Mastocytomas are one of the most frequent tumors in dogs and the most frequent skin tumors. Aggressive mast cell tumors are usually resistant against established therapy and the prognosis is grave.
Current research aims for discovers of new targets in neoplastic mast cells (MC) and the development of targeted drugs. Heme oxygenase-1 (HO-1), also known as heat shock protein 32 (Hsp32), is a survival molecule which is expressed in various neoplastic cells. In this study we examined the expression of Hsp32 in primary canine neoplastic MC as well as in the canine mastocytoma cell line C2. Expression of the Hsp32 protein was examined by immunocytochemistry and Western Blot-Analysis, and expression of Hsp32 mRNA by RT-PCR and Northern Blot-Analysis. To investigate the functional role of Hsp32, two Hsp32-targeting drugs, pegylated zinc-protoporphyrin (PEG-ZnPP) and styrene maleic acid-micelle-encapsulated ZnPP (SMA-ZnPP), were used in our experiments.
As assessed by immunocytochemistry and Western Blot-Analysis, C2 cells, as well as primary neoplastic MC expressed the Hsp32 protein in a constitutive manner. Moreover, we found that C2 cells express Hsp32 mRNA. Exposure of C2 cells to hemin resulted in an upregulation of Hsp32 expression. Both Hsp32-targeting drugs were found to inhibit the proliferation of C2 cells and of primary neoplastic MC. The growth-inhibitory effects of PEG-ZnPP and SMA-ZnPP were dose-dependent (IC50: 1-20 µM) and associated with apoptosis. In addition, we found that SMA-ZnPP cooperates with midostaurin in inhibiting the proliferation of C2 cells. In conclusion, Hsp32 is an important survival factor and interesting new target in neoplastic MC. Clinical trials with Hsp32-targeted drugs are now required to examine the in vivo antineoplastic efficacy of these agents.