Human Cancers Derived from Either Genetic or Lifestyle Factors Are Initiated by Impaired Estrogen Signaling

Background: Genetic studies have found that a germline BRCA1 gene mutation is the origin of highly increased cancer risk. Clinical studies have suggested that increased cancer risk in type-2 diabetes may be attributed to unhealthy lifestyle factors and bad habits. Purpose: Patients with either BRCA1 gene mutation or type-2 diabetes similarly exhibit increased cancer risk, insulin resistance, and fertility disorders. It was suggested that these three alterations derive from a common genomic failure, and its recognition may shed light on the unsolved secret of cancer. Results: (1) Germline mutations on ESR1, BRCA1, and CYP19A genes encoding estrogen receptor alpha (ERα), genome safeguarding BRCA1 protein, and CYP19 aromatase enzyme cause genomic instability. BRCA1 and ESR1 gene mutations specifically cause breast cancer, while error in the CYP19A gene leads to cancers in the endometrium, ovaries, and thyroid. (2) ERα, BRCA1, and CYP19 aromatase proteins are transcription factors creating the crucial DNA stabilizer circuit driven by estrogen regulation. Liganded ERα drives a second regulatory circuit to also control cell proliferation, in partnership with various growth factors. In a third regulatory circuit, liganded ERα drives cellular glucose supply in close interplay with insulin, IGF-1, and glucose transporters. (3) Impaired expression or activation of each transcription factor of the triad leads to defective estrogen signaling and endangers regular cell proliferation, insulin sensitivity, and fertility. (4) Impaired estrogen signaling caused by either genetic or lifestyle factors alarms the hypothalamus, which issues neural and hormonal commands throughout the body to restore estrogen signaling. (5) When the compensatory actions cannot restore estrogen signaling, the breakdown of genomic regulation leads to cancer initiation. (6) Lifestyle factors that upregulate estrogen signaling decrease cancer risk, while downregulating estrogen signaling increases it. Conclusions: Increased cancer risk, insulin resistance, and infertility all originate from defective estrogen signaling.