Diabetes mellitus has been proposed as an epidemiological risk factor for human liver cancer development. One reasonable possibility is that this is attributable to hyperinsulinemia compensatory for obesity-related insulin resistance. However, diabetes mellitus is a complex disease with multiple abnormal conditions essentially caused by hyperglycemia. Therefore, it is not evident whether hyperinsulinemia is prerequisite for the elevated cancer risk. To gain a clue to answer this question, we characterized chemically induced hepatocarcinogenesis in diabetic model mice genetically deficient for insulin. Akita inbred mice originating from the C57BL/ / 6 strain carry a heterozygous germline mutation of the insulin II gene and suffer from inherited insulin deficiency and diabetes in an autosomal dominant manner. They were mated with normal C3H/ /HeJ mice with high sensitivity to liver carcinogenesis and the resultant F 1 littermates, which were either normal or insulin deficient, were exposed to diethylnitrosamine and induced hepatocellular tumors were evaluated for number, size, proliferative activity, and apoptosis. Unexpectedly, both mean and total volumes of hepatocellular tumors in the insulin-deficient animals were more than twofold larger than those in the normal controls, with no significant difference in tumor number. The tumors in insulin-deficient mice showed a significantly lower frequency of apoptosis but no alteration in cell proliferation. In conclusion, our results indicate that insulin-independent liver tumor promotion occurred in diabetic mice. Clearly, insulin-independent mechanisms for the human case also deserve consideration. (Cancer Sci 2010; 101: 65-72) A lthough persistent hepatitis B or C viral infection is the rate-limiting risk factor for human liver carcinogenesis in most cases, (1,2) the process can be significantly modified by other agents, one of which is diabetes mellitus (DM). Epidemiological studies have consistently confirmed DM, mostly of type 2, to be associated with approximately twofold elevation in the frequency of hepatocellular carcinonomas. (3)(4)(5) This is a serious social issue, as the number of DM patients is now rapidly increasing, particularly in developed countries. Similar epidemiological data have also been reported for cancers of the pancreas, endometrium, colorectum, and breast, (6) while data for the lung, for instance, are controversial, (7,8) suggesting organ-specific influences. As one recent study proposed that DM promotes human hepatocarcinogenesis associated with chronic hepatitis C, but not hepatitis B, DM-related liver cancer risk may also depend on the type of viral infection. The mechanism of liver cancer promotion by DM is not well understood, mainly because DM of type 2 is a consequence of a complex pathogenesis involving both obesity-related insulin resistance and eventual impaired insulin secretion. Nevertheless, many investigators have proposed that compensatory hyperinsulinemia associated with insulin resistance seen in pre-DM or early DM pop...