Tumor-induced STAT3 hyperactivation creates a dendritic cell differentiation block by significantly downregulating protein kinase C βII expression (P2003)

Abstract

Tumor mediated blockade of dendritic cell (DC) differentiation is a large component of cancer induced immunosuppression, contributing to tumor outgrowth. Tumors impair DC differentiation via factors that hyperactivate STAT3 in DC progenitors, though the mechanisms by which this occurs are largely unknown. PKCβII is essential in DC differentiation, and we test here if tumor driven STAT3 hyperactivation downregulates PKCβII expression, and if this is the mechanism underlying impaired DC differentiation. Myeloid cells from tumor bearing mice have significantly decreased PKCβII expression. Culture in tumor conditioned media (TCM) significantly decreased PKCβII expression and significantly impaired phorbol ester driven DC differentiation in human monocytes and in a human cell line model (KG1). In KG1, this was dependent on PKCβII downregulation: enforced PKCβII expression preserved phorbol ester driven DC differentiation in the face of TCM. Interestingly, PKCβII overexpression and/or activation also antagonized TCM-driven STAT3 activation. STAT3 drives this decreased PKCβII expression: TCM induced STAT3 activation and drove 4.5 fold higher STAT3 binding to the PKCβ promoter (compared to media control, p<0.001). Mutation of a STAT3 consensus binding site in the promoter eliminates the ability of STAT3 to impair PKCβ promoter activity. Together, these observations argue that tumor driven STAT3 hyperactivation inhibits DC differentiation by downregulating PKCβII.