BACKGROUND Protein kinase C (PKC), centrally involved in signal transduction, has been implicated in colorectal carcinogenesis. The purpose of this study was to identify specific PKC isoform alterations associated with colorectal cancer metastases to the liver in an orthotopic transplantation nude mouse model. MATERIALS AND METHODS Solid subcutaneous tumors from colorectal cancer cell lines were established in dorsal sites of nude mice (3 mice per cell line) by subcutaneous injection of 10(7) cells (> 90% viable). Orthotopic transplantation cecal tumors, representative of each passage (S1-5) were examined for specific PKC isoform messenger ribonucleic acid (mRNA) expression. In addition, a cell line representative of passage S5 was established, characterized by light and electron microscopy, karyotype, clonogenicity, doubling time, and assayed for total PKC activity and PKC isoform mRNA expression. RESULTS After the fifth (S5) sequential orthotopic transplantation passage of the human colorectal cancer cell line, SW620, a highly metastatic clone was obtained. Relative to parental cells, metastatic SW620-S5 cells were less differentiated and demonstrated many more chromosomal abnormalities and lower clonogenicity. Total PKC activity was elevated in metastatic cells. In addition, specific PKC isoform mRNA alterations were noted: PKC-n (L) was abundantly expressed in the metastatic clone but absent from the parental cell line; PKC-alpha, delta and theta expression increased with serial orthotopic transplantation passages; PKC-delta remained unchanged, while PKC-beta was absent. CONCLUSIONS Metastases-specific PKC isoform alterations may serve as novel markers of metastases and treatment targets via specific PKC isoform modulation.