The p16INK4A tumor suppressor regulates cellular oxidative stress

Abstract

Mutations or deletions in the cyclin-dependent kinase inhibitor p16INK4A are associated with multiple cancer types, but more commonly found in melanoma tumors and associated with familial melanoma predisposition. Although p16 is thought to function as a tumor suppressor by negatively regulating the cell cycle, it remains unclear why genetic compromise of p16 predisposes to melanoma over other cancers. Here we describe a novel role for p16 in regulating oxidative stress in several cell types, including melanocytes. Expression of p16 was rapidly upregulated following UV-irradiation, and in response to H2O2-induced oxidative stress in a p38 stress-activated protein kinase-dependent manner. Knockdown of p16 using siRNA increased intracellular reactive oxygen species (ROS) and oxidative (8-oxoguanine) DNA damage which was further enhanced by H2O2 treatment. Elevated ROS were also observed in p16-depleted human keratinocytes, and in whole skin and dermal fibroblasts from Cdkn2a-deficient mice. Aberrant ROS and p38 signaling in Cdkn2a-deficient fibroblasts were normalized by expression of exogenous p16. The effect of p16 depletion on ROS was not recapitulated by knockdown of retinoblastoma protein (Rb) and did not require Rb. Finally, p16-mediated suppression of ROS could not be attributed to potential effects of p16 on cell cycle phase. These findings suggest a potential alternate Rb-independent tumor-suppressor function of p16 as an endogenous regulator of carcinogenic intracellular oxidative stress. Compared to keratinocytes and fibroblasts, we also found increased susceptibility of melanocytes to oxidative stress in the context of p16 depletion, which may explain why compromise of p16 predisposes to melanoma over other cancers.

A short acidic motif in ARF guards against mitochondrial dysfunction and melanoma susceptibility

“…13). Previous studies have implicated p16 in the control of the general levels of cellular ROS 37 . However, the observed increase in superoxide in ARF/ p16-depleted cells could be attributed to ARF deficiency alone, as it could be rescued by re-expression of ARF, but not p16 ( Supplementary Fig.…”

Melanoma genotypes and phenotypes get personal

“…11 The p16INK4a locus, besides regulating cell cycle activity, may also regulate production of reactive oxygen species (ROS) that may occur in the absence of exogenously induced oxidative stress. 12 The role for ROS in generating DNA mutations in MM, regardless of the exposure to sunlight, has gained momentum due to recent findings with red-hairbearing mice (so-called ginger mice), in which pheomelanin predominates over eumelanin. 13 Using genomically distinct strains of mice, it was demonstrated that there might be an ultraviolet radiation-independent pathway to MM carcinogenesis.…”

Genomics of Human Malignant Melanoma

“…The key regulatory molecule involved in this mechanism is p16 INK4a , which inhibits the activity of CDK4/6 kinases in a dose-dependent manner and consequently causes cell cycle arrest [Chin et al, 2006]. Recently, it has been shown that p16 INK4a functions as an alternate RB-independent regulator and suppressor of UV-induced DNA damage by reactive oxygen species and reduces the effect of oxidative stress in melanocytes via the p38 stress-activated protein kinase [Jenkins et al, 2011]. Frequent point mutations in the coding region of the CDKN2A locus typically target and inactivate p16 INK4a , while preserve p14 ARF in 25% to 40% of melanoma-prone families and 0.2% to 2% of sporadic melanomas [Chin et al, 2006;Meyle & Guldberg, 2009].…”

“…In normal melanocytes, free radicals are completely inactivated via the redox function of eumelanin. Both reactive oxygen species (ROS) and reactive nitrogen species (RNS) have been identified in melanoma [Jenkins et al, 2011]. The effect of NO in tumour progression is dose dependent; a high NO concentration can lead to apoptosis via its effect on multiple apoptosis-related proteins, such as p53 and Bcl-2, and growth inhibition, whereas a low NO concentration may contribute to tumour growth and angiogenesis [Palmieri et al, 2009].…”