Pro-B-cell-specific transcription and proapoptotic function of protein kinase Cη

Using a subtractive cloning scheme on cDNA prepared from primary pro-B and pre-B cells, we identified several genes whose products regulate apoptosis. We further characterized one of these genes, encoding protein kinase Cη (PKCη). PKCη transcripts were readily detected in pro-B cells but were absent in pre-B cells. Although both a full-length and a truncated form of PKCη were detectable in bone marrow pro-B cells, transition to the pre-B-cell stage was associated with increased relative levels of truncated PKCη. We found that PKCη is proteolyzed in apoptotic lymphocytes, generating a kinase-active fragment identical to the truncated form which is capable of inducing apoptosis when expressed in a pro-B cell line. Caspase-3 can generate an identical PKCη cleavage product in vitro, and caspase inhibitors prevent the generation of this product during apoptosis in transfected cell lines. Inducible overexpression of either the full-length or truncated form of PKCη results in cell cycle arrest at the G₁/S transition. These results suggest that the expression and proteolytic activation of PKCη play an important role in the regulation of cell division and cell death during early B-cell development.