl-2. Consistent with this result, activation of caspase-3 was suppressed in 32D/Cbl-Delta Y371 cells cultured in the absence of IL-3 compared with 32Dcl3 cells cultured under the same conditions.