Umm, you have to have cancer 1st, before IGF will accelerate its proliferation... Thats the difference between "getting cancer" and "proliferating existing cancer". IGF does not "cause" cancer, it just activates receptors on cancer cells, telling them to do the same thing it does for myoblasts (muscle stem cells).
IGF-1 can cause antiapoptotic effects that make non cancerous cells cancerous, which then proliferates those cells into a hyperproliferation, which in turn can precede with the formation of an adenoma(tumor)
"The end results of these opposing effects of IGF-1 may be several-fold. First, there is increased proliferation and thus epithelial cell turnover within tissues. Second,
the antiapoptotic effects cause an imbalance in the usual tight control between proliferation and cell death and result in hyperproliferation. This is the first stage in the development of many cancers and has been particularly well demonstrated in colorectal tumorigenesis in which it precedes the formation of colonic adenomas. Third, such an imbalance between cell proliferation and death would favour, even slightly, survival of stem cells that had undergone early genetic 'hits'.[4] This would increase the pool of damaged cells available for second and subsequent hits. Higher levels of IGF-1 would be expected to activate survival pathways that would make programmed cell death of damaged cells slightly less probable. When applied overall to a large number of 'at risk' cells over many years, even a small influence in favour of survival of such cells could accelerate carcinogenesis, although not initiate cancer development per se."
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